Sequential Polling of Devices

Our systems usually consist of multiple remote sites, connected by low-bandwidth (~56k) radios or leased phone lines. Each site, when communicating, requires about 10 Kbps of bandwidth. What we would like to be able to do is sequentially poll each site on a regular interval.

I know we can do this using scan classes and the old prime number trick to reduce overlaps, but short of creating a new module that creates “device polling groups”, is there any other way to force ignition to only talk to one device at a time?

Maybe a better understanding of how a scan class executes would help as well… do multiple devices inside a scan class get queried in any particular order, or does the whole class execute (communicate) at the same time?

When I have projects that have low bandwidth (radio, leased lines) I use a dedicated PLC to do the network communications. This gives you absolute control over network communications. Using a dedicated PLC for networking communications will also give you the fastest data throughput since you won’t have to worry about OPC polling collisions or retries.

I too have the same issue I am trying to work with a 30 station remote network over slooooow radio, I have been looking into installing a PLC just for the connectivity issue, can you recommend any particular brands. We are using Alan Bradley in our Hydro operations but the Hydro department is looking at a different solution.

I don’t use Allen-Bradley much, but you’ll want a PLC that speaks the same protocol as your remote units and has enough memory to store all the remote read and write data required for your remote units.

Here’s a pdf by Kepware on how to optimize OPC communication using their OPC Server. Granted it’s about their OPC Server software, but there’s good general information about optimization in there.