Communications ProtocolsOne way to think of Communications Protocols is a standard procedure for regulating data transmission. Communication is typically between computers, or some electronic devices.

Communications ProtocolsAll communications between devices require that the devices agree on the format and characteristics of the data.

The set of rules defining these is called a protocol.

The communications protocol must define the following: Rate of transmission (in baud or bps) Whether transmission is to be synchronous or asynchronous Whether data transmission is half-duplex or full-duplex mode Ways to encode and decode dataSignal levelsDynamic Signal Characteristics (Tr, Tf, PW, etc.)Handshaking techniques (# of signals, timing, signal levels, etc.)Interrupt SystemHardware used in the system.

Communications Protocols

In addition, protocols can include sophisticated techniques for detecting and recovering from transmission errors (techniques such as Reed-Solomon codes, BCH codes, the binary Golay code, a binary Goppa code, a Viterbi decoder and more).

Protocols can also describe encoding / decoding data and handshaking techniques.

Before discussing any specific protocols, first lets define a standard

Standards WHAT IS A STANDARDAn agreed upon set of rules describing both the structure and the operation characteristics of a communication technique.They are created by organizations representing many industrial segments from both a regulatory position as well as a commercial position. Examples of organizations are Electronics Industry Alliance (EIA) (RS). (One of a set of standards from the Electronics Industries Association for hardware devices and their interfaces - RS-232 is a well-known standard for transmitting serial data by wire.)Institute of Electrical and Electronic Engineers (IEEE): Example of a standard from IEEE is the GPIB - IEEE 488International Standards Organization (IS0)

Why Have StandardsCompatibility among devices from different manufacturers.Standardization of devices, driving cost downHelps reduced design time/cost.Users know what they are getting in terms of operational performance NO SURPRISES

Standards In Bus Structuresor Communications ProtocolsSeveral structures in use today.

Structures evolve because requirements change as technology changes.

Different structures required for different applications.

**This is not all inclusive, rather only a sample.

PrecautionsAlways be cautious about buying a device or product that advertise NEAR specific BUS performance characteristics.The specific characteristic you want will probably not be available.

Now lets look at some examples of SpecificCommunication ProtocolsProtocol for connecting hardware together

Parallel Structure

Serial Structure

Developed by manufacturers like HP

RequirementsMany times it is required to interconnect many devices to one system on a single bus10 to 20 devices.If each was to be handled by a single C port, this would require extensive I/O on the computer.

RequirementsThe alternative:Use a bus that has some established operation that allows the C to I/F with minimal I/O.

Possible SolutionsSeveral structures or protocols would lend themselves to satisfy this requirement:Parallel structure IEEE 488 Standard busSerial Controller Area Network (CAN bus)Serial USBSerial Mil Std 1553 BusSerial Firewire IEEE 1384However some are more efficient and have the necessary features already designed in.

IEEE 488We will first look at a solution that is most related to Data Acquisition systemDeveloped by HP as an 8 bit parallel, byte serial I/F bus to overcome the problem of lack of standardizationIt is most efficient to handle a complex hardware arrangement.

IEEE HistoryThe IEEE-488 bus was developed to connect and control programmable instruments.It would provide a standard interface for communication between instruments from different sources. Hewlett-Packard originally developed the interfacing technique, and called it HP-IB. The interface quickly gained popularity in the computer industry because the interface was so versatile.The IEEE committee renamed it GPIB (General Purpose interface Bus).

IEEE 488 Purpose of this protocol is to provide a standardized parallel communication method to connect 2 to 15 devices to enhance data transfer.

IEEE-488 OverviewAlmost any instrument can be used with the IEEE-488 specification, because it says nothing about the function of the instrument itself, or about the form of the instrument's data. The specification defines a separate component, the interface device, that can be added to the instrument. The signals passing into the interface from the IEEE-488 bus and from the instrument are defined in the standard. The instrument does not have complete control over the interface. Often the bus controller tells the interface what to do. The Active Controller performs the bus control functions for all the bus instruments.

IEEE 488 FeaturesIEEE 488 is a cable based bus with a special connector which allows several instruments to be connected in a star or linear configuration or in any combination. Each cable end has a combined male and female connector to allow parallel connection of cables at any device. Some of the key features of the IEEE 488 interface are: - Up to 15 devices may be connected to one bus. - Total bus length may be up to 20m and the distance between devices may be up to 2m. - One byte (8 bit) of digital information is sent in parallel each time. - Message transactions are hardware handshake using special bus lines. - Maximum data rate is 1 Mbyte/s. (8 Mbytes with HS488 bus)

IEEE 488 FeaturesThe limitations in maximum distance and number of units may be avoided by using special units - bus extenders, bus expanders - from various companies. National Instruments suggested a new transfer protocol HS488 which allows up to 8 Mbyte/s transfer rate on the usual GPIB-cable.

