8051 Serial Communication Compatibility: Can an 8051 in Mode 1 Communicate with an 8051 in Mode 3?

The central question is whether one 8051 UART configured in Mode 1 can communicate correctly with another 8051 configured in Mode 3. The short answer is: they are not fully symmetric or naturally compatible as equivalent peers, but limited one-way or constrained communication can occur under specific conditions.3

Mode 1 transmits a frame of 10 bits: 1 start bit, 8 data bits, and 1 stop bit. Mode 3 transmits 11 bits: 1 start bit, 8 data bits, a programmable ninth bit, and 1 stop bit.2 Since the framing formats differ, direct communication is not generally considered standard-compatible in both directions.2

However, because both modes are asynchronous serial modes and both can use a variable baud rate generated from Timer 1, the timing can be matched.2 The real issue is frame interpretation, not baud generation.2 In particular:

• A Mode 1 receiver expects only start, 8 data bits, and stop.
• A Mode 3 transmitter inserts an extra ninth data bit before the stop bit.2
• Therefore, a Mode 1 receiver may misinterpret the Mode 3 ninth bit as the stop bit, causing the actual stop bit to spill into the next timing window and corrupt framing.2

By contrast, a Mode 3 receiver can often accept a Mode 1 transmission more gracefully, because in Mode 3 the receiver samples start bit, 8 data bits, a ninth bit into RB8, and then a stop bit.2 If a Mode 1 transmitter sends only start + 8 data + stop, then the Mode 3 receiver effectively treats the Mode 1 stop bit as the received ninth bit, leaving no true stop bit in the expected location. This again breaks the intended frame structure.2 Thus, standard reliable communication between Mode 1 and Mode 3 is not guaranteed and should be considered incompatible in normal design practice.3

The academically correct conclusion is therefore:

Footnotes

1. 8051 UART | 8051 Controller - Overview of 8051 serial modes, SCON bits, and baud-rate generation. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

2. 8051 Microcontroller UART (Serial Communication) | Everything You Need to Know | JunctionByte - Explains Mode 1 and Mode 3 framing and the role of the ninth bit. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹

3. Intel 8051AH Serial UART (Standard Interface without T2 as BRG) Simulation Details - Describes SCON semantics including TB8, RB8, RI, and mode behavior. ↩ ↩² ↩³ ↩⁴

4. 8051 Serial Port - Summarizes Mode 1 and Mode 3 bit structure and baud-rate properties. ↩ ↩² ↩³ ↩⁴

5. 80C51/87C51/80C52/87C52 80C51 8-bit microcontroller family (PDF) - Manufacturer-style datasheet details on RB8, SM2, and the mode definitions. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸

To understand the incompatibility rigorously, we must compare the bit-level behavior of the 8051 serial hardware. The SCON register selects the serial mode using bits SM0 and SM1.2 The standard mapping is:

In Mode 1, the receive-side RB8 bit stores the stop bit.2 In Modes 2 and 3, RB8 stores the received ninth data bit.2 This is a crucial distinction because the hardware meaning of the sampled bit differs across modes. A communication link is only robust when both transmitter and receiver agree on what each bit position means.2

Formally:

$$\text{Mode 1 frame} = \{S, D_0, D_1, \dots, D_7, P_{stop}\}$$ $$\text{Mode 3 frame} = \{S, D_0, D_1, \dots, D_7, D_8, P_{stop}\}$$

where $S$ is the start bit, $D_0 \dots D_7$ are the ordinary data bits, $D_8$ is the programmable ninth bit, and $P_{stop}$ is the stop bit.2

Because these frame lengths differ by one bit, the receiver state machine becomes misaligned if one endpoint assumes Mode 1 while the other assumes Mode 3.2

Footnotes

1. 8051 UART | 8051 Controller - Overview of 8051 serial modes, SCON bits, and baud-rate generation. ↩ ↩² ↩³ ↩⁴ ↩⁵

2. Intel 8051AH Serial UART (Standard Interface without T2 as BRG) Simulation Details - Describes SCON semantics including TB8, RB8, RI, and mode behavior. ↩ ↩² ↩³ ↩⁴

3. 8051 Serial Port - Summarizes Mode 1 and Mode 3 bit structure and baud-rate properties. ↩ ↩²

4. 8051 Microcontroller UART (Serial Communication) | Everything You Need to Know | JunctionByte - Explains Mode 1 and Mode 3 framing and the role of the ninth bit. ↩ ↩² ↩³ ↩⁴

5. 80C51/87C51/80C52/87C52 80C51 8-bit microcontroller family (PDF) - Manufacturer-style datasheet details on RB8, SM2, and the mode definitions. ↩ ↩² ↩³

Bit-Level Interpretation in Both Directions

The easiest way to answer the original question is to inspect both communication directions separately.

