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Module 5: External Communication Interface

20257m

Discuss the RS232 communication interface with neat diagrams.

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RS232 Communication Interface

Standard: EIA RS232C, single-ended, ±3-15V (MAX232 level shifter), 25-pin DB25 or 9-pin DB9, up to 20kbps (115k with modern), 15m distance. Asynchronous start-stop bits, full-duplex via TXD/RXD.

Signals DB9: 2 RXD, 3 TXD (as Q114), 5 GND, 7 RTS,8 CTS (flow), 4 DTR,6 DSR,1 CD,9 RI. TTL from MCU via MAX232 charge-pump converts 5V↔±10V.

Frame: 1 start (0), 5-8 data, parity optional, 1-2 stop (1). Baud via Timer1 as before.

Diagram description: 8051 TxD→MAX232 T1IN→T1OUT→ DB9 Pin2/3, RxD opposite via R1IN→R1OUT. Capacitors 1µF×4 for MAX232.

Programming: SCON mode1, REN, TI/RI handling. Use interrupts for reception. Example transmission as Q37.

Advantages: Simple point-to-point PC link; disadvantages: short distance, single driver.

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