IEEE 488 ConfigurationIs a 24 wire bus that has8 control lines 8 data lines8 ground and shieldsSpecific mechanical connection (24 pin connector)Operates at 500KHz.8 bit wide data bus

IEEE488 Connector Configuration

IEEE488 logicSignal drivers must be open collector logic which allows for a parallel, multidrop, connection of all devices. Logical TRUE and data 1 is defined for voltages +2V (ca. TTL-levels).

IEEE488 System Controller and Active ControllerAt power-up time, the IEEE-488 interface that is programmed to be the System Controller becomes the Active Controller in charge.The System Controller has several unique capabilities including the ability to send Interface Clear (IFC) and Remote Enable (REN) commands. IFC clears all device interfaces and returns control to the System Controller. REN allows devices to respond to bus data once they are addressed to listen. The System Controller may optionally Pass Control to another controller, which then becomes Active Controller.

IEEE488 Listeners, Talkers and ControllersThere are 3 types of devices that can be connected to the IEEE-488 bus (Listeners, Talkers, and Controllers).

MasterSlaveSlave

IEEE488 Listeners, Talkers and ControllersThe Active Controller may pass control to another controller which in turn can pass it back or on to another controller. A Listener is a device that can receive data from the bus when instructed by the controller.A Talker transmits data on to the bus when instructed. The Controller can set up a talker and a group of listeners so that it is possible to send data between groups of devices as well.

IEEE488 Listeners, Talkers and ControllersSome devices include more than one of these functions. The standard allows a maximum of 15 devices to be connected on the same bus. A minimum system consists of one Controller and one Talker or Listener device (i.e., a microcontroller with an IEEE-488 interface and a voltmeter with an IEEE-488 interface). It is possible to have several Controllers on the bus but only one may be active at any given time.

IEEE488 Data LinesThe lines DIO1 through DIO8 are used to transfer addresses, control information and data.

The formats for addresses and control bytes are defined by the IEEE 488 standard. Data formats are undefined and may be ASCII (with or without parity) or binary.

DIO1 is the Least Significant Bit (note that this will correspond to bit 0 on most computers).

IEEE488 Handshake LinesThree lines asynchronously control the transfer of message bytes between devices.

The process is called a 3-wire interlocked handshake. This handshaking process guarantees that the bytes on the data lines are sent and received without any transmission errors and is one of the unique features of the IEEE-488 bus

IEEE488 Handshake LinesThe three handshake lines (NRFD, NDAC, DAV) control the transfer of message bytes among the devices and form the method for acknowledging the transfer of data. These lines are identified as:

NRFD (Not Ready for Data) handshake line is asserted by a Listener (pulled low) to indicate it is not yet ready for the next data or control byte. Note that the Controller will not see NRFD released (i.e., ready for data) until all devices have released it.

NDAC (Not Data Accepted) handshake line is asserted by a Listener to indicate it has not yet accepted the data or control byte on the data lines. Note that the Controller will not see NDAC released (i.e., data accepted) until all devices have released it.

DAV (Data Valid) handshake line is asserted (pulled low) by the Talker to indicate that a data or control byte has been placed on the data lines and has had the minimum specified stabilizing time. The byte can now be safely accepted by the devices.

IEEE488 HandshakingThe handshaking process is outlined as follows: When the Controller or a Talker wishes to transmit data on the bus, it sets the DAV line high (data not valid), and checks to see that the NRFD and NDAC lines are both low, and then it puts the data on the data lines. When all the devices that can receive the data are ready, each releases its NRFD (not ready for data) line. When the last receiver releases NRFD, and it goes high, the Controller or Talker takes DAV low indicating that valid data is now on the bus.

In response each receiver (listner) takes NRFD low again to indicate it is busy and releases NDAC (not data accepted) when it has received the data. When the last receiver (listner) has accepted the data, NDAC will go high and the Controller or Talker can set DAV high again to transmit the next byte of data.

IEEE488 HandshakingNote that if after setting the DAV line high, the Controller or Talker senses that both NRFD and NDAC are high, an error will occur. Also if any device fails to perform its part of the handshake and releases either NDAC or NRFD, data cannot be transmitted over the bus. Eventually a timeout error will be generated.

The speed of the data transfer is controlled by the response of the slowest device on the bus, for this reason it is difficult to estimate data transfer rates on the IEEE-488 bus as they are always device dependent.

