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Module 5: External Communication Interface

20207m

Determine if an 8051 in mode 1 can communicate with an 8051 in mode 3.

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Can 8051 Mode 1 communicate with Mode 3?

No, not directly without adaptation.

  • Mode 1: 10-bit frame: 1 start + 8 data + 1 stop, variable baud via Timer1.
  • Mode 3: 11-bit frame: 1 start + 8 data + 1 programmable 9th bit (TB8/RB8) + 1 stop, variable baud.

Receiver in Mode 3 expects 9th bit; transmitter in Mode 1 does not send it (or sends stop as 9th). Baud rates may match but frame mismatch causes framing errors and RB8 misinterpretation. Multiprocessor bit SM2 handling also differs.

If forced: Mode 3 receiver can be configured to ignore 9th bit (SM2=0, and software ignores RB8), and Mode 1 transmitter's stop bit will be sampled as 9th bit. With SM2=0, communication can succeed with 8 data bits but 9th bit is garbage. For robust communication, both sides must be same mode (both Mode 1 or both Mode 3). Mixed mode is unreliable and not recommended.

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