DH+ comm problems

[tbalt 0]

Hi everybody, I have a project where I'm connecting a PLC5 to a Panelview 1000 through DH+. That works fine but what is stumping me is we also have a slc5/04 on the same DH+ network that is also talking to the same panelview 1000. They all have different node addresses and are all talking at 57.6K baud but the communication from my pc to the 5/04 keeps dropping. Both the plc5 and 5/04 are having no problem talking to the panelview and the communication from the plc to the plc5 doesn't drop just the 5/04. If I connect the pc to the 5/04 by itself the problem is gone so it has to be cabling or something simple hopefully. I know we used to use a terminating resistor on remote I/O back in the old plc5 days, could that be the problem and if so what would the resistor size be? I hope you can picture what I'm doing.

[BobLfoot 302]

Check the manuals, but I am fairly sure you need End of Line resistors on DH+.

[newpageboba 7]

Follow the same rules as Remote I/O. At 56KB rate, you need two 150 ohm terminating resistors at each end. Also, make sure it is daisy chain or trunk/drop method. Do not use star wiring.

[tbalt 0]

Thanks for the reply, are you meaning two 150 ohm resistors one at each end or two 150 ohm resistors at each end? Also what is meant by trunk/drop method? Thanks

[Mickey 71]

See Pic for Resistors and daisy chain method. Truck line would be a bus with drops off of the bus. You can do a truck line method but you must keep the drops short. Best to stay with a daisy chian network.

[OkiePC 14]

I was told, last week by a Rockwell engineer, that the minimum segment length is 12 feet. This is supposed to help avoid reflected wave noise. I had always heard it was ten feet, and had numerous systems with shorter cables, but have always followed that rule when adding new nodes or making repairs. I have seen your exact symptoms caused by missing end node resistors, so I expect that will make a difference for you. If it doesn't there are some diagnostic counters you can dig into if you have a status file assigned to the PLC channels. Paul

[tbalt 0]

Thanks guy's that is the way I have it wired but without the resistors so I will give it a try. Tom

[Guest]

and he meant "just ONE 150-ohm resistor at EACH end" ... so a total of only TWO resistors in the installation ...

[Guest]

and this isn't an issue with the Original Poster's question - but I thought I'd mention something that I've noticed lately ... I've got a lot of different Remote I/O equipment that I use in the lab for student exercises ... in many cases I've never worried about using the terminating resistors at all ... just plug-and-play from one PLC-5 to another SLC-500, etc., etc. with NO problems ... same thing with Data Highway Plus ... no resistors - no problems ... but ... whenever I plug the SAME stuff into a 1756-DHRIO module on my ControlLogix systems, I ALWAYS need the resistors - or else I get serious problems ... either very intermittent communications - or even NO communications ... I even noticed this ... I tried to make a DH+ connection from my PCMK card - by plugging the cable to the 8-pin mini-DIN connector on the front of the DHRIO ... I could NOT get the connection to work - UNTIL - I finally plugged a spare three-pin connector into the Channel A port - with a 150-ohm resistor across the two outside pins ... BINGO! ... it worked fine ... (note that there was no other cable plugged into the Channel A port when I started) ... so ... in my personal opinion, the DHRIO module seems to be a LOT pickier about its cable connections ... not that there's anything wrong with that ... (yes, we should ALWAYS do it "right") ... but it's something that I've definitely noticed in the lab ... just thought I'd pass it along ...

[JeffKiper 9]

I can testify to what Ron has said about the terminating resistors. I was in his lab using some 1756-DHRIO and Block I/O my machine was running along and then it would stop responding for a second or so. I just knew Ron let me make intentional misguided step to help the learning sink in. After about 10 min. or so of troubleshooting the light bulb went off (resistors?). The comms where flaking in and out. The Block I/O was set to retain lat state for Outputs. The resistors made me and the system very happy. The end.

[paulengr 44]

Trunk/drop IS in the RIO manual but they specifically say that it's strictly for the older cabling system with trunk/drop connectors. It was ugly and expensive. Better to stick with daisy chains.