IEEE488 Operation

MasterSlaveSlave

IEEE488 Operation The following block diagram depicts bus operation

Master

Listner

Listner

Listner

Listner

Talker

Listner

IEEE488 OperationSee timing diag next slide

Start

Master Assigns Talker

Talker sends DATA VALID command (DAV)

Talker puts data on data bus

Listeners configure data bus as inputs

Talker configures data bus as output

Master gives up bus to the talker

Master Assigns Listeners

As each Listener receives data, it acknowledges that it ahs read the data

When last Listener receives data, it signals the Talker

Is there more data??

Talker puts new data on Bus

Talker indicates data is valid

Talker notifies Master that data exchange is complete

Master takes over bus

Master assigns Talker and Listeners and cycle repeats

End

No

Yes

HANDSHAKE

Handshake ProcessNote: No time scale shown; asynchronous transmission.

CONTROL SIGNALS

IEEE488 Interface Management LinesInterface Management Lines The five interface management lines (ATN, EOI, IFC, REN, SRQ) manage the flow of control and data bytes across the interface.

ATN (Attention) signal is asserted by the Controller to indicate that it is placing an address or control byte on the data bus. ATN is released to allow the assigned Talker to place status or data on the data bus. The Controller regains control by reasserting ATN; this is normally done synchronously with the handshake to avoid confusion between control and data bytes.

EOI (End or Identify) signal has two uses. A Talker may assert EOI simultaneously with the last byte of data to indicate end-of-data. The Controller may assert EOI along with ATN to initiate a parallel poll. Although many devices do not use parallel poll, all devices should use EOI to end transfers (many currently available ones do not). IFC (Interface Clear) signal is asserted only by the System Controller in order to initialize all device interfaces to a known state. After releasing IFC, the System Controller is the Active Controller.

IEEE488 Interface Management LinesREN (Remote Enable) signal is asserted only by the System Controller. Its assertion does not place devices into remote control mode; REN only enables a device to go into remote mode when addressed to listen. When in remote mode, a device should ignore its local front panel controls.

SRQ (Service Request) line is like an interrupt: it may be asserted by any device to request the Controller to take some action. The Controller must determine which device is asserting SRQ. The requesting device releases SRQ when it is serviced.

Service Request (Interrupt)When a device on the IEEE 488 bus requests service, it does so through an interrupt-like activity using the SRQ lineThe controller reacts to this by talking to the device and providing the necessary serviceDepending on how many devices are on the bus, different techniques are employed.

Service Request (Interrupt)If there are fewer than 9 devices, typically each device is assigned one of the data lines as a interrupt response.The controller does a parallel poll in which the data bus is viewed and the interrupting equipment indicates via the data line assigned to it.

Service Request (Interrupt)If there are >8 devices, the controller must do a serial poll.A serial poll is one in which the controller interfaces with each device, one at a time, to identify the cause of the interrupt.Typically a slower process

IEEE488 SummaryThe IEEE-488.1 standard greatly simplified the interconnection of programmable instruments by clearly defining mechanical, hardware, and electrical protocol specifications.

For the first time, instruments from different manufactures were connected by a standard cable.

This standard does not address data formats, status reporting, message exchange protocol, common configuration commands, or device specific commands.

The IEEE-488.2 standard enhances and strengthens the IEEE-488.1 standard by specifying data formats, status reporting, error handling, controller functionality, and common instruments commands.

It focuses mainly on the software protocol issues and thus maintains compatibility with the hardware- oriented IEEE-488.1 standard. IEEE-488.2 systems tend to be more compatible and reliable.

IEEE488 Connector Configuration

IEEE488 Connector Configuration

*What is computer interfacing?Computer interfacing: the art of connecting computers and peripherals. In a lot of circumstances, it looks more like magic than art. It is not uncommon that you end up removing all unnecessary hardware from your computer to get that communication device to work. Despite all plug-and-play internal hardware solutions for the PC, connecting a number of external devices still requires some amount of technical knowledge and experience.

*Office(opt)USB2.0Monitor2-4GbpsOffice(core)etc10/100/1000Ethernet802.11xUSB HUBINTERFACING COMPONENTS

*In computer interfacing it is often difficult to find the right cable for a specific purpose. Although the USB interface tries to solve this problem, there are many situations where you need to search for the right cable. This can be the case when you need a RS-232 or parallel cable to connect a device to your computer. There is also information about modular cables and cables for connecting PLC's if you happen to work in the industrial automation business

*A typical parallel port on the back of your computer

*Standard Parallel PortThis is the original parallel port, it's been used on all PC computers since the very beginning and has mostly remained unchanged over the years. When IBM came up with the first PC, they opted to go along the lines of the the most prominent printer manufacturer at the time, Centronics. This printer manufacturer had developed a set of control signals that was up to the task of controlling computer printers and since many printer manufacturers had been adopting this now standard design, it was the obvious choice to make.

