Re: Two quick RS-485 questions

From: Meindert Sprang (
Date: 05/03/04

Date: Mon, 3 May 2004 07:26:30 +0200

"Lewin A.R.W. Edwards" <> wrote in message
> Hi all,
> I have to implement a very simple multidrop RS485 application,
> carrying slow 2400bps or even 1200bps data from a PC to a half-dozen
> devices about 300 feet away. The data flow is unidirectional; the
> devices don't talk back. Low cost is a significant factor in this. I
> plan to stick an RS485 driver on an AVR, weld on the small additional
> logic I need, and put it in a box.
> Two quickies:
> 1. What's the cheapest assembled RS232 to RS485 converter I can
> recommend to the customer? (I'm not supplying this piece to him. I am
> going to test with an SBC that has RS485 built-in).

I think a current loop might be your cheapest option. You would need a
proper 'driver' at the TX side while all receivers are merely cheap
optocouplers at a fraction of the price of an RS-422/485 receiver. And as a
bonus, you'll get galvanic isolation for free.

> 2. Can I use cat5 network cable for the cable run? I ask because both
> I and the customer have metric truckloads of the stuff lying about.

With 2400 bps and a current loop you could even use fence wire. So yes, cat5
will be fine :-)