1. Mode 3 transmitter → Mode 1 receiver

The Mode 3 transmitter sends:

$$0 + 8\text{ data bits} + \text{TB8} + 1$$

A Mode 1 receiver expects:

$$0 + 8\text{ data bits} + 1$$

So the Mode 1 receiver is likely to treat TB8 as the stop bit.2 If TB8 happens to be 1, the receiver may see what looks like a valid stop bit, but the actual stop bit still arrives afterward and disrupts synchronization for the next frame. If TB8 is 0, the receiver may detect a framing-related problem or fail to set status as expected.2 Therefore, this direction is not reliable.

2. Mode 1 transmitter → Mode 3 receiver

The Mode 1 transmitter sends:

$$0 + 8\text{ data bits} + 1$$

A Mode 3 receiver expects:

$$0 + 8\text{ data bits} + \text{9th bit} + 1$$

Here, the receiver can sample the transmitted stop bit into RB8 as if it were the ninth data bit.2 But then the receiver still expects a true stop bit after that, which does not exist in the Mode 1 frame.2 So again, the frame does not satisfy the receiver’s expected structure.

This explains why textbooks and training material treat Mode 1 and Mode 3 as distinct UART formats rather than interchangeable ones.2

Footnotes

1. 8051 Microcontroller UART (Serial Communication) | Everything You Need to Know | JunctionByte - Explains Mode 1 and Mode 3 framing and the role of the ninth bit. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

2. 80C51/87C51/80C52/87C52 80C51 8-bit microcontroller family (PDF) - Manufacturer-style datasheet details on RB8, SM2, and the mode definitions. ↩ ↩² ↩³ ↩⁴

3. Intel 8051AH Serial UART (Standard Interface without T2 as BRG) Simulation Details - Describes SCON semantics including TB8, RB8, RI, and mode behavior. ↩ ↩²

4. 8051 Serial Port - Summarizes Mode 1 and Mode 3 bit structure and baud-rate properties. ↩

Why People Sometimes Think They Can Communicate

Some confusion arises because Modes 1 and 3 share several features:

1. Both are UART-style asynchronous serial modes.2
2. Both can use Timer 1 for variable baud-rate generation.2
3. Both use the same TXD and RXD pins.

These similarities may suggest compatibility, but UART interoperability requires agreement on the entire frame format, not just the wire and baud rate.2 In communication theory terms, the frame format is part of the protocol. If one endpoint assumes 10 bits per character and the other assumes 11, the protocol is mismatched.

A useful engineering analogy is standard PC UART settings. Two devices set to the same baud rate but different formats—such as 8N1 versus 9-bit custom framing—are not truly compatible. The 8051 Mode 1 versus Mode 3 issue is of the same type.2

Footnotes

1. 8051 UART | 8051 Controller - Overview of 8051 serial modes, SCON bits, and baud-rate generation. ↩ ↩² ↩³

2. 8051 Serial Port - Summarizes Mode 1 and Mode 3 bit structure and baud-rate properties. ↩ ↩² ↩³

3. 8051 Microcontroller UART (Serial Communication) | Everything You Need to Know | JunctionByte - Explains Mode 1 and Mode 3 framing and the role of the ninth bit. ↩ ↩²

4. 80C51/87C51/80C52/87C52 80C51 8-bit microcontroller family (PDF) - Manufacturer-style datasheet details on RB8, SM2, and the mode definitions. ↩

Final Engineering Conclusion

An 8051 in Mode 1 and an 8051 in Mode 3 should be treated as incompatible for normal serial communication, even though they may share the same baud rate generator and physical UART pins.2 The incompatibility arises because:

• Mode 1 uses a 10-bit frame.
• Mode 3 uses an 11-bit frame with a programmable ninth bit.2
• The hardware meaning of RB8/TB8 differs between the modes.2
• A receiver configured for one mode does not interpret the bit stream from the other mode in the intended way.2

Therefore, the correct answer to the question “Determine if an 8051 in mode 1 can communicate with an 8051 in mode 3” is:

No, not reliably in standard hardware UART operation.
They can match baud rate, but their frame structures differ, so dependable bidirectional communication requires both 8051 devices to use the same serial mode.3

Footnotes

1. 8051 UART | 8051 Controller - Overview of 8051 serial modes, SCON bits, and baud-rate generation. ↩ ↩²

2. 8051 Microcontroller UART (Serial Communication) | Everything You Need to Know | JunctionByte - Explains Mode 1 and Mode 3 framing and the role of the ninth bit. ↩ ↩² ↩³ ↩⁴

3. 8051 Serial Port - Summarizes Mode 1 and Mode 3 bit structure and baud-rate properties. ↩ ↩²

4. 80C51/87C51/80C52/87C52 80C51 8-bit microcontroller family (PDF) - Manufacturer-style datasheet details on RB8, SM2, and the mode definitions. ↩ ↩² ↩³

5. Intel 8051AH Serial UART (Standard Interface without T2 as BRG) Simulation Details - Describes SCON semantics including TB8, RB8, RI, and mode behavior. ↩