use of terminating resistors in communication networks

rejoe.koshy

Member
R
Join Date
Dec 2011
Location
kolkata
Posts
195
hi,
Do i need to connect terminating resistors to the first & last node on a DH+/RIO network???Is it mandatory for the network to work??
 
I've seen networks DH/RIO work without them but I always install them. They ship them with the processors and comm cards for a reason. I will be question #2 from Tech Support right after " are you using Blue Hose".
 
The answer is yes it is mandatory.

If you do not follow the installation guidelines, you dont know if it will continue to work even if it seems to work for now.
I have seen DH+ and RIO work without resistors or even with plain screened cable because of lack of the proper Belden cable. But I have never left an installation without everything done according to the guidelines.
 
By default use them but there will be some cases where they can cause problms and have to be left out. Having too many can also cause issues.

If you follow and install them by spec you will be in good shape 99% of the time.

Some devies have resistors built in and can be turned on or off by dip switch or jumpers on pins so make sure your devices do not have this before you install a resistor.
 
JesperMP said:
This you have to describe a bit more detailed. Am interested.
Click to expand...

Most times it is where a device requires more signal voltage than the line can deliver and by removing the resistors you can increase the voltage. Also if the propogation delau is less than one bit width then you really don't need terminating resistors. This value is primarily affected by cable length but you have to know the propogation velocity from the cable to calculate it. most times it can be found in the detailed spec for the cable.

JesperMP said:
You mean more than the ones at the two extreme ends of the daisychain, or .. ?
Click to expand...

Yes. I have found a few installs where the device had a built in resistor and the installer had put resistors on the end of the cable also and it caused problems. Some devices with internal resistors are turned on from factory and some are not. 90% ship with it off which is good.
 
Last edited:
I have find it depends on what speed you are running the DH+.
At the lower speed 57K you may get by without it, At the higher speed of 230K I have always used the resistors. Just remember that the different baud rates requires different resistors. 57 (150 ohm) vrs. 230 (88 ohm?). I do however agree that's its a good practice to always use them.
 
My experience is that removing a resistor never solved a comms problem for me, but installing one HAS solved a couple problems.

Using them is the best practice.
 
Just for kicks I did a litle digging. Check out tech note 18644 on the rockwell knowledge base tehy recommend removing the resistors in the case at hand.

I have had to remove them a few times to get gateways and other equipment to comm.
 
On 485 and 422 multidrop systems it is quite common to run with no resistors also. All depends on system cable,length,nodes and as a previous poster said network speed.
 
MasterBlaster said:
My experience is that removing a resistor never solved a comms problem for me, but installing one HAS solved a couple problems.

Using them is the best practice.
Click to expand...

The Plc Kid said:
On 485 and 422 multidrop systems it is quite common to run with no resistors also. All depends on system cable,length,nodes and as a previous poster said network speed.
Click to expand...

Calculating the values for resistors on 485/RTU networks is a bit of electrical alchemy, IHMO.

Best to isolate first, and worry about terminations second.
 
Use the resistors at both ends. If you don't you'll end up with standing waves over the cable, a phenomenon that is better known with radio hams than with PLC users. The damage that can be done depends on the frequencies in use, the length of the cable, the place(s) where equipment is connected and so on. It is very possible you have never had any problems so far, but the moment all conditions are at the worst, you'll be convinced to use terminating resistors for the rest of your life, believe me. And don't think it won't happen with DH+ because of the limited frequencies. Don't forget to take into account that these signals are digital and therefor you need to count in also the harmonics.

Kind regards,
 

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