EEVblog Electronics Community Forum

EEVblog => EEVblog Specific => Suggestions => Topic started by: neal.sprouse on May 04, 2010, 12:40:09 pm

Title: RS-485
Post by: neal.sprouse on May 04, 2010, 12:40:09 pm

A blog on RS-485 would be great. I thinking RS-485 might be a solution for a project I am working on. What transceiver IC would you recommend? Cable termination? Multiple devices on the same bus?


Title: Re: RS-485
Post by: tecman on May 09, 2010, 06:37:36 pm
I have used 485 too many times to count.  The most common xceiver is the 75176.  Works quite well but is not as robust as others.  Exceeding the bus voltage with an external source will generally fry them.  For a high end xceiver use a MAX485 or MAX 1485.  They will take a whole log more abuse than the 75176, but cost 4x-5x more.

Termination is needed only at higher bauds and longer cable lengths, or if you have multiple nodes.  Use 120 ohm with a series 0.1uf cap.  Resistor alone will work with 2 nodes, but with more the loading with more nodes, the DC resistance can swamp the signal.

The spec supports 128 or more devices on the bus.  We have used over a dozen w/o problem.  If you do have issues, often a lower baud will be a quick fix, otherwise terminations, cable quality, etc start to become factors.