Communication from one outstation to another outstation

[PLCs_4_Me]

Hi,

I have been tasked with figuring out how to communicate from one outstation (SCADAPack 357 Master ) to another outstation (SCDAPack 357 Slave) I want to be able to Read a Holding Register from the Slave Device. They are physically 1/2 a mile away from each other. They both communicate via Modbus RTU to our ClearSCADA server with Sierra wireless Routers (RV50) through com 2 on the PLC's. I did read somewhere that I shouldn't involve ClearSCADA to try to get two outstations to talk to each other or could I use ClearSCADA to send data from one outstation to another.  I feel I can't use the same comm. port (com 2 on the PLC) to communicate to both ClearSCADA Server and to the other Slave PLC. Or am I wrong on this. Any help is greatly appreciated. If any more information is needed, please let me know.

SCADA Expert ClearSCADA 2015 R2

Version 6.77.5882

[BevanWeiss]

You should try to avoid having points of failure within your control system, and introducing ClearSCADA into such a system introduces a point of failure.

If you have a single Modbus RTU port at each end then this becomes more troublesome to avoid however.

Modbus RTU really only handles a single master polling for data.  It can be possible to multiplex it if you can do some kind of 'token' handling, but that's often more trouble than it's worth.

If you can migrate to ModbusTCP, then it will make it easier, since you can almost always get a couple of ModbusTCP clients connecting at the same time, and they are largely independent.  (Some low end devices do only allow a single ModbusTCP connection however... that makes it complicated again).

So a few things to think about:

1. Can you migrate the outstations to use TCP / IP communications?  That makes multi-master much easier

2. What is the criticality of the communications?  If it stops for a few hours whilst a ClearSCADA server may be offline, is that an issue..  For things in water networks that's normally not acceptable, for things in sewer networks it's often ok.. other industries tend to fall more in the 'not acceptable' realm.

I tend to use TCP/IP communications, either DNP3 over TCP when dealing with telemetry devices, or EthernetIP/CIP or ModbusTCP when dealing with plant floor comms.  The end devices (RTUs/PLCs) support peer-to-peer communications on these protocols (either acting as a DNP3 master, or a ModbusTCP client, or EthernetIP/CIP scanner), the other end then acts as a DNP3 slave, ModbusTCP server, or EthernetIP/CIP adapter allowing data to be written / read from it.

In this way, as long as the comms path between the two devices is alive, and the two devices are.. then there is comms.  It doesn't require any additional intermediate device decreasing the availability of the system.

[Joel_Weder]

I agree with Bevan that you don't want to involve ClearSCADA in this communication link unless absolutely necessary. However, I do have another possible solution...

The SCADAPack 357 has more than one serial port, so you could add a point-to-point serial radio link between the two RTU's eg using Trio KR900 (serial only) or JR900 (ethernet/serial) license-free radios.

A SCADAPack can actually operate as both a Modbus Master and Slave at the same time, on different ports. OR it can act as both a Master and Slave on a single port, as long as its MSTR (serial) or MSIP (ethernet) function block is only triggered when the RTU is not currently handling a request as a slave on that port. (I hope that makes sense!)

So what I'd do, with a pair of radios in a point to point link, is to configure an MSTR function block in one RTU. It would be configured to operate at whatever recurring rate you need, to either send Modbus data to the other RTU, or poll it from that RTU as required. The MSTR block would be configured to talk via an unused serial port on the RTU.

[PLCs_4_Me]

Thanks for the reply Bevan Weiss.

1. Can you migrate the outstations to use TCP / IP communications?  That makes multi-master much easier

     The RV50 does have an ethernet port. I am working on getting that port to work with the PLC-Router-ClearSCADA via                   Modbus TCP. 

2. What is the criticality of the communications?  If it stops for a few hours whilst a ClearSCADA server may be offline, is that an issue..  For things in water networks that's normally not acceptable, for things in sewer networks it's often ok.. other industries tend to fall more in the 'not acceptable' realm.

     It is not that critical. We have bigger issues if the server is offline.

All we need is for the Outstation to poll just one holding register for the Master outstation. This is for a Sewer Pump Station. Basically our problem is we have pump station "A" that pumps into pump station "B". Then pump station "B" pumps into pump station "C". When we have issues at pump station "C", we only have roughly an hour response time before it overflows. Our thoughts are to monitor the wet well level at pump station "C" and send that info to pump stations "A" and "B". If it reaches a certain level - lets say 80 inches - have pump station "A" and "B'

change there regular on-off levels so as to allow the level at pump station "C" to rise at a lower rate, giving us a longer response time. We do not do that at the moment because of odor complaints, we do not allow the sewer to sit longer in the wet well due to H2S. 

[PLCs_4_Me]

Joel_Weder, 

Thanks for your reply Joel

I will definitely look into point-to-point serial radio link between the two RTU's eg using Trio KR900 (serial only) or JR900 (ethernet/serial) license-free radios.

Since this is just particular to these pump stations, this might be the best route to take.

Thanks for the help.