*The parallel port socket on your computer uses 25 pins. On most peripherals, the 36 pins Centronics version is used. Both connector pinouts are shown here. The centronics socket is named after the company that introduced the first dot matrix printer in 1970, but after IBM and Epson took over the dot matrix printer market (later followed by Hewlett Packard in the laser and deskjet printer segment) most people only associate the word centronics with the port interface itself, not with a manufacturer.

* But when IBM constructed it's PC they opted to not use the true Centronics connector which was a 36 conductor Amphenol connector (also known as the Centronics connector). IBM opted for a 25 pin D shell connector also called a DB-25 connector. Since then, printer manufacturers have always used Centronics connectors and PC manufacturers have been using DB-25 connectors. This is the reason why you need this special adapter cable that is known as a printer cable and is now a standard accessory.

* Following are pictures of the connectors pin view and it's signal assignments, also are a description of each signal.

Each signal is identified by its pin number on a DB-25 and Centronics 36 pin connector and its signal name.

*Parallel DB 25 pinCentronics pin

*Parallel printer cable

LineDB 25 male (computer)CStrobe1 1Data bit 02 2Data bit 13 3Data bit 24 4Data bit 35 5Data bit 46 6Data bit 57 7Data bit 68 8Data bit 79 9Acknowledge10 10Busy11 11Paper out12 12Select13 13Autofeed14 14Error15 32Reset16 31Select17 36Signal ground18 33Signal ground19 19 + 20Signal ground20 21 + 22Signal ground21 23 + 24Signal ground22 25 + 26Signal ground23 27Signal ground24 28 + 29Signal ground25 16 + 30ShieldCover Cover + 17

*Standard Centronics Parallel Cable Db-25 To Centronics 36DB-25 PIN Male (PC) Centronics 36 Male CENTRONICS 36 MALE CENTRONICS 36 FEMALE 1 --------------------------------------> 1 Strobe * 2 2 Data bit 0 + 3 3 Data bit 1 + 4 4 Data bit 2 + 5 6 Data bit 3 + 6 6 Data bit 4+ 7 7 Data bit 5 + 8 8 Data bit 6 + 9 9 Data bit 7 +

* 10

* Note!! * denotes and active low signal. This pin out depicts the newer bi-directional parallel port with input and output capabilities often used with external tape drives and accessory devices. If pins 31 or 32 are grounded on a cable the printer will fail to come ready when attached to the PC. This is common on low cost parallel printer cables.

*Strobe The strobe line is the heart of the parallel port, it tells the printer when to sample the information of the data lines, it is usually high and goes low when a byte of data is transmitted. The timing is critical for the data to be read correctly, all bits on the data lines must be present before the strobe line goes low, to insure data integrity when the printer samples the data lines. The time needed for each byte is about half a microsecond then the the strobe line goes low for about one microsecond and then the data is usually still present for another half microsecond after the strobe goes high. So the total time needed to transmit a full byte is around two microseconds.

*DataThese 8 lines carry the information to be printed and also special printer codes to set the printer in different modes like italics, each line carries a bit of information to be sent, the information here travels only from the computer to the printer or other parallel device. These lines function with standard TTL voltages, 5 volts for a logical 1 and 0 volts for a logical 0.

*AcknowledgeThis line is used for positive flow control, it lets the computer know that the character was successfully received and that it's been dealt with. It's normally high and goes low when it has received the character and is ready for the next one, this signal stays low for about 8 microseconds.

*BusyAs seen previously (strobe line), each byte takes about 2 microseconds to be sent to the printer, this means the printer is receiving about 500,000 bytes per second (1 sec divided by 2 microseconds).No printer can print this fast, so they came up with a busy line. Each time the printer receives a byte this line will send high to tell the computer to stop sending, when the printer is done manipulating the byte (printing, putting it in the buffer or setting it's internal functions) it then goes back low, to let the computer know that it can send the next byte.

*Paper EndAlso referred to as Paper Empty, this line will go high when you run out of paper, just like the paper out light on your printer, this way the computer will know and can tell you of the problem. When this happens the busy line will also go high so the computer stops sending data. Without this line when you would run out of paper the busy line would go high and the computer would seem to be hanged.

*SelectThis line tells the computer when it is selected (or online), just like the light on your printer. When the select line is high the printer is online and is ready to receive data, when it's low the computer will not send data.

