quick and dirty rs232 to rs485?


Thread Starter


A quick and dirty method for converting RS232 to RS485. For a testing environment. Can't budget for a 'off the shelf' converter as once testing is finished its no longer required(ever).

Want to use an RS485 device with PC (RS232)
Is there a chip (Maxim perhaps) that will acomplish this.
Hold on. You can get a 232-to-485 converter from Black Box for about $100 US. It is p/n IC620A (specify sex) and you can order from http://www.blackbox.com. It appears on page 627 of their current catalog.

Walt Boyes
Walt Boyes--MarketingPractice Consultants
21118 SE 278th Place - Maple Valley, WA 98038
425-432-8262 home office - 253-709-5046 cellphone
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Dave Jackson

Thanks for the reply but I don't want to run to that cost. If I bought the device it would probably never be used again after this project.

Thats why I want and quick and dirty throw away method.

Andrea Fiorino


There is the possibility to build a simple and dirty RS232/RS485 converter using only two chips and few other components:

nr.1 MAXIM MAX232 - RS232 to TTL Rx / Tx interface
nr.1 MAXIM MAX491 - RS485 to TTL Rx / Tx interface
nr.4 1uF 25V ceramic capacitor to be connected to MAX232 (see data sheet)
nr.2 2.2Kohm 1/4W resistor to polarize correctly RS485 Rx+ & Rx- inputs

For 2 wires RS485 connections the RS232/RTS must be used to drive MAX491 Tx 3state enabling.

The power supply must be 5Vdc

Don't hesitate to contact me for eventually other info.



Daniel M. Kamers

> For 2 wires RS485 connections the RS232/RTS must be used to drive MAX491 Tx 3state enabling.

Be aware that, if you are using an 16550-compatible UART, your software will have to control RTS in a slightly different way of that using a "pure" 232 port. You cant just drop RTS at the end of transmission. You will must have to consider a little "delay", in such a way that the last characters get actually transmitted before "drop the line".

IMHO, there are good and cheap 232-485 converters that do the hard work for you, as recomended
in the EIA-485 specifications.

Best regards.

Daniel M. Kamers
Eng. de Desenvolvimento
Reivax Automação e Controle
fone/fax: +55(48)3349000
e-mail: [email protected]

Darold Woodward

Sounds like fun, but if you don't work for free, I very much doubt that you can create and test for less than the $59USD cost of a converter from B&B Electronics. Even if you did work for free, any time you are working on something, you aren't working on something else so there is a cost involved.

Darold Woodward PE
Schweitzer Engineering Laboratories
[email protected]