wireless profibus, profinet

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Procedure for the Stabilization of Wireless PROFIBUS FIELDBUS Connections used in industrial Automation

December 29, 2011, Dipl.Ing. Thomas Schildknecht, Managing Board Schildknecht AG, Germany 71711 Murr, Haugweg 26 [email protected]

FIELDBUS transmission such as PROFIBUS & Profinet is a fast cyclic data transfer between a

master, usually PLC, and one or several slaves such as input-output terminals, frequency

converters, laser scanners, measuring devices, operator panels etc.. This local data transfer has

firmly established in automation engineering and is successfully applied by millions. The

emergence of wireless transmission standards such as WLAN, Bluetooth, DECT, Zigbee

(802.15.4) also provides the realisation of wireless FIELDBUS transmission. For more than ten

years, Schildknecht AG located near Stuttgart /Germany has been successfully offering wireless

data transmission systems for automation engineering. In several thousand successful

applications, valuable experience with practically all wireless standards available on the global

market for the licence-free ISM and SRD bands have thereby been gained. In this article, the

PROFIBUS cable interface of wireless transmission is described. The implementation of this

procedure resulted in the patent application DE10 207 041 621 A.

Among the pioneers of “wireless automation”, Schildknecht has nearly twenty years

experience in wireless coupling of PLC´s and ten years with wireless PROFIBUS and is now in the fourth device generation. Thereby, hardware and software functions have been

realised which in the manufacturer’s opinion are indispensable for a successful application in automation engineering. Schildknecht is experienced in PROFIBUS tunnelling via the following wireless technologies:

WLAN 2.4 GHz 802.11b,g WLAN 2.4GHz 802.11 proprietary wireless technology WLAN 5.8GHZ 802.11h DECT 1.9GHZ Bluetooth 2.4GHz 802.15.1 802.15.4, Zigbee 2.4GHz 4xx, 868MHz, 915Mhz proprietary wireless technologies Upbanded DECT 2.4GHz Nanonet 2.4Ghz

There are a number of publications available of Schildknecht and colleagues and

competitors about the above-mentioned wireless technologies in general. However, there is

hardly any information about the differences in hardware, software and transfer strategies of

those technologies. Exactly those differences, however, may be decisive whether a wireless

system works successfully and reliably in the industrial application. For the user, the

difference becomes noticeable in more or less frequent or longer bus interruptions such as

PROFIBUS errors.

Procedure for the stabilisation of a wireless data link


Job Definition

The user, applying a wireless PROFIBUS solution, expects the wireless link providing the full

availability like a cabled solution. For mobile applications such as forklift trucks, warehouse

operator devices, suspended electric railways, cranes, movable plant and machine parts,

wireless transmission systems have been used for a long time already that are based on laser,

infrared, inductive or slotted wave guide data transmission, or trailing cables and slip-rings

with contact transfer are used for data transmission. Wireless transmission may be considered

an alternative technology here which in the one or the other application may replace

technologies applied so far. It is probably impossible to comply with the demand to

completely replace technologies used so far, on the one hand due to the approx. 1000 times

lower transmission rate via a wireless channel in comparison to a cable and on the other due

to the technical and regulatory limitation of available radio bands. At the same time, an

electromagnetic wave is required to use air, the worst possible and most incalculable means of

transmission. This is why the know-how under which conditions wireless FIELDBUS

transmissions may be successfully applied is still very limited among manufacturers as well.

What is the user and planner required to do to be able to successfully operate a wireless FIELDBUS transmission system?

1. Analysis whether a wireless transmission might solve the automation job anyway 2. Acceptance of wireless technologies already applied at a factory or hall

3. Planning, which wireless technology is suitable and may be used 4. Durable monitoring of the wireless transmission

For these technical and organisational measures we would like to refer to the guideline 2185

sheet 2 “Coexistence of industrial wireless technologies“, published by the VDI. Meanwhile, this guideline also receives international attention and has already cleared the first hurdle into

the American IEC. Together with some professional colleagues, Thomas Schildknecht is the

co-author of this guideline.

