RS485 Implementing a Daisy-chain

jecottrell · Joined: 16 Jan 2005 Posts: 559 Location: Tucson, AZ

I'm diving into RS485 for the first time and I've read many of the posts here on CCS and spent time Googling as well. I've come up with some questions that I haven't been able to answer and thought the vast experience here might be able to point me in the right direction.

First, I'm not sure whether RS485 is necessarily the correct answer for my situation, but it appears to be the most robust and ubiquitous of the options.

The planned configuration is one master, many slaves (20ish max), distances initially less than a couple of meters total, but could grow to tens of meters, not necessarily a terribly noisy environment but here will possibly be some high current PWM happening on the slaves and not large amounts of data being moved, mostly just commands from master to slaves.

My unanswered questions so far are:

1. Interconnecting each slave in a daisy-chain: would a pair of RJ45 connectors be a satisfactory solution? The goal is to allow the end user to simply add another slave by plugging in another cable into the last unit.

2. Ground loops and reference for the RS485: There may be multiple power supplies in the network and I'm trying to figure out grounding. Won't the ground(ref) that is run with the A & B wires end up creating a large loop problem?

Thanks for your time and ideas,

John

Ttelmah · Joined: 11 Mar 2010 Posts: 19499

Yes.
Opto-couple the RS485 drivers if you want really good reliability, and the devices may be on different supplies.
Remember the need to bias the rails to the 'off' state when not being driven.
Depending on the data rate you intend to use, you may need to have termination at each 'end' of the bus. Consider a plug in terminator that goes into the last RJ45, makes it simple to move, no jumpers involved, and easy to remember.
Twisted pair cable.

Best Wishes

newguy · Joined: 24 Jun 2004 Posts: 1907

Don't discount CAN. With RS485 you have to take care of bus contention manually (not really a problem if each slave only replies to the master when it is commanded to), but CAN takes care of that in hardware....you never have to worry about it.

Regarding interference, I have direct experience with an automated welding system with 4 welders, each @ 130-250A, and the welding leads running alongside the CAN cable for 60'. No interference issues.

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

John P · Joined: 17 Sep 2003 Posts: 331

RS-485 drivers and receivers are designed to work with common-mode variations--usually grounds that aren't consistent. You'd need to be in a pretty unusual environment to run into problems with only a couple of meters of cable.

A lot of systems with a master and slaves end up with two data channels (i.e. 4 wires), one channel for the master to talk to the slaves, and one for the slaves to respond. There are various schemes to make sure the slaves talk one at a time, but the master can always send out messages.

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

temtronic · Posted: Sat Feb 25, 2012 10:03 am

RS-485 seems to be the 'defacto' standard for halfduplex,multidrop communications these days, though there are other methods.
One problem,though not unique to it, can be loss of communiction due to wire breakage( one of the 'petals' dies).
There are solutions to this, one I've used for 20+ years up to 15 miles one way trip.
Just something to be aware of.You should have some slave code 'incase of failure to communiate'.

Humberto · Posted: Sat Feb 25, 2012 11:49 am

John P · Joined: 17 Sep 2003 Posts: 331