interfacing 8251 with 8086

8/18/2019 Interfacing 8251 With 8086

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8086 HOW-TO GUIDE

Interfacing 8251 with

8086


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n the Technical Community Today! http://www.pantechsolutions

Contents at a Glance

8086 Trainer Board .......................................................... 3

8251 (USART) ................................................................... 3

Interfacing 8251 with 8085 .............................................. 4

Pin Assignment with 8086 ................................................ 5

Circuit Diagram to Interface 8251 with 8086 .................... 6

Assembly program to interface 8251 with 8086 ................ 7


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8086 Trainer Board

The 8086 trainer kit is specifically designed to help

students to master the required skills in the area of

embedded systems. The kit is designed in such way that al

the possible features of the microprocessor will be easily

used by the students. MICROPROCESSOR offers more

advantages than conventional discrete hardwired logic

circuits. The major advantage is its programmability to adapt

to different environment and in addition it reduces the PCB

space.

8251 (USART)

The RS232C interface of PS-TIMER & USART

comprises of the universal synchronous/asynchronous

receiver/transmitter 8251 (USART), RS232C driver max 232.

The 8251A is used here as a peripheral device for serial

communication and is programmed by the CPU to operate


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using virtually any serial data transmission technique. The

USART accepts data characters from the CPU in parallel

format and then converts them into a continuous serial datastream for transmission. Simultaneously, it can receive serial

data streams and convert them into parallel data characters

for the CPU. The CPU can read the status of the USART at any

time. These include data transmission errors and control

signals.

Interfacing 8251 with 8085

Microprocessor don’t have the direct seria

communications, so to communicate the data serially to a

device we need a driver to send a character serially, here we

use 8251. 8251 is a driver which converts the parallel data to

a serial data the pin detail and control pins are given so

according to that we can develop the hardware.

Obviously, 8251 is not directly compatible with these

signal levels. Standard method to interfaceRS232C and TTL


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levels is with MC1488 quad TTL-to-RS232C drivers and

MC1489 quad RS232C-to-TTL receivers. Of the 25 handshake

signals provided by the RS232C standard, we will discuss onlyfour signals which are used in our design. They are the RTS,

CTS, RxD and TxD signals.

Pin Assignment with 8086

8085 8255 8255 PIN DIAGRAM

C O

N T R O L L L I N E S

13 A1

29 RD

30 WR

24 CLK

3 RST

From

decoderCS

8 2 5 5 –

D A T A

L I N E S

12 D0

13 D1

14 D2

15 D3

16 D4

17 D5

18 D6

19 D7

VCC 40 26 Address line and data lines are Decoded

using LatchGND 20 7


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Circuit Diagram to Interface 8251 with 8086

U1

8251

RESET

21 CLK

20

C/D

12

RD

13 WR

10

CS

11

RXD

3

RXC

25TXC

9

TXD

19

D0

27 D128 D2

1 D3

2 D4

5 D5

6 D6

7 D7

8

RXRDY

14

DSR22

CTS

17RTS

23

G N D

4

V C C

2 6

15

15

16

16

18

18

24

24

RST

D7

D6

D5

D4

D3

D2

D1

D0

PCLK

WR

P1

COM2

5

9

4

8

3

7

2

6

1

VCC

11

OUT2

19

VCC C1

0.1UF

RD

U2

MAX232

C1+

1

C1-

3

C2+

4

C2-

5

V+2

V -

6

R1OUT

12 R2OUT

9

T1IN

11

T2IN

10

R1IN

13

R2IN

8

T1OUT

14

T2OUT

7

G N D

1 5

V C C

1 6

C4

22MFD

C5

22MFD

C3

22MFD

C6

22MFD

R1OUT

R1OUT

T1IN

T1IN

C8

22MF

VCC


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Assembly program to interface 8251 with 8086********************************************************************************

Title : to transmit a character from 8251 to the HyperTerminal using 8086

********************************************************************************

MEMORY OPCODE PNEMONICS

1100 Be 00 15 MOV SI,1500H

1103 B0 36 MOV AL,36H

1105 BA 06 FF MOV DX,FF06

1108 EE OUT DX,AL

1109 B0 40 MOV AL,40H

110B BA 04 FF MOV DX,FF04

110E EE OUT DX,AL

110F B0 01 MOV AL,01H

1111 BA 04 FF MOV DX,FF04

1114 EE OUT DX,AL

1115 B1 05 RELOAD: MOV CL,05H

1117 BA 12 FF CHECK: MOV DX,FF12

111A EC IN AL,DX

111B 24 02 AND AL,02H

111D 74 F8 JZ CHECK

111F BA 10 FF MOV DX,FF10

1122 EC IN AL,DX

1123 88 04 MOV [SI],AL

1125 46 INC SI

1126 3C 3F CMP AL,3FH

1128 75 EB JNZ RELOAD

112A FE C9 DEC CL

112C 75 E9 JNZ CHECK

112E CD 02 INT 02

1130 CD 02 INT 02


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ADDRESS DATA

1500 48H,45H,4CH,4CH

1504 4FH,2DH,38H,30H (HELLO-8086)

1508 38H,35H,0AH,0DH

150C END


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