Strategy of wireless Transmission

An extremely important aspect has not yet been mentioned in any press release or guideline: Wireless solutions of different providers substantially differ according to the question on which strategy a wireless transmission is based.

In the event of those few technical solutions available on the market anyway, PROFIBUS or a

RS485 asynchronous sign is sent byte by byte by wireless transmission via WLAN, Bluetooth

or a proprietary wireless narrow-band technology. At the same time, the PROFIBUS frame

arriving via the cable interface is packed into a corresponding wireless frame of the respective

wireless standard, transmitted in a wireless way and unpacked again at the wireless receiver

and then forwarded to the DP slave by cable. Equivalent solutions are available for Ethernet /

Profinet as well.

Picture 1 shows the time progress of a normal transparent wireless FIELDBUS transmission, we

call it further “strategy 1”. The FIELDBUS master sends a data package which is packed into the

respective wireless protocol in the wireless modem and is output again at the receiver after a time

T1. The FIELDBUS slave usually replies to this command with the required data which arrives

at the FIELDBUS master again after the time T6. After the time T3 the



FIELDBUS master is ready to send new data again. Since the response time of the slave

across the wireless link is not constant, T3 must be chosen accordingly large to provide reserves for delays of the wireless link. The time T1 is the pure delay time of the wireless

link which can be in the range of 5ms to 10ms.

For the user, however, the substantially longer cycle time T2 is important, since it is decisive within which time the FIELDBUS terminal receives new data. In the event of transparent transmission, T2 is approx. 100ms (@187,5kbit, Tslot 16000).

Picture 1: Time course of a FIELDBUS transmission across a wireless link

The times are defined as follows:

T1 Transmission delay wireless link undisturbed T2 Updating time/cycle time FIELDBUS after the undisturbed wireless link T3 Updating time/cycle time FIELDBUS before wireless link T4 Transmission delay wireless link disturbed

T5 Updating time/cycle time FIELDBUS after disturbed wireless link T6 TSDR response time of slave

In strategy 1 of wireless transmission the FIELDBUS transmission at the master is output

significantly slowed down and added longer delays between the telegrams. This requires

amending the PROFIBUS timing settings. In PROFIBUS applications this is achieved by an

increase of the TSLOT time and a decrease of the data rate to 187,5kbit at the most. Practice

has shown that a decrease to even 93,75kbit or 19,2kbit as well as a fictive increase of the

number of repeaters and an increase of the PROFIBUS retries is required in the profibus

properties. The adaptation of the TSLOT has to be tried individually depending on the

PROFIBUS telegram length, i.e. the connected periphery, the used wireless engineering, the

condition of the wireless channel and the number of PROFIBUS participants connected. This

is exclusively done by Try & Error. There are solutions of competitors available requiring

times of 16000 Tbit (100-300 are normal) here, which is equal to a complete deflection of the



PROFIBUS timings. The PROFIBUS cycle time is substantially reduced thereby,

measurements have shown that in undisturbed operation 100 to 150ms are possible at best.

In Profinet applications, it is a bit easier since it is possible to adjust the reduction ratio for each participant, an additional waiting period during which the participant is not addressed. However, Profinet is not construed for a signal interruption of 200ms either and will head for bus-error condition also.

In a wired solution, no further PROFIBUS participants can be connected with the PLC

(profibus master) if the PROFIBUS timing settings are amended. Manufacturers of such system use this procedure in order to be able to work with wireless modules that are

technically simpler and consequently cheaper. This means a disadvantage for the customer who receives an instable FIELDBUS via a wireless link.

Picture 2 shows the influence of a transmission interference on the FIELDBUS leading to a bus error. The black arrows represent the wireless link which has to repeat the data in the event of interference. Picture 2: PROFIBUS error caused by delays during wireless transmission.

In this simple transmission procedure of strategy 1, due to the characteristics of the system PROFIBUS errors happen in regular intervals in a reproducible manner.