*Auto FeedNot all printers treat the carriage return the same way, some will just bring the print head to the beginning of the the line being printed and some will also advance the paper one line down (or roll the paper one line up). Most printers have a DIP switch or some other way to tell your preference of how to interpret the carriage return. The auto feed signal lets your computer do the job for you, when it puts this signal low, the printer will feed one line when it gets a carriage return, by holding the signal high the software must send a line feed along with the carriage return to obtain the same effect.

*ErrorThis is a general error line, there is no way of knowing the exact error from this line. When no errors are detected, this line is high, when an error is detected it goes low. Some of the errors that can arise through this line are: cover open, print head jammed, a broken belt by detecting that the head does not come back to it's home position or any other error that your printer can detect.

*Initialize PrinterThis line is used to reinitialise the printer, the computer will accomplish this by putting the line, which is normally high, to it's low state. This is very useful when starting a print job, since special formatting codes might have been sent to the printer on the last job, by reinitialising the printer you are sure of not messing up the whole thing, like printing the whole document in italics or something.

*Select InputMany computers give the option of letting the computer the option of putting the printer online or not, by putting this signal high the printer is kept in it's offline state and putting it low the printer is online and will accept data from the computer. Many printers have a DIP switch to let decide if the computer can control the online state, when the switch is active it will keep this line always low, thus keeping the computer from putting the printer offline.

*GroundThis is a regular signal ground and is used as a reference for the low signal or logical 0.

* Newer Parallel Ports are standardized under the IEEE 1284 standard first released in 1994. This standard defines 5 modes of operation which are as follows:1. Compatibility Mode 2. Nibble Mode 3. Byte Mode4. EPP Mode (Enhanced Parallel Port). 5. ECP Mode (Extended Capabilities Mode).

* The aim was to design new drivers and devices which were compatible with each other and also backwards compatible with the Standard Parallel Port (SPP). Compatibility, Nibble & Byte modes use just the standard hardware available on the original Parallel Port cards while EPP & ECP modes require additional hardware which can run at faster speeds, while still being downwards compatible with the Standard Parallel Port.

*Port AddressesThe Parallel Port has three commonly used base addresses.The 3BCh base address was originally introduced used for Parallel Ports on early Video Cards. This address then disappeared for a while, when Parallel Ports were later removed from Video Cards. They has now reappeared as an option for Parallel Ports integrated onto motherboards, upon which their configuration can be changed using BIOS.

* LPT1 is normally assigned base address 378h, while LPT2 is assigned 278h. However this may not always be the case as explained later. 378h & 278h have always been commonly used for Parallel Ports. The lower case h denotes that it is in hexadecimal. These addresses may change from machine to machine.

* AddressNotes3BCh - 3BFhUsed for Parallel Ports which were incorporated on to Video Cards. Doesn't support ECP addresses.

378h - 37Fh Usual Address For LPT 1.

278h - 27Fh Usual Address For LPT 2.

* When the computer is first turned on, BIOS (Basic Input/Output System) will determine the number of ports you have and assign device labels LPT1 and LPT2 to them. BIOS first looks at address 3BCh. If a Parallel Port is found here, it is assigned as LPT1, then it searches at location 378h. If a Parallel card is found there, it is assigned the next free device label. This would be LPT1 if a card wasn't found at 3BCh or LPT2 if a card was found at 3BCh. The last port of call, is 278h and follows the same procedure than the other two ports. Therefore it is possible to have a LPT2 which is at 378h and not at the expected address 278h.

* What can make this even confusing, is that some manufacturers of Parallel Port Cards, have jumpers which allow you to set your Port to LPT1, LPT2. Now what address is LPT1? - On the majority of cards LPT1 is 378h, and LPT2, 278h, but some will use 3BCh as LPT1, 378h as LPT1 and 278h as LPT2. Life wasn't meant to be easy. The assigned devices LPT1 and LPT2 should not be a worry to people wishing to interface devices to their PC's. Most of the time the base address is used to interface the port rather than LPT1 etc. However should you want to find the address of LPT1 or any of the Line Printer Devices, you can use a lookup table provided by BIOS. When BIOS assigns addresses to your printer devices, it stores the address at specific locations in memory, so we can find them.

* LPT Addresses in the BIOS Data Area Start Address Function0000:0408LPT1's Base Address

0000:040ALPT2's Base Address

*How Serial Ports Work?Considered to be one of the most basic external connections to a computer, the serial port has been an integral part of most computers for more than 20 years. Although many of the newer systems have done away with the serial port completely in favour of USB connections, most modems still use the serial port, as do PDAs and digital cameras. Few computers have more than two serial ports.

*Two serial ports on the back of a PC

* Almost nothing in computer interfacing is more confusing than selecting the right RS232 serial cable. This page is intented to provide information about the most common serial RS232 cables in normal computer use. Essentially, serial ports provide a standard connector and protocol to let you attach devices, such as modems, to your computer. All computer operating systems in use today support serial ports, because serial ports have been around for decades. Parallel ports are a more recent invention and are much faster than serial ports.

