MVI56-DH485 Communications
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mrholdcroft
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Monday 24 July 2006 8:28:23 am |
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MVI56-DH485 Communications
I have a MVI56-DH485 module communicating to 6 SLC5/02 processors. The Prosoft module and ControlLogix PLC are replacing an DCM module and SLC5/05 that was used before. The DH-485 network hasn't changed. I have everything set up and communicating but I keep seeing drop outs of the data coming form the SLC5/02s on the DH-485 network. I also see the active node statuses come and go randomly for all the SLC5/02s (some worse than others). I don't know if I have timing issues or if the network itself always had problems and the MVI56-DH485 is just more sensitive to old problems.
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jsanders
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Monday 24 July 2006 9:26:04 am |
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RE:MVI56-DH485 Communications
Without having many more details about your specific application, it is hard to posit a exact solution. The A-B DH485 protocol, being a low speed, token-passing protocol, has always been suseptable to traffic overload resulting in network errors. Any node on the network is allowed to create messages and poll any other node for data. The number of nodes putting messages on the network and the timing used in sending those messages can have an effect on the number of errors experienced.
SLC 5/02s do not have the 'horsepower' to handle DH-485 messages as quickly as other SLCs. They are more easily overloaded with traffic. As their scan times go up, the communications responsiveness goes down. Communications servicing happens AFTER ladder scan and typically the comm servicing gets only a small percentage of the overall scan time to complete its task. Any messages that can't be handled get trapped in a buffer awaiting an opportunity to be sent; this is true for messages created by the logic of that 5/02 as well as any response messages that have to be returned to other nodes who sent messages to that SLC. If the total number of messages that the SLC must send exceeds its ability to send them, errors inevitably result.
Speaking in general terms, whenever you start seeing recurring network errors on a DH-485 network, since the maximum bandwidth is fixed, about the only thing you can do to eliminate the errors is to cut back on the amount of messages being sent in a given amount of time. If you are using our MVI56-DH485 module as a Master and sending commands to other nodes, you can easily use the Poll Interval in each command to slow things down by putting an acceptable amount of poll delay into the commands. You can also use this feature to prioritize your commands by using shorter Poll Intervals for commands used to get/send more critical data; and then use longer Poll Intervals for commands that send/receive less critical. Such strategies can prevent error-creating communications log-jams.
For additional strategies or for more help with your specific application, please contact ProSoft Technical Support. You can e-mail to Support@psft.com...or you could call (661) 716-5100.
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