Picture 3 shows the behaviour of a simple PROFIBUS wireless link of a competitor

If in strategy 1 wireless systems PROFIBUS retries are set in the PROFIBUS parameters, this

results in bus errors even in the event of short transmission interferences, since the retries

accrued are output again at the bus at a wrong time or too late. Picture 4 shows this one with

a state measured by means of a PROFIBUS diagnosis tool (Profitrace of the company

Procentec). The red arrow shows the moment in which the PROFIBUS master awaits the

response via the wireless link and repeats the attempted transmission using a “repeat“. After

126 ms (interference of the wireless link), two response telegrams arrive from the slave

across the wireless link. The master interprets this behaviour as incorrect, replies with a sync

after 693ms, activates the bus error and initialises communication again (Set Parameter, Get

Diagnostic, Check Config), although the response telegrams of the slave have not gone lost at

all.

It can be observed that the actualisation of the PROFIBUS telegrams after the wireless link is approx. only 100ms (approx. 70ms SD2 + ca. 26ms ACK). As for the PROFIBUS bus speed, 187,5kbit was set. Picture 4 Profitrace analysis of the PROFIBUS telegrams at the master of



Picture 5 Analysis of PROFIBUS telegrams after wireless link

Picture 5 shows that all data is internally buffered by Bluetooth until the wireless link is built

up again. In the event of a transparent PROFIBUS transmission by strategy 1 without filtering

and control of this data transmission, this leads to problems. The illegal frame at the time

4939,30ms contains the collected data garbage of the wireless link. Without the strongly

defined keeping to the pause time between two PROFIBUS frames or a slip output of a

frame, the state leads to a bus error. Similar effects are also expected regarding a Profinet

(Ethernet) wireless transmission using WLAN or Bluetooth.

Transparent wireless Profinet Transmission

As regards WLAN 802.11, the CSMA procedure involves a further problem. In this

procedure, a wireless system waits until other wireless systems do no longer occupy this

band. In a TCP/IP transmission, this is not dramatic since the TCP-layer is responsible for the

control, sorting and if required for the repetition of telegram packages. In Profinet

applications, however, data is repeated in cyclic intervals. If the wireless channel is occupied

by another wireless system now, the receiver recognizes the channel occupation and waits. In

the event of just three non-overlapping channels in a WLAN application in the 2.4GHz band,

the resource wireless band width becomes overloaded within shortest time. Even one further

system at the same wireless channel involves bus errors.



Picture 6 The WLAN CSMA procedure serves for temporary storage of cyclic data and for sending them all at once. Just a simple analogue wireless radio video camera may involve a durable blocking of a wireless Profinet transmission.



DATAEAGLE wireless Data Transmission Device strategy

Based on the experiences gained by several thousands of successful applications with

wireless PROFIBUS during the past more than ten years, the company Schildknecht has

chosen a completely different approach in which the FIELDBUS data is pre-processed in the

wireless module. The aim of pre-processing is to occupy the wireless channel as less as

possible and to enable access control and control of the cable interface and the wireless

medium. Both media, FIELDBUS as well as the wireless link, are usually half-duplex. In

Bluetooth applications, due to partly automatic repetitions at the wireless interface, access

control at the cable interface represents a great problem. In some thesis works and

dissertations about “wireless PROFIBUS“, authors therefore assumed that from the technical

point of view a deterministic FIELDBUS transmission via standard wireless technologies

would be impossible. However, if pre-processing and corresponding storage and control is

integrated in each wireless module, compliance with the bus timing is possible also in the

event of interferences, even at high PROFIBUS speeds of 500kbit and 1.5Mbit, and also the

use of standard wireless technologies. This principle is shown in picture 7. The central point

is that it is possible to operate the PROFIBUS with a high baudrate using the standard timing.

Unlike a wired connection, no modifications in the control project or the settings are required.

