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.

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 OUT FFH : Dummy mode word OUT FFH MVI A, 40H : Reset command word OUT FFH : Reset 8251A MVI A, CAH : Mode word initialization OUT FFH MVI A, 11H : Command word initialization OUT FFH CHECK: IN FFH 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

 INTERFACING DIAGRAM