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RS-232 Transmit Data (TXD) Pin Identification

RS-232 Transmit Data (TXD) Pin Identification

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May 19, 2026

In the standard RS-232 DE-9 connector used on DTE devices, the TXD signal is on pin 3.3 Therefore, the correct answer is:

(iv) Pin 3.2

This question is common because RS-232 pin numbering depends on whether one is discussing a DTE-facing connector, a DCE-facing connector, or a cable-end perspective. In the most widely taught PC-style DE-9 pinout, pin 2 is RXD, pin 3 is TXD, and pin 5 is signal ground.3

A compact view of the core signal relationship is shown below:2

PinSignalDirection on DTE
2RXDInput
3TXDOutput
5GNDReference ground

The key conceptual rule is that TXD means data transmitted by the local device, so on a standard DTE serial port, that function is assigned to pin 3.2

Footnotes

  1. EIA/TIA-232, 422, and 485 Serial Networks - Educational reference showing DE-9 DTE assignments with pin 3 as Transmitted Data and pin 2 as Received Data. ↩ ↩2 ↩3 ↩4 ↩5

  2. The RS232 Standard - CAMI Research - Explains DTE/DCE labeling differences that often cause confusion in RS-232 pin identification. ↩ ↩2

  3. RS232 Cables, Wiring and Pinouts - Pinout reference listing DB9/DE-9 pin 3 as TD/TXD and pin 2 as RD/RXD. ↩ ↩2 ↩3 ↩4

  4. RS232 Pin Out | Connector Reference Guide - Practical guide noting common three-wire RS-232 use and contrasting male/female connector contexts. ↩

What is RS232 and What is it Used for?

Exam Answer

For the usual RS-232 DE-9 connector pinout used on PCs and DTE devices, TXD is on pin 3.2

Footnotes

  1. EIA/TIA-232, 422, and 485 Serial Networks - Educational reference showing DE-9 DTE assignments with pin 3 as Transmitted Data and pin 2 as Received Data. ↩

  2. RS232 Cables, Wiring and Pinouts - Pinout reference listing DB9/DE-9 pin 3 as TD/TXD and pin 2 as RD/RXD. ↩

Why the Answer Is Pin 3

The confusion often arises because DCE devices may present complementary labeling relative to DTE devices.2 However, standard RS-232 teaching materials and pinout references consistently show that for the DE-9 DTE interface, pin 3 = Transmitted Data (TD/TXD) and pin 2 = Received Data (RD/RXD).3

Several technical references support this mapping:

  • A serial pinout guide lists pin 3 as TD/TXD and pin 2 as RD/RXD for RS-232 on DB9/DE-9.
  • Texas Instruments’ RS-232 interface documentation shows the DB9 DTE interface with RD on pin 2 and TD on pin 3.
  • Educational material on EIA/TIA-232 states that the DTE assignment on a DE-9 connector places Transmitted Data on pin 3.

Thus, among the options:

  • (i) Pin 2 → incorrect, because pin 2 is normally RXD on DTE.2
  • (ii) Pin 1 → incorrect, because pin 1 is usually DCD.2
  • (iii) Pin 4 → incorrect, because pin 4 is usually DTR.2
  • (iv) Pin 3 → correct.3

Footnotes

  1. The RS232 Standard - CAMI Research - Explains DTE/DCE labeling differences that often cause confusion in RS-232 pin identification. ↩

  2. RS232 Pin Out | Connector Reference Guide - Practical guide noting common three-wire RS-232 use and contrasting male/female connector contexts. ↩

  3. EIA/TIA-232, 422, and 485 Serial Networks - Educational reference showing DE-9 DTE assignments with pin 3 as Transmitted Data and pin 2 as Received Data. ↩ ↩2 ↩3 ↩4 ↩5 ↩6

  4. RS232 Cables, Wiring and Pinouts - Pinout reference listing DB9/DE-9 pin 3 as TD/TXD and pin 2 as RD/RXD. ↩ ↩2 ↩3 ↩4 ↩5 ↩6

  5. Interface Circuits for TIA/EIA-232-F (Rev. A) - Texas Instruments - Vendor technical document showing the DB9 DTE interface with RD on pin 2 and TD on pin 3. ↩ ↩2 ↩3

How to Determine the TXD Pin in RS-232 Questions

  1. 1
    Step 1

    First determine whether the question refers to the common 9-pin RS-232 DE-9 connector or the older 25-pin version. In modern educational and PC contexts, the 9-pin DE-9 is usually intended.2

    Footnotes

    1. EIA/TIA-232, 422, and 485 Serial Networks - Educational reference showing DE-9 DTE assignments with pin 3 as Transmitted Data and pin 2 as Received Data. ↩

    2. RS232 Cables, Wiring and Pinouts - Pinout reference listing DB9/DE-9 pin 3 as TD/TXD and pin 2 as RD/RXD. ↩

  2. 2
    Step 2

    Most textbook and exam questions assume a DTE-oriented PC serial port. In that convention, pin 2 is RXD and pin 3 is TXD.3

    Footnotes

    1. EIA/TIA-232, 422, and 485 Serial Networks - Educational reference showing DE-9 DTE assignments with pin 3 as Transmitted Data and pin 2 as Received Data. ↩

    2. The RS232 Standard - CAMI Research - Explains DTE/DCE labeling differences that often cause confusion in RS-232 pin identification. ↩

    3. Interface Circuits for TIA/EIA-232-F (Rev. A) - Texas Instruments - Vendor technical document showing the DB9 DTE interface with RD on pin 2 and TD on pin 3. ↩

