Varistor selection for RS485
Varistor selection for RS485
(OP)
Can you please help me select the correct varistors to try and use on our RS485 communication circuit. I believe we are running into a induced voltage on the lines causing them to loose communication. I was thinking about installing a varistor in the circuit on each of the two lines to ground to try and dissipate any of this excess voltage. I believe the max voltage on rs485 is 5 volts so the closer we can get the better, without causing unintended faults. I would like to modify one of our varistor boxes we have on hand and remove the 8 varistors that are currently in it, and replace them with 4 pairs of different size varistors. I think a 5V, 6V, 8V and a 10V would be a sufficient test group to use. I have already gone to mouser.com and selected a couple of varistors that I would pick but I am not a expert in this area, so that is were I could use your help. I am not sure how the clamping voltage, capacitance, and amperage are going to effect the varistor and it effectiveness. I have added the shopping cart that i picked out.
http://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=716b46c9fc
http://www.mouser.com/ProjectManager/ProjectDetail.aspx?AccessID=716b46c9fc





RE: Varistor selection for RS485
Check why (and also IF) there is excess voltage and then eliminate the reason for it.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Varistor selection for RS485
Dan - Owner

http://www.Hi-TecDesigns.com
RE: Varistor selection for RS485
RS485 is differential-mode, make sure you haven't accidentally connected to a source or ground -- the Tx and Rx should be floating. If voltage is being induced, shield cable and ground shield properly.
Good on ya,
Goober Dave
RE: Varistor selection for RS485
RE: Varistor selection for RS485
TTFN
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RE: Varistor selection for RS485
RE: Varistor selection for RS485
1) A number of app notes (check out troubleshooting & grounding notes) at
http://www.robustdc.com/application-notes/
2) B&B Electronics' RS-485 technical page:
http://www.bb-elec.com/technical_library.asp
RE: Varistor selection for RS485
Not really. RS485 is 2 signal wires, each referenced to a common. The signal is transmitted in the difference between the wires. We already had a thread discussing this before. I think we agreed it was common mode wiring with a differential for data or something like that. RS485 is not a true 2-wire differential signal like say POTS or 10base-T.
The device will be using earth ground as the common if you only have the 2 wires going to the device. And earth ground as a reference is prone to local ground differences causing problems such as you are describing. Well, unless you have left an isolated port floating and in that case you need to run a common wire to it.
The Robust DC notes are an excellent read. Personally, I would recommend grounding the circuit in only one place and using isolated devices everywhere else. Run a shielded cable with 2 twisted pairs. Use one pair for the signals and one of 2 wires from the second pair as the common.
RE: Varistor selection for RS485
You're right, the data is differential but there is a common reference. My bad.
Good on ya,
Goober Dave
RE: Varistor selection for RS485
So, can anyone give a good reason for RS485 not being differential?
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Varistor selection for RS485