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Serial Port Modes Supporting Multi-Processor Communication (8051 UART Modes 0–3)

Serial Port Modes Supporting Multi-Processor Communication (8051 UART Modes 0–3)

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Sep 12, 2026

In the classic 8051 serial port, multi-processor type communication is implemented using the special 9-bit UART framing where an extra bit (the “9th bit”, i.e., TB8/RB8TB8/RB8) can be used to mark address bytes vs data bytes. This capability is tied to the multi-processor mode control bit SM2 and the fact that 9th-bit signaling exists only in serial modes 2 and 3 (where TB8 and RB8 are meaningful).

Specifically:

  • Modes 2 and 3 support multi-processor communication, controlled by SM2, and use the 9th bit for addressing/selection of slaves. 2
  • Modes 0 and 1 are not multiprocessor modes; the 9th-bit mechanism for addressing is not available in those modes. 2

Therefore, among the options:

  • (i) Modes 0 & 1 → No, multiprocessor addressing requires modes 2/3. 2
  • (ii) Modes 1 & 2 → Only mode 2 qualifies (mode 1 does not).
  • (iii) Modes 2 & 3 → Yes, both qualify as multiprocessor modes. 2
  • (iv) All of the above → No, because modes 0/1 are not multiprocessor modes. 2

Correct answer: (iii) Modes 2 & 3. 2

Footnotes

  1. Multiprocessor Communication Using 8051 Microcontroller - States 8051 operates in multiprocessor mode for serial communication mode-2 and mode-3; describes TB8=1 for address and TB8=0 for data. ↩ ↩2 ↩3 ↩4

  2. 8051 UART Communication Tutorial - Step-by-step Guide - Shows SM0/SM1 mode table and explains SM2 as enabling multiprocessor communications in modes 2 and 3; also notes TB8/RB8 used in modes 2 and 3. ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7

  3. Serial Communication (SCON bit definitions) - Provides SCON bit definitions and explicitly indicates TB8/RB8 are used only in modes 2 and 3; includes SM2 multiprocessor mode control. ↩ ↩2

Key idea

Multi-processor communication on the 8051 UART relies on 9-bit frames (address vs data) and is therefore associated with modes 2 & 3, not modes 0 & 1. 2

Footnotes

  1. Multiprocessor Communication Using 8051 Microcontroller - States 8051 operates in multiprocessor mode for serial communication mode-2 and mode-3; describes TB8=1 for address and TB8=0 for data. ↩

  2. 8051 UART Communication Tutorial - Step-by-step Guide - Shows SM0/SM1 mode table and explains SM2 as enabling multiprocessor communications in modes 2 and 3; also notes TB8/RB8 used in modes 2 and 3. ↩

Mapping the options to hardware features

The 8051 serial control register SCON uses mode-select bits SM0/SM1 to choose between modes 0–3, and SM2 as the multiprocessor enable. In particular, SM2 is the multiprocessor mode control bit, and it applies to modes 2 and 3. 2

Also, TB8/RB8 are explicitly “used in modes 2 and 3”, indicating the presence of the 9th-bit mechanism required for multiprocessor addressing. 2

Decision table

OptionIncludesMultiprocessor capability present?Reason
(i) Modes 0 & 10, 1NoMultiprocessor mode uses 9th-bit framing in modes 2/3, not modes 0/1.
(ii) Modes 1 & 21, 2Not fullyOnly mode 2 supports multiprocessor communication; mode 1 is not a multiprocessor mode. 2
(iii) Modes 2 & 32, 3YesMultiprocessor operation is for modes 2 and 3. 2
(iv) All of the above0–3NoBecause modes 0/1 are not multiprocessor modes. 2

Footnotes

  1. 8051 UART Communication Tutorial - Step-by-step Guide - Shows SM0/SM1 mode table and explains SM2 as enabling multiprocessor communications in modes 2 and 3; also notes TB8/RB8 used in modes 2 and 3. ↩ ↩2 ↩3 ↩4 ↩5 ↩6

  2. Serial Communication (SCON bit definitions) - Provides SCON bit definitions and explicitly indicates TB8/RB8 are used only in modes 2 and 3; includes SM2 multiprocessor mode control. ↩ ↩2 ↩3 ↩4

  3. Multiprocessor Communication Using 8051 Microcontroller - States 8051 operates in multiprocessor mode for serial communication mode-2 and mode-3; describes TB8=1 for address and TB8=0 for data. ↩

How multiprocessor communication relates to the 8051 serial modes

  1. 1
    Step 1

    Choose the framing mode using the SM0/SM1 bits in SCON (modes 0–3).

