Free Electronic Circuits & 8085 projects

Electronic projects with circuit diagram and 8085 microprocessor projects.

Transmit message using 8251

Statement: Write a assembly program to transmit a message from an 8085 to a CRT terminal for the following requirements and draw the interfacing diagram.

i) A message of 50 characters is stored as ASCII characters (without parity) in memory locations starting at 2200H.

ii) Baud rate x 16

iii) Stop bits 2

Solution Description:

  • CRT terminal uses normal RS 232C standard serial communication interface. Therefore, to transmit data to CRT it is necessary to have RS 232C interface at the sending end.
  • Fig. shows the interfacing of 8251 with RS 232C to 8085.
  • As shown in the Fig. three RS-232C signals (TxD, RxD are Ground) are used for serial communication between the CRT terminal and the 8085 system.
  • Line drivers and receivers are used to transfer logic levels from TTL logic to RS-232C logic.
  • For RS-232C the voltage level +3V to +15V is defined as logic 0 and voltage level from -3V to -15V is defined as logic 1.
  • The line driver, MC 1488, converts logic 1 of TIL to approximately -9V and logic a of TIL to  approximately +9V. These levels at the receiving end are again converted by the line receiver, MC1489, into TTL compatible logic.

66.a-Transmit message using 8251

66.bTransmit message using 8251

66.cTransmit message using 8251

Source program:

  • LXI H, 2200H : Initialize memory pointer to pointer the message
  • MVI C, 32H : Initialize counter to send 50 characters
  • MVI A, 00H
  • OUT FFH : Dummy mode word
  • MVI A, 40H : Reset command word
  • OUT FFH : Reset 8251A
  • MVI A, CAH : Mode word initialization
  • MVI A, 11H : Command word initialization
  • ANI 0lH : Check TxRDY
  • JZ CHECK : Is TxRDY I? if not, check again
  • MOV A, M : Get the character in accumulator
  • OUT FEH : Send character to the transmitter
  • INX H : Increment memory pointer
  • DCR C : Decrement counter
  • JNZ CHECK : if not zero, send next character
  • HLT : Stop program execution

Flowchart for Source Program

66.dTransmit message using 8251


66eTransmit message using 8251 

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