*The RS232 connector was originaly developed to use 25 pins. In this DB25 connector pinout provisions were made for a secondary serial RS232 communication channel. In practice, only one serial communication channel with accompanying handshaking is present. Only very few computers have been manufactured where both serial channels are implemented. It can be used to query the modem status while the modem is on-line and busy communicating. On personal computers, the smaller DB9 version is more commonly used today. Note, that the protective ground is assigned to a pin at the large connector where the connector outside is used for that purpose with the DB9 connector version.

*Where the definition of RS232 focussed on the connection of DTE, data terminal equipment (computers, printers, etc.) with DCE, data communication equipment (modems),

RS232 DB9 pin assignment DEC MMJ pin assigment

* The name "serial" comes from the fact that a serial port "serializes" data. The advantage is that a serial port needs only one wire to transmit the 8 bits (while a parallel port needs 8). The disadvantage is that it takes 8 times longer to transmit the data than it would if there were 8 wires. Serial ports lower cable costs and make cables smaller. Before each byte of data, a serial port sends a start bit, which is a single bit with a value of 0. After each byte of data, it sends a stop bit to signal that the byte is complete. It may also send a parity bit.

* Serial ports, also called communication (COM) ports, are bi-directional. Bi-directional communication allows each device to receive data as well as transmit it. Serial devices use different pins to receive and transmit data -- using the same pins would limit communication to half-duplex, meaning that information could only travel in one direction at a time. Using different pins allows for full-duplex communication, in which information can travel in both directions at once.

*RS232 null modem cables The easiest way to connect two PC's is using an RS232 null modem cable. The only problem is the large variety of RS232 null modem cables available. For simple connections, a three line RS232 cable connecting the signal ground and receive and transmit lines is sufficient. Depending of the software used, some sort of handshaking may however be necessary. Use the RS232 null modem selection table to find the right null modem cable for each purpose. For a Windows 95/98/ME Direct Cable Connection, the RS232 null modem cable with loop back handshaking is a good choice.

*RS232 null modem cables with handshaking can be defined in numerous ways, with loopback handshaking to each PC, or complete handshaking between the two systems. The most common null modem cable types are shown here. See Figure 1.0

Figure 1.0

Connector 1Connector 2Function23Rx Tx32Tx Rx55Signal ground

*If you read about null modems, this three wire null modem cable is often talked about. Yes, it is simple but can we use it in all circumstances? There is a problem, if either of the two devices checks the DSR or CD inputs. These signals normaly define the ability of the other side to communicate. As they are not connected, their signal level will never go high. This might cause a problem. The same holds for the RTS/CTS handshaking sequence. If the software on both sides is well structured, the RTS output is set high and then a waiting cycle is started until a ready signal is received on the CTS line. This causes the software to hang because no physical connection is present to either CTS line to make this possible. The only type of communication which is allowed on such a null modem line is data-only traffic on the cross connected Rx/Tx lines.

*This does however not mean, that this null modem cable is useless. Communication links like present in the Norton Commander program can use this null modem cable. This null modem cable can also be used when communicating with devices which do not have modem control signals like electronic measuring equipment etc. As you can imagine, with this simple null modem cable no hardware flow control can be implemented. The only way to perform flow control is with software flow control using the XOFF and XON characters.

*UART, an introduction

An UART, universal asynchronous receiver / transmitter is responsible for performing the main task in serial communications with computers. The device changes incomming parallel information to serial data which can be sent on a communication line. A second UART can be used to receive the information. The UART performs all the tasks, timing, parity checking, etc. needed for the communication. The only extra devices attached are line driver chips capable of transforming the TTL level signals to line voltages and vice versa.

*UART types

Serial communication on PC compatibles started with the 8250 UART in the IBM XT. In the years after, new family members were introduced like the 8250A and 8250B revisions and the 16450. The last one was first implemented in the AT. The higher bus speed in this computer could not be reached by the 8250 series. The differences between these first UART series were rather minor. The most important property changed with each new release was the maximum allowed speed at the processor bus side. The 16450 was capable of handling a communication speed of 38.4 kbs without problems. The demand for higher speeds led to the development of newer series which would be able to release the main processor from some of its tasks. The main problem with the original series was the need to perform a software action for each single byte to transmit or receive. To overcome this problem, the 16550 was released which contained two on-board FIFO buffers, each capable of storing 16 bytes. One buffer for incomming, and one buffer for outgoing bytes.

