#509 MAX3232 RS232 to TTL Module

Using a MAX3232 Module with CH340 adapter for USB to RS-232 serial communication, with examples using screen on MacOSX.

Notes

I want to talk to a device on an RS232 DE-9 serial port, and it seems I have modules on hand for that!

This is a quick exercise of combining two modules to allow RS232 serial communication over USB:

• a MAX3232 RS232 to TTL Serial Module,
• with a CH340G-based USB to TTL serial adapter

See LEAP#406 for details of the USB to TTL serial adapter. Here I’m going to take a closer look at the MAX3232 module, which can be found for pennies on aliexpress or ebay:

RS-232

RS-232 is a vintage serial standard that used to be ubiquitous on PC equipment for point-to-point peripheral communication, especially modems and mouses. It almost became extinct, but appears to have had a but of the resurgence on the back of IoT/embedded systems.

RS-232 is a serial communication protocol, where a time-series of bits (data bits, optional parity bit, and a number of stop bits) is encoded using differential voltage levels for ‘0’ and ‘1’. The RS-232 standard does not define the data encoding.

What makes RS-232 a little daunting to directly integrate with are the voltage levels on the line:

This is where RS-232 driver chips like the MAX3232 come into play: they handle all the voltage conversion and allow control and communication at CMOS or TTL voltage levels.

DE-9 Pinout

The RS-232 standard recommended D-subminiature 25 pin connectors (DB-25), but the 9-pin DE-9 became the most common for computer applications.

pedantic side note: almost everyone refers to the 9-pin connectors as “DB-9”, but I just learned from the wikipedia page they are officially “DE-9”, with B/E referring to the different shell sizes.

Flow Control

In modern applications, a minimal “3-wire” RS-232 (transmit, receive, ground) connection is often used. This configuration does not use any hardware flow control.

A “5-wire” configuration adds hardware flow control with RTS and CTS lines.

Null Modem

Usually when using RS-232 to connect to computers/microprocessors, a Null modem (cross-over) connection is required. This basically has transmit and receive crossed i.e. what one transmits, the other wants to receive.

NB: straight-through connections are for traditional uses with peripherals - e.g. a computer talking to a modem.

About the MAX3232

The MAX3232 provides level conversion for 2 lines in and 2 lines out, so it can support 3-wire or 5-wire RS-232. It can operate with 3.0V to 5.5V TTL, with transmission speeds up to 1Mbps.

MAX3232 Module Construction

The MAX3232 module is a standard 3-wire configuration, with a DE-9 female connector. Here’s a quick transcription of the module circuit. It follows the recommended examples provided in the datasheet:

Connecting the USB Adapter

Connecting with Screen

I’m using MacOSX, so (as with Linux) the most handy console utility is screen.

The device I am connecting to for a test requires 9600 baud, with 8 data bits, no parity and 1 stop bit. The CH340G adapter was connected on /dev/tty.wchusbserial14210 device, so a screen session is started with the command:

$ screen /dev/tty.wchusbserial14210 9600,cs8,-parenb,-cstopb,-hupcl

…and communication is up and running without any problem:

The USB adapter and MAX3232 module connected to the serial device:

Credits and References

• MAX3232 RS232 to TTL Serial Port Converter Module DB9 Connector - aliexpress seller
• MAX3232 product info and datasheet
• RS-232 - wikipedia
• D-sub 9 Connector Pinout
• Null modem - wikipedia
• screen notes
• Serial Communication – RS232 Basics - maxembedded
• Beginning Embedded Electronics - 4 - UART and Serial Communication - sparkfun tutorial