RS485 cable

DLMUK

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Join Date
Jun 2013
Location
Southampton
Posts
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Hi

I have been reading stuff online about the "characteristic resistance" of cable which is best for RS485.

A lot of material says you should have 120ohm when using long lengths. Other web sites say that lower values are OK but you have to match the termination resistors to the character resistance.

Does anybody have any experience of using RS485 devices over cables with different characteristic resistance?

Cheers,
 
We have many RS485 networks (big and small) and we always use the 120 ohm termination resistors and also use one twisted pair cable with screen.
 
DLMUK said:
Hi

I have been reading stuff online about the "characteristic resistance" of cable which is best for RS485.

A lot of material says you should have 120ohm when using long lengths. Other web sites say that lower values are OK but you have to match the termination resistors to the character resistance.

Does anybody have any experience of using RS485 devices over cables with different characteristic resistance?

Cheers,
Click to expand...

Simple, just use Belden 9841 or 9842

http://www.farnell.com/datasheets/33905.pdf
 
Thanks - do you ever set them up in a star configuration?

Read a lot saying it is not best practice to do it but on initial tests it seems to work ok.

Thanks,
 
DLMUK said:
Thanks - do you ever set them up in a star configuration?

Read a lot saying it is not best practice to do it but on initial tests it seems to work ok.

Thanks,
Click to expand...

Nope, star won't work (well it will in certain circumstances with VERY short stubs as you have probably porved on your bench) its not accepted practice and will cause more issues than it will solve.
 
We had to use one old star configuration recently. It works, but it wasn't fun.. Only if you really have to and no guarantees..
 
grnick50 said:
If you want to make star networks with modbus you can use a Modbus hub
Click to expand...
I don't agree. You SHOULD use a Modbus hub if a star configuration is needed for some reason. Most of the times this is because the client thinks he can save some cable costs that way.

In regard to the terminating resistors and the cable impedance: don't forget that for using square wave signals up to 12MBit/s there are significant sine components of up to 132MHz in your signal. This means you're dealing with high-frequency signals, thus also with standing waves if you don't terminate the cables correctly.

It's the same discussion as with Ethernet, but there we only use point-to-point connections with every port properly terminated. Nobody questions that we use the proper cable there. Only us, automation guys, are sometimes (a lot of times in fact) asked to do with cheap solutions.
 

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