* This 40-pin Dual Inline Package (DIP) chip is a variation of the National Semiconductor NS16550D UART chip.

*The Serial ConnectionThe external connector for a serial port can be either 9 pins or 25 pins. Originally, the primary use of a serial port was to connect a modem to your computer. The pin assignments reflect that. Let's take a closer look at what happens at each pin when a modem is connected.

*Close-up of 9-pin and 25-pin serial connectors

* 9-pin ConnectorCarrier Detect - Determines if the modem is connected to a working phone line. Receive Data - Computer receives information sent from the modem. Transmit Data - Computer sends information to the modem. Data Terminal Ready - Computer tells the modem that it is ready to talk. Signal Ground - Pin is grounded.

* Data Set Ready - Modem tells the computer that it is ready to talk. Request To Send - Computer asks the modem if it can send information. Clear To Send - Modem tells the computer that it can send information. Ring Indicator - Once a call has been placed, computer acknowledges signal (sent from modem) that a ring is detected.

*25-pin ConnectorNot Used Transmit Data - Computer sends information to the modem. Receive Data - Computer receives information sent from the modem. Request To Send - Computer asks the modem if it can send information. Clear To Send - Modem tells the computer that it can send information. Data Set Ready - Modem tells the computer that it is ready to talk.

* Signal Ground - Pin is grounded. Received Line Signal Detector - Determines if the modem is connected to a working phone line. Not Used: Transmit Current Loop Return (+) Not Used Not Used: Transmit Current Loop Data (-) Not Used Not Used Not Used

* Not Used Not Used Not Used Not Used: Receive Current Loop Data (+) Not Used Data Terminal Ready - Computer tells the modem that it is ready to talk. Not Used

* Ring Indicator - Once a call has been placed, computer acknowledges signal (sent from modem) that a ring is detected. Not Used Not Used Not Used: Receive Current Loop Return (-)

*Introduction to How USB Ports WorkJust about any computer that you buy today comes with one or more Universal Serial Bus connectors on the back. These USB connectors let you attach everything from mice to printers to your computer quickly and easily. The operating system supports USB as well, so the installation of the device drivers is quick and easy, too. Compared to other ways of connecting devices to your computer (including parallel ports, serial ports and special cards that you install inside the computer's case), USB devices are incredibly simple!

*

*What is USB?Anyone who has been around computers for more that two or three years knows the problem that the Universal Serial Bus is trying to solve -- in the past, connecting devices to computers has been a real headache! Printers connected to parallel printer ports, and most computers only came with one. Things like Zip drives, which need a high-speed connection into the computer, would use the parallel port as well, often with limited success and not much speed.

* Modems used the serial port, but so did some printers and a variety of odd things like Palm Pilots and digital cameras. Most computers have at most two serial ports, and they are very slow in most cases. Devices that needed faster connections came with their own cards, which had to fit in a card slot inside the computer's case. Unfortunately, the number of card slots is limited and you needed a Ph.D. to install the software for some of the cards.

* The goal of USB is to end all of these headaches. The Universal Serial Bus gives you a single, standardized, easy-to-use way to connect up to 127 devices to a computer. Each device can consume up to a maximum of 6 megabits per second (Mbps) of bandwidth, which is fast enough for the vast majority of peripheral devices that most people want to connect to their machines. Just about every peripheral made now comes in a USB version. A sample list of USB devices that you can buy today includes:

* Printers Scanners Mice Joysticks Flight yokes Digital cameras Web cams Scientific data acquisition devices Modems Speakers Telephones Video phones Storage devices such as Zip drives Network connections

* Connecting a USB device to a computer is simple -- you find the USB connector on the back of your machine and plug the USB connector into it.

* The rectangular socket is a typical USB socket on the back of the computer.

* A typical USB connector for a device, called an "A" connection

* If it is a new device, the operating system auto-detects it and asks for the driver disk. If the device has already been installed, the computer activates it and starts talking to it. USB devices can be connected and disconnected at any time. Many USB devices come with their own built-in cable, and the cable has an "A" connection on it. If not, then the device has a socket on it that accepts a USB "B" connector.

* A typical "B" connection

* The USB standard uses "A" and "B" connectors to avoid confusion: "A" connectors head "upstream" toward the computer. "B" connectors head "downstream" and connect to individual devices.By using different connectors on the upstream and downstream end, it is impossible to ever get confused -- if you connect any USB cable's "B" connector into a device, you know that it will work. Similarly, you can plug any "A" connector into any "A" socket and know that it will work.

*Running Out of Ports?Most computers that you buy today come with one or two USB sockets. With so many USB devices on the market today, you easily run out of sockets very quickly. For example, on the computer that I am typing on right now, I have a USB scanner, a USB Digital camera and a USB mouse. My computer has only two USB connectors on it, so the obvious question is, "How do you hook up all the devices?"

