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How to Interface 8251 with 8085 Lab Trainer Kit

8085 Trainer Board

PS-TRAINER-8085 microprocessor trainer kit is proposed to smooth the progress of learning and developing designs of microprocessor from Intel. It has the Facility to connect PC’s 101/104 Keyboard, to enter user programs in Assembly languages. User verifies the programs through LCD or PC. User friendly Firmware confirms facilitating the beginners learns operations of a microprocessor quickly.

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 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 data stream 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 serial 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

Pin Assignment with 8051



 

8085

8255

8251 PIN DIAGRAM

CONTROL L LINES

13

A1

          

29

RD

30

WR

24

CLK

3

RST

From decoder

CS

8251 – DATA LINES

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 Latch

GND

20

7

 

Circuit Diagram to Interface 8251 with 8051





Assembly program to interface 8251 with 8085

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

Title : Program to transmit a character from 8085 through Bluetooth

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

ADDRESS

OPCODE

                NEMONICS

9100 

3E B6

                MVI A,B6

9102

D3 23

                OUT 23

9104

3E 40

                MVI A,B6

9106

D3 22

                OUT 22

9108

3E 01

                MVI A,01

910A

D3 22

                OUT 22

910C

0E 05 

 REPET :  MVI C,05

910E

DB 01

 CHECK:  IN 01   

9110

E6 04

                ANI 04

9112

CA 0E 91

                JZ  CHECK

9115

21 00 92

START :  LXI H,LOOK UP

9118

06 0C

                MVI B,0C

911A

7E

REPT1:   MOV A,M

911B

D3 00

                OUT 00

911D

11 03 03

                LXI D,0303

9120

00

 DELAY:   NOP

9121

1B

                 DCX D

9122

7B

                 MOV A,E

9123

B2

                ORA D

9124

C2 20 91

                JNZ DELAY

9127

23

                INX H

9128

05

                DCR B

9129

C2 1A 91

                JNZ REPT1

912C

C3 15 91

                JMP START

912F

FE 3F

                CPI 3F

9131

C2 0C 91

                JNZ  REPET

9134

0D

                DCR C

9135

C2 0E 91

                JNZ CHECK

9138

CF

                RST 1

 



LOOK UP TABLE:

ADDRESS     DATA

 

9200     48H,45H,4CH,4CH

 

9204     4FH,2DH,38H,30H (HELLO-8085)

 

9208     38H,35H,0AH,0DH

 

920C     END