    Footnotes

    1. 8051 UART Communication Tutorial - Step-by-step Guide - Shows SM0/SM1 mode table and explains SM2 as enabling multiprocessor communications in modes 2 and 3; also notes TB8/RB8 used in modes 2 and 3. ↩

  2. 2
    Step 2

    The 9th-bit control/receive bits are TB8 and RB8, which are explicitly used in modes 2 and 3. 2

    Footnotes

    1. 8051 UART Communication Tutorial - Step-by-step Guide - Shows SM0/SM1 mode table and explains SM2 as enabling multiprocessor communications in modes 2 and 3; also notes TB8/RB8 used in modes 2 and 3. ↩

    2. Serial Communication (SCON bit definitions) - Provides SCON bit definitions and explicitly indicates TB8/RB8 are used only in modes 2 and 3; includes SM2 multiprocessor mode control. ↩

  3. 3
    Step 3

    Set SM2 to enable multiprocessor operation; sources describe SM2 as the multiprocessor enable for modes 2 and 3. 2

    Footnotes

    1. Multiprocessor Communication Using 8051 Microcontroller - States 8051 operates in multiprocessor mode for serial communication mode-2 and mode-3; describes TB8=1 for address and TB8=0 for data. ↩

    2. 8051 UART Communication Tutorial - Step-by-step Guide - Shows SM0/SM1 mode table and explains SM2 as enabling multiprocessor communications in modes 2 and 3; also notes TB8/RB8 used in modes 2 and 3. ↩

  4. 4
    Step 4

    Multiprocessor mode uses the 9th bit to distinguish address bytes (e.g., slave selection) from data bytes for slave response.

    Footnotes

    1. Multiprocessor Communication Using 8051 Microcontroller - States 8051 operates in multiprocessor mode for serial communication mode-2 and mode-3; describes TB8=1 for address and TB8=0 for data. ↩

Conceptual progression from normal serial to multiprocessor

Shift register framing

Mode 0

Fixed-style serial framing; no multiprocessor 9th-bit addressing feature. "

Footnotes

  1. 8051 UART Communication Tutorial - Step-by-step Guide - Shows SM0/SM1 mode table and explains SM2 as enabling multiprocessor communications in modes 2 and 3; also notes TB8/RB8 used in modes 2 and 3. ↩

8-bit UART framing

Mode 1

Standard 8-bit asynchronous UART; multiprocessor addressing requires modes 2/3. "

Footnotes

  1. 8051 UART Communication Tutorial - Step-by-step Guide - Shows SM0/SM1 mode table and explains SM2 as enabling multiprocessor communications in modes 2 and 3; also notes TB8/RB8 used in modes 2 and 3. ↩

9-bit UART (multiprocessor)

Mode 2

Multiprocessor capable; uses TB8/RB8 and SM2 enables multiprocessor behavior. 2"

Footnotes

  1. Multiprocessor Communication Using 8051 Microcontroller - States 8051 operates in multiprocessor mode for serial communication mode-2 and mode-3; describes TB8=1 for address and TB8=0 for data. ↩

  2. 8051 UART Communication Tutorial - Step-by-step Guide - Shows SM0/SM1 mode table and explains SM2 as enabling multiprocessor communications in modes 2 and 3; also notes TB8/RB8 used in modes 2 and 3. ↩

9-bit UART with variable baud (multiprocessor)

Mode 3

Also multiprocessor capable with the same 9th-bit addressing structure. 2"

Footnotes

  1. Multiprocessor Communication Using 8051 Microcontroller - States 8051 operates in multiprocessor mode for serial communication mode-2 and mode-3; describes TB8=1 for address and TB8=0 for data. ↩

  2. 8051 UART Communication Tutorial - Step-by-step Guide - Shows SM0/SM1 mode table and explains SM2 as enabling multiprocessor communications in modes 2 and 3; also notes TB8/RB8 used in modes 2 and 3. ↩

Common confusion checks

Knowledge Check

Question 1 of 3
Q1Single choice

Which serial modes possess the potential to support the multi-processor type of communication?

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