* The easy solution to the problem is to buy an inexpensive USB hub. The USB standard supports up to 127 devices, and USB hubs are a part of the standard.

* A typical USB four-port hub accepts 4 "A" connections.

* A hub typically has four new ports, but may have many more. You plug the hub into your computer, and then plug your devices (or other hubs) into the hub. By chaining hubs together, you can build up dozens of available USB ports on a single computer. Hubs can be powered or unpowered. As you will see on the next page, the USB standard allows for devices to draw their power from their USB connection.

* Obviously, a high-power device like a printer or scanner will have its own power supply, but low-power devices like mice and digital cameras get their power from the bus in order to simplify them. The power (up to 500 milliamps at 5 volts) comes from the computer. If you have lots of self-powered devices (like printers and scanners), then your hub does not need to be powered -- none of the devices connecting to the hub needs additional power, so the computer can handle it. If you have lots of unpowered devices like mice and cameras, you probably need a powered hub. The hub has its own transformer and it supplies power to the bus so that the devices do not overload the computer's supply.

*The Universal Serial Bus has the following features: The computer acts as the host. Up to 127 devices can connect to the host, either directly or by way of USB hubs. Individual USB cables can run as long as 5 meters; with hubs, devices can be up to 30 meters (six cables' worth) away from the host.The bus has a maximum data rate of 12 megabits per second.

* Any individual device can request up to 6 Mbps (obviously, you cannot really have more than one device requesting 6 Mbps or you would exceed the 12-Mbps maximum for the bus). A USB cable has two wires for power (+5 volts and ground) and a twisted pair of wires to carry the data. On the power wires, the computer can supply up to 500 milliamps of power at 5 volts.

* Low-power devices (such as mice) can draw their power directly from the bus. High-power devices (such as printers) have their own power supplies and draw minimal power from the bus. Hubs can have their own power supplies to provide power to devices connected to the hub. USB devices are hot-swappable, meaning you can plug them into the bus and unplug them any time. Many USB devices can be put to sleep by the host computer when the computer enters a power-saving mode.

* The devices connected to a USB port rely on the USB cable to carry power and data. Inside a USB cable: There are two wires for power -- +5 volts (red) and ground (brown) -- and a twisted pair (yellow and blue) of wires to carry the data. The cable is also shielded.

* When the host powers up, it queries all of the devices connected to the bus and assigns each one an address. This process is called enumeration -- devices are also enumerated when they connect to the bus. The host also finds out from each device what type of data transfer it wishes to perform:

* Interrupt - A device like a mouse or a keyboard, which will be sending very little data, would choose the interrupt mode. Bulk - A device like a printer, which receives data in one big packet, uses the bulk transfer mode. A block of data is sent to the printer (in 64-byte chunks) and verified to make sure it is correct. Isochronous - A streaming device (such as speakers) uses the isochronous mode. Data streams between the device and the host in real-time, and there is no error correction.

* The host can also send commands or query parameters with control packets. As devices are enumerated, the host is keeping track of the total bandwidth that all of the isochronous and interrupt devices are requesting. They can consume up to 90 percent of the 12 Mbps of bandwidth that is available. After 90 percent is used up, the host denies access to any other isochronous or interrupt devices. Control packets and packets for bulk transfers use any bandwidth left over (at least 10 percent).

* The Universal Serial Bus divides the available bandwidth into frames, and the host controls the frames. Frames contain 1,500 bytes, and a new frame starts every millisecond. During a frame, isochronous and interrupt devices get a slot so they are guaranteed the bandwidth they need. Bulk and control transfers use whatever space is left.

*The USB 2.0 spec promises a speed increase by a factor of 10 or 20, while maintaining backward compatibility with older devices and using the same cables. This sort of speed will make it possible to connect almost anything to your computer via USB, including external hard drives and video cameras. Speed of USB 480Mbps

Thats AllFolks

LectureCAN USB RS 232

What is the Controller Area Network (CAN)Controller Area Network (CAN) is a serial communications network Originally designed for the automotive industry, but has also become a very popular bus in industrial automation as well as other applications. The CAN bus is primarily used in embedded systems It is a two-wire, half duplex, high-speed network system, well suited for high speed applications using short messages. Its robustness, reliability and large following in the semiconductor industry are some of the benefits with CAN.It is a broadcast message transmission system

Broadcast

CAN ApplicationsIndustrial applicationsMarine Control and Navigation systemsElevatorsMachine toolsToysCopiersMedical equipmentAgricultural machineryFactory automationEtc.

Thats AllFolks

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