Transmit message using 8251 - Free 8085 Microprocessor projects

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.