On a quest! cryouts over cellular modems!

With the ever growing popularity of using cellular modems to monitor industrial devices, it seems as though alarm cryout functions are becoming more popular.

There are many devices that can send out voice, text, or email alerts, but not many can communicate back to a scada system and then let the scada system handle the emails, alerts, or callouts as needed.

I have tried two different device setups from a manufacturer with no luck. all I see is large amounts of data being transferred when it shouldnt, which totally defeats the purpose of using a cryout function.

My thought is to have a plc or rtu of some type at the location to be monitored. On that plc or rtu, have the ethernet port setup as a modbus tcp master. Then at your central gathering station, have a plc or rtu that has two comm ports, 1 as a modbus slave to receive the modbus tcp messages from the field, and another that you would poll as a modbus slave to gather information from the field devices.

Now, basically you would block off a certain amount of registers per location that the location's plc would write to in the central gathering stations plc. of course each location device would have to be programmed to write to certain registers, but that seems manageable. Does this sound like something that is doable?

My first concern would be making sure that the central gathering stations plc is capable of having more than one master write to it. other than that, all of the heartbeat and timeout stuff could easily be handled in logic.

Also, another concern would be handling retries from the location side if the cellular network is down. Also, how well would the system handle the slow ping times that you often see on cellular.

Am I totally off base here or does this seem doable?

We offer a service like this to our customers but the receiving end is our web server.

Long story short, we take a device capable of creating and manipulating files, usually a csv file or a sql import. I'm partial to the Red Lion's because they are just awesome. If I need a screen then their G3's work nicely, if I don't need a screen then a Data Station works well.

The remote devices package the files which include everything you need to render your screen on the SCADA end and send it to our web server in set time increments, usually once a minute. They can also send alarms immediately. This information is decoded into a web interface.

This give the customer a "real time" view of their remote system. If you try to poll your devices instead of having them send the information out then you will quickly learn the frustrations of lag from a cellular system. It is WAY worse than dial up. One of our systems takes nearly 30 seconds to respond because of it's remote location.

The remote systems also download a file from our web server each time which can contain updates to tags.