  3. 3
    Step 3

    Use the essential three-wire mapping: RXD, TXD, and GND. For DE-9 DTE, these are pins 2, 3, and 5 respectively.2

    Footnotes

    1. RS232 Cables, Wiring and Pinouts - Pinout reference listing DB9/DE-9 pin 3 as TD/TXD and pin 2 as RD/RXD. ↩

    2. Interface Circuits for TIA/EIA-232-F (Rev. A) - Texas Instruments - Vendor technical document showing the DB9 DTE interface with RD on pin 2 and TD on pin 3. ↩

  4. 4
    Step 4

    Pin 1 corresponds to DCD and pin 4 corresponds to DTR, so they cannot be the transmit-data line in the standard DTE pinout.2

    Footnotes

    1. EIA/TIA-232, 422, and 485 Serial Networks - Educational reference showing DE-9 DTE assignments with pin 3 as Transmitted Data and pin 2 as Received Data. ↩

    2. RS232 Cables, Wiring and Pinouts - Pinout reference listing DB9/DE-9 pin 3 as TD/TXD and pin 2 as RD/RXD. ↩

  5. 5
    Step 5

    After matching signal names to pin numbers, choose pin 3 as the transmit-data line.2

    Footnotes

    1. EIA/TIA-232, 422, and 485 Serial Networks - Educational reference showing DE-9 DTE assignments with pin 3 as Transmitted Data and pin 2 as Received Data. ↩

    2. RS232 Cables, Wiring and Pinouts - Pinout reference listing DB9/DE-9 pin 3 as TD/TXD and pin 2 as RD/RXD. ↩

Common Source of Confusion

Some references discuss cable ends or DCE labeling, which can make pin 2 appear associated with transmitted data in a different context. For standard DE-9 DTE pinout questions, TXD is pin 3.2

Footnotes

  1. The RS232 Standard - CAMI Research - Explains DTE/DCE labeling differences that often cause confusion in RS-232 pin identification. ↩

  2. RS232 Pin Out | Connector Reference Guide - Practical guide noting common three-wire RS-232 use and contrasting male/female connector contexts. ↩

RS-232 Signal Context

Serial communication in RS-232 depends on clearly defined signal roles. The most essential lines are:

  • TXD: outgoing data from the transmitting endpoint.
  • RXD: incoming data to the receiving endpoint.
  • GND: shared electrical reference needed for valid signaling.

For the classic DTE interface, data flows outward on pin 3 (TXD) and inward on pin 2 (RXD).3 This arrangement enabled personal computers and terminals to connect to modems and similar DCE equipment through straightforward or crossed serial cabling, depending on the application.2

A useful memory aid is:

  • 2 = Receive
  • 3 = Transmit
  • 5 = Ground2

Footnotes

  1. RS232 Pin Out | Connector Reference Guide - Practical guide noting common three-wire RS-232 use and contrasting male/female connector contexts. ↩ ↩2 ↩3 ↩4

  2. RS232 Cables, Wiring and Pinouts - Pinout reference listing DB9/DE-9 pin 3 as TD/TXD and pin 2 as RD/RXD. ↩ ↩2 ↩3

  3. EIA/TIA-232, 422, and 485 Serial Networks - Educational reference showing DE-9 DTE assignments with pin 3 as Transmitted Data and pin 2 as Received Data. ↩ ↩2

  4. Interface Circuits for TIA/EIA-232-F (Rev. A) - Texas Instruments - Vendor technical document showing the DB9 DTE interface with RD on pin 2 and TD on pin 3. ↩ ↩2

PinSignal
1DCD
2RXD
3TXD
4DTR
5GND
6DSR
7RTS
8CTS
9RI

This is the common modern exam-oriented mapping for a DTE serial port.3

Footnotes

  1. EIA/TIA-232, 422, and 485 Serial Networks - Educational reference showing DE-9 DTE assignments with pin 3 as Transmitted Data and pin 2 as Received Data. ↩

  2. RS232 Cables, Wiring and Pinouts - Pinout reference listing DB9/DE-9 pin 3 as TD/TXD and pin 2 as RD/RXD. ↩

  3. Interface Circuits for TIA/EIA-232-F (Rev. A) - Texas Instruments - Vendor technical document showing the DB9 DTE interface with RD on pin 2 and TD on pin 3. ↩

Essential RS-232 DE-9 Pins for Basic Communication

Comparison of importance in basic 3-wire serial communication

Clarifications and FAQs

Final Answer and Concept Summary

The correct option is (iv) Pin 3.3

This conclusion follows from the standard DE-9 RS-232 DTE pin assignment, where:

  • Pin 2 = RXD
  • Pin 3 = TXD
  • Pin 5 = Ground3

For learners, the most important conceptual distinction is between the common DE-9 DTE pinout and the older DB-25 pinout, because DB-25 uses pin 2 for TXD, while DE-9 uses pin 3 for TXD. Many multiple-choice mistakes come from mixing those two connector families.

Footnotes

  1. EIA/TIA-232, 422, and 485 Serial Networks - Educational reference showing DE-9 DTE assignments with pin 3 as Transmitted Data and pin 2 as Received Data. ↩ ↩2

  2. RS232 Cables, Wiring and Pinouts - Pinout reference listing DB9/DE-9 pin 3 as TD/TXD and pin 2 as RD/RXD. ↩ ↩2 ↩3

  3. Interface Circuits for TIA/EIA-232-F (Rev. A) - Texas Instruments - Vendor technical document showing the DB9 DTE interface with RD on pin 2 and TD on pin 3. ↩ ↩2

Knowledge Check

Question 1 of 4
Q1Single choice

Which pin on the standard RS-232 DE-9 connector is used for Transmit Data (TXD) on a DTE device?

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