From: "Jim Pennell"
Subject: Re: RS-485 termination
Date: Wed, 23 Oct 2002 21:49:22 -0700
Organization: MindSpring Enterprises
References: <3DB580A9.C3C4AC4A@scazon.com> <3DB64E6D.7E42A7A8@scazon.com>
X-Server-Date: 24 Oct 2002 04:49:25 GMT
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Well, they would be in parallel, sort of, if you have separate runs to
each receiver, but the problem is, since each receiver is high impedance,
then you have long "open ended" lines running out from the source and when
you send digital signals in them it tends to create a lot of ringing and
signal distortion on the edges of each data transition.
If you provide a termination at each receiver, it would reduce SOME of
the ringing, but each termination would have to be relatively a high
resistor value since you have to figure out what all of the terminations
work out to back at the transmitter and it has to be somewhere near 100 ohms
to keep the transmitter happy.
This gets complicated and downright nasty, since the values would have to
change depending on how many lines and loads you have. Not to mention that
each line is seeing 100 ohms at the transmitter, but probably something like
1K ohm at the receiver end, assuming you are trying to run 10 receivers in
this example system.
So, even if you get the load on the transmitter to be 100 ohms, you still
have a pretty big mismatch going to each receiver. Not good.
The whole idea for RS485 is a single cable run with a single termination
on the far end. This keeps the signal distortion to a minimum and allows
fairly high data rates over a long cable run.
As for the ground wire, you need it to make sure the voltages going into
the receivers are within the survival limits of the chip. If you ran it
without an electrical ground wire, you could easily have a setup where there
is, for example, a 60 Hz ground loop which could result in the signals that
show up on the receiver input pins going +/- 20 or 30 volts or more
depending on a lot of uncontrollable things in the building ground wires and
power supply construction for each device.
I have seen interface chips literally blown up due to AC ground loops.
Now, if the system was using opto-isolators, then you could indeed make
it a two wire system and not worry about absolute voltage levels. Opto
isolator systems are more complicated, and a simple ground wire is a much