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.

 INTERFACING DIAGRAM

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