Picture 7: Functional block realised in hard- and software for the stabilisation of the FIELDBUS transmission



The function block in picture 7 marked in light blue consists of a combined hard- and software solution which is firmly integrated in the wireless modem. This function block is responsible for the following tasks:

Processing and temporary storage of all PROFIBUS telegrams Bus arbitration of response telegrams Filtering of required telegrams Firewall for telegrams that are not required FDL telegrams handling Token telegrams handling Monitoring of filtering time Own PROFIBUS participant for diagnosis data

Mode of DATAEAGLE strategy Operation In uninterrupted wireless operation, the control unit sends a PROFIBUS telegram via the

wireless link (1) which is answered by the PROFIBUS slave (2) and output after transmission

from the wireless modem to the PROFIBUS master (3). The PROFIBUS master outputs

redundant data in cyclic intervals until a change in state occurs (4), represented in the

drawing as a change in colour. The wireless device returns the direct response (5) to the

master, still with the old data of the slave which has been temporarily stored. In the event of an interference of the wireless channel (7) the wireless modem tries to

transfer the data until a connection is possible again (6). Due to the separation of the wireless

link by the function block in the wireless modem, the wireless system at the bus is non-

reactive. A PROFIBUS error at the master is excluded even in the event of interferences of

the wireless link. Experience has shown that transmission interferences occur in the range of

50 to 500 milliseconds.

FDL request and token telegrams sent by the PROFIBUS master as broadcast (8) may be

filtered out if required. Answers of the slaves to these broadcast telegrams that do no longer

comply with the timing are avoided and therefore may not lead to collisions at the bus and

bus errors activated thereby. (9) shows the example of a token or FDL broadcast at the time at

which the response of the wireless slave is actually arrived. For this reason, wireless

transmission systems transmitting PROFIBUS using strategy 1, without this function

block will regularly create bus errors, which is a system-immanent error. The great

advantage of this pre-processing is the retention of the high PROFIBUS data rate of up to

1,5Mbit without additional pause times. In practice, at this data rate the wireless system is

required to process a PROFIBUS telegram every millisecond, the longest inactive time on the

PROFIBUS is just 200us and is called Tslot.

Due to a transmission interference, the transmission time T1 increases to T4 and the updating

time from T2 to T5. This leads e.g. to the fact that the output signal of a bus terminal is switched

with a delay equivalent to this time. In Blutooth applications, this time is between 15 to 30ms in

uninterrupted state and increases by the duration of the transmission interference.



Picture 11 Typical percentage distribution of the response times DATAEAGLE using

wireless PROFIBUS via Bluetooth: NLOS no line of sight 20m, good possibilities for

reflection of the wireless link. The resolution is chosen very low in order to notice even individual higher updating times. In this example, the measuring time was approx. ten

minutes. In practice, longer response times are likely to occur.



Picture 12 shows a typical wireless PROFIBUS transmission via standard WLAN

802.11.g. The total loss of telegrams (255ms) is particularly noticeable. That means in praxis, that 802.11g is under the same conditions compared to a Bluetooth technology less

robust and produces higher delays.

Without the function block realised in the DATAEAGLE the PROFIBUS connection would

require to be slowed down by an enlargement of the TSLOT and a reduction of the bus speed to

93,75kbit to such an extent that no new data is sent by the PROFIBUS master for 90ms.

Setting the T-Slot time is exclusively done according to the "Try –and Error" principle.

Thanks to the DATAEAGLE concept, however, data is updated every 23ms to 50ms and delays of up to 89ms are seldom and do not lead to bus errors. Times stated are not absolute

but depend on the respective application. The example above shows a bus terminal with

two bytes writing / reading.

