# RS-232: put a transceiver between UART and cable

Understand why an RS-232 port cannot connect straight to a logic-level UART.

**Applies to:** Signal path only. Choose a transceiver and supply circuit from its datasheet; DTE/DCE roles determine the cable wiring.

## When to use it

Interfacing a microcontroller with an instrument or legacy serial port.

## Why it works this way

The transceiver translates the electrical levels and polarity while the UART handles byte framing.

![RS-232: put a transceiver between UART and cable — Signal path only. Choose a transceiver and supply circuit from its datasheet; DTE/DCE roles determine the cable wiring.](https://valleytech-black-wire-guide.pages.dev/library/wiring/rs232-interface.svg)

[Open / save the original SVG](https://valleytech-black-wire-guide.pages.dev/library/wiring/rs232-interface.svg)

## Connection reference

| Layer | Job | Check |
| --- | --- | --- |
| MCU UART | TX / RX logic | I/O voltage |
| RS-232 transceiver | Translate electrical levels | Supply, capacitors, channel direction |
| Serial cable | Connect endpoints | DTE/DCE and pin-numbered connector views |

## Make the connection

1. Identify both port roles and whether hardware flow control is required.
2. Use the equipment pinouts to select a straight-through or null-modem cable.
3. Match baud and framing after checking the electrical connection.

## Check before powering

- Never apply an RS-232 signal directly to a GPIO input.
- A DE9 shell does not prove a port is RS-232; similar connectors carry CAN and other interfaces.

## Matching module references

This is a general interface guide; match your own hardware documentation.

## Sources & review

Source documents inspected; independent electrical and bench review pending. Reviewed 2026-09-27.

- [Analog Devices: RS-232 fundamentals](https://www.analog.com/en/resources/technical-articles/fundamentals-of-rs232-serial-communications.html) — DTE / DCE and RS-232 electrical interface; checked 2026-09-27

Original writing and diagram by Kal / Valleytech Solutions, CC BY 4.0. Manufacturer artwork and documentation retain their own rights.

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