DH+ and RIO Terminal Resistors

All,

I have a PLC with a DH/RIO card. Ch A is connected to an HMI via DH+. Ch B is RIO connected to a 1794-ASB module. That is the extent of any networking.

Do I require terminal resistors on both ends of the DH+ and RIO media? (On the PLC and on the HMI / ASB?)

Thanks.

Yes. The PLC-5 was very tolerant of missing termination resistors, but from what I have read, the newer controllogix modules are much more picky. Note that the network needs to be a daisy chain (no star networks) and that resistors are ony required at the end nodes.

Refer to the proper manuals for resistor sizing. I believe it is still dependent on baud rates, but may not be exactly the same as it was with the PLC-5.

Thanks.

Just to clarify, when you mean end nodes, you mean not only at the device end (in this case the ASB and the HMI) but also at the DH RIO card itself?

Yes, if there are only two nodes, then both of them. Each channel of the the DH/RIO card is a node, the ASB is a node, and the HMI is a node. Always two resistors per DH+/RIO network, and only at he end nodes (where only one blue hose is landed). You have two networks so you're gonna need four resistors.

Is it safe to assume you're using a 1756-DHRIO card?

Yes, 1756-DHRIO. I have the manuals with the resistor requirements, I just was unclear after reading the manual.

Thanks for the info.

So before I could get the terminal resistors on the media, I went online to configure a few things. I am actually receiving (and can send) data over RIO even without the terminal resistors.

Are the resistors more of something recommended rather than a must have? For larger networks I've seen issues with the terminal resistor being in the wrong place, however for smaller networks do the resistors make that big of a difference?

Thanks again.

RIO and DH+ are usually very tolerant of wiring issues including missing termination resistors. Technically, they're required. I have worked on machines that were missing the resistors for years without ever having problems. I have worked on other machines that began to have timeouts and retries in the error logs and adding the correct resistors fixed the problem.

I even had one machine where the installer used a star-daisy chain to another star on RIO and it worked flawlessly. He did this to avoid two rather lengthy cable pulls. I disapproved, but I couldn't find any errors at all once the machine had been running for over a week, so I left it alone.

I have not used the 1756-DHRIO module without the resistors and have heard others say that this module is less likey to tolerate incorrect termination.

I would highly recommend that you use the resistors. You don't want to encounter problems down the road and catch the blame for it.

Okie is right, don't tempt fate. I have seen major issues with missing or incorrect Resistors on RIO and DH+. As Okie said, they are amazing tolerant, but in the future something could change, and everything could go sideways. AB would not supply them with the Cards if you didn't need them. Install the correct Resistor for the speed you are running at both end points. Also, I have seen bad Shield termination cause problems. Cut the Shield neatly, and Shrink Wrap it just short of the end so you can terminate it in the 3 Pin Connector. Then Shrink Wrap over the end of the Cable. I usually label the ends with where it goes to at the other end.

Stu.....

I second OKIE PC information.

I too have soem RIO networks that are star daisy chain and some have 3-4 resistos in the circuit and some have none.

Some have them mid run. Some we have had problems with and some we don't. We try to fix these as they are discovered.

We have had some devicenet networks without resistors. Too many resistors. Resistors in the wrong place or resistor is the wrong value and some work perfect and some don't.

The long and the short of it is it just depends on too many factors to calculate.

Best advice has been given put them in per spec and if at any point in time there are issues you don't have to worry about that part.

Just because you can do something and just because it does work does not necessarily make it a good idea.

I can work in a energized panel with no gloves or arc flash gear. I have done it many times in the past and never had a problem. But why tempt fate.

The most common wiring problem I see is the shield drain wire shorting out against the termination resistor lead, but that's not a reason to omit the resistor !

I always carry shrinkwrap tubing, wire strippers, and a torch when I'm troubleshooting RIO. Especially with the modular plugs used on PLC-5, SLC and Logix controllers, you can get a bad connection inside the screw clamp and it won't be obvious from the outside.

At one of the very first Automation Fair events (1997?) I'm told they had RIO running over an actual barbed wire fence.

An unterminated RIO or DH+ network will probably work, especially if it's short, but cannot be relied upon as it grows and gets more heavily loaded. It's not like RS-485 where you have to build bias into the system, or like Profibus where they require power at the termination end.

82 ohms for 230.4 kb/s networks, 150 ohms for 115.2 or 57.6.

Appreciate the info, guys.

Thanks.

Ken, funny about the barbed-wire fence! I would have liked to see that...

So I just added terminal resistors (150 Ohm @ 57.6k) and I noticed something rather odd:

I am connecting through the programming terminal port.

Before, when I didn't have the terminal resistor in place, I could see both the controller AND the connected device HMI on the DH+ network.

I added both resistors and I try to reconnect through the programming port on the DHRIO card. I can see the HMI fine, but not the controller.

But when I disconnect the DH+ cable from Ch A (I am still connected to the programming port) I am able to see the controller again.

Did I mess something up when I terminated the cable or is the programming port unable to see the controller when you have terminated ends?