El-O-Matic P-CDPS and Profibus DP


Thread Starter

Johan Nel

We have configured the network and setup the node addresses of the valves. The problems we experience is intermittent. Sometimes a valve will just fail to communicate and other times it will accept a command to open but does not send the 24Vdc signal to the solenoid. We have now constructed a test program to test the devices one at a time. But the results are the same. The installation used siemens profibus dp cable and weidmuller spur line connectors. The maximum length of the network is about 50m. All the spur lines are shorter than 1m. Is there anybody that can give me a couple of ideas as I am fresh out.
Intermittent communications sounds like a problem with your network, but that doesn't explain why a valve accepts the command to open, but doesn't actually energize the solenoid. You may have multiple problems, or maybe your communications problems are causing this other problem. In any rate. You did not say if you have the network terminated at both ends. Profibus requires active termination at both ends. You didn't say which devices were at either end of the network. If you have valves at either end, check to see if they have a DIP switch will turn on the termination. If not, check the connector. If the connector does not have a terminator switch, then you will need to get an active terminator. The only one I know if is made by Siemens. I know the 317-DP2 does not have a DIP switch to turn on a terminator, so if the PLC is at an end of the network, then you will need to use a connector with a terminator or an active terminator.

Also, unless you are running at a very slow baud rate, Profibus-DP should be run with the cable daisy-chained between the slaves, not with spur lines. Try setting the bus at it's slowest setting and seeing if the communication improves. If it does, then keep on speeding it up until you find the point that you start to see problems again. Then you will know what speed you can run the bus at given your wiring. If all you have on the bus is valves, you are not passing a lot of data, so I wouldn't think you would need to be running it very fast anyway.

One more thing, if the first two suggestions don't solve your problem, check the grounding of the Profibus cable.

Good Luck.

Curt Wuollet

Check power quality. Throw a large electrolytic capacitor across the power terminals and see if the problem goes away.



Michael Griffin

In reply to Johan Nel - Are you actually getting Profibus communications error messages, or are you just observing no response from the valves? From the information that you have provided, I would suspect that the problem may be in the valve or valve driver, and not in the communications node.

The valve should have power which is separate from the communications. Check the valve power, DC common, and grounding. Check isolation, common interconnections, polarities, etc. of everything (power and communications).

Are you sure the valve isn't trying to turn on? Is there a status light on the valve that will tell you what the valve is doing? If the light(s) is going on and off when it is supposed to, but the valve isn't moving then it could be a valve problem (not necessarily a bad valve, but an incorrect connection or configuration).

Check the wiring very carefully against the manufacturer's recommended installation practice. I am not familiar with the El-O-Matic in particular, but I have see cases where incorrect electric power connections have resulted in current flowing in paths that don't show up on an electrical schematic or block diagram. Things may not work, even if everything looks OK when checked with a voltmeter. Generally the problems were related to power systems being isolated when they shouldn't have been, or not isolated when they should have.