Monitoring Filter Time A longer failure of the wireless connection shall have the same consequence on the PROFIBUS

connection like in a wired connection. The advantage is that all monitoring structures existent in

the control system may be used also in the event of a transmission interference without any

amendments. For this reason, a filter function has been integrated in the function block. Within

an adjustable filter time it is possible to maintain the PROFIBUS transmission even in the event

of a short transmission interference. This helps to avoid that short transmission interferences of

usually 100ms to 300m involve a bus error and consequently a failure of the system. It can take

60 seconds and longer until a bus error is acknowledged by the operator and removed again. In

the event of a poor wireless link enforcing one times a delay of 300ms per minute, the system’s

availability is zero per cent. A really realistic practical scenario, above all in WLAN applications

as the wireless technology.

Bus error enforced by filter function after expiry of filter time

Picture 8 shows the example of an enforced bus error after expiry of the filter time



The filter time is started upon arrival of a telegram via the wireless link. After expiry of the

adjustable filter time T8 the function block does no longer automatically reply to the PROFIBUS telegram of the wireless slave (9). This leads to an enforced bus error. So the

wireless PROFIBUS link behaves like a wired connection. In the Siemens S7, the OB86 is

addressed and the loss of the PROFIBUS participant is reported to the cycle program.

Token Administration

Token telegrams serve for the allocation of masters at the bus to a communication time slot. A PROFIBUS master is a PLC (AG), an operator panel (OP) or a programming device (PG). For a wireless connection of these devices, the procedure described here requires extension by a further function block in the wireless equipment of the slave. Both function blocks representatively accept the partner tokens existent at the respective bus segment and process them representatively as well. This is called token mirroring. Once

a participant is removed, the partner device is instructed via status information to delete the token.

Picture 9: Active participants after the wireless link by token mirroring

Diagnosis Possibilities of the wireless Link

For long-time monitoring of the wireless link, a concept for the integration into the

automation solution has been realised. In the wireless data system an own PROFIBUS

participant has been realised in addition to the transparent PROFIBUS transmission, that can be integrated into the control system via a supplied GSD-file. This method enables access

from the cycle program to the data of the wireless link. Optical monitoring by LEDs in the wireless system as exclusively used by competitors is not practicable.



The DATAEAGLE wireless data system for example supplies current measurement readings such as the cycle or the updating time T5 about the wireless link, the number of retries Z1, cold and warm starts of the wireless link and the connected PROFIBUS participants as well

as service data in the event of failure.

Picture 10: Function diagnosis slave



Conclusion “simple Radio Systems“ of Competitors

A completely transparent wireless transmission of cyclic FIELDBUS telegrams such as PROFIBUS and Profinet is extremely error-prone and leads to bus errors in irregular intervals.

The FIELDBUS master (PLC) takes a non-reacting slave of the circulation list and

activates the OB 86. Depending on the PLC used, it is tried after 20 to 60 seconds to connect the participant with the bus again. Bus errors subject to the system operator’s

acknowledgement remain existent until acknowledgement.

It is possible to just balance delays of the wireless link of up to approx. 100ms even using simple wireless solutions by changing the Tslot and a reduction of the bus speed. However, bus conflicts with token and FDL may not be avoided in principal. It is a static problem how often this will lead to errors. Customers report about experiences of one time per hour.

Wired participants may not be connected at the master together with radio modems. Thus, a separate component assembly is required to be used for the wireless link, with a separate wiring up to the radio modem.

Conclusion DATAEAGLE Concept

Because of the way the system operates, bus errors due to interferences on the wireless link do not occur.

The functional transparency enables Profisafe via PROFIBUS as well, allowing the connection of all error-proof Profisafe components along the wireless link.

Thanks to the diagnosis slave, the integration into the automation concept and a long-time monitoring of the wireless link is possible.

Literature

Thomas Vogel (2003): Wireless PROFIBUS über WLAN 802.11(b) Andreas Willig (2002): Investigations on MAC and Link Layer for a wireless PROFIBUS over IEEE 802.11



Wireless Profinet, Siemens S7 PLC Wireless wireless siemens PLC Wireless PLC Data Linc Simatic S7 wirelsss Profisafe SIMATIC S7-1500

For more information about the DATAEAGLE

Solutions please contact M2M Solutions, Inc.

info @ m2m-dynamics.com

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