M580 safety communication to Foxboro DCS through VSAT

[Abanoub]

Hi,

I am working on a project as a tendering engineer, The primary aim of this project is to deploy RTUs in four valve rooms spread along a 50km Pipeline. These RTUs will be responsible for gathering essential data and transmitting it to the existing Foxboro DCS at CCR.

Proposed RTU: We are proposing M580-safety to meet the customer's requirement for a SIL 2 RTU.

Communication Infrastructure:

• Primary: A VSAT terminal will be installed in each valve room and at CCR.
• Secondary (backup): Wi-Max
• Communication Protocol: Modbus TCP

Below you may find the proposed system architecture.

I kindly request your input, as well as any recommendations or potential obstacles you foresee with this proposed solution.

Also, will Modbus be sufficient or we should rely on DNP3?

Thank you for your kind support,

[dnordenberg1]

I'm not a SIL expert but I think you will have problems with making a modbus communication comply with SIL. For example, you can poll a PLC using modbus and see "frozen" values without the CPU even being in RUN mode, how do you know the data is actual then? You will probably have to "invent" lots of safety checks and these checks will probably have to be verified and certified by a third party then. Or do you mean that each valve room only needs SIL locally and modbus is just for remote monitoring and not safety related?

[Abanoub]

Yes exactly, the initial requirement from the customer was to have two controllers; one SIL certified to perform the safety-related control "ESD" and the other one acting as RTU which retrieves the data from the ESD controller to be sent to the DCS.

We proposed M580s to have only one controller in each valve room instead of two, especially that we have a very limited IO count.

Assuming that the data transferred between M580s and DCS will not be safety-related and will be used for monitoring and basic control.

[dnordenberg1]

Nice, then that part would be fine then 🙂

For protocol choice, any of those would be totally fine. DNP3 requires an extra communication module and I see no real benefit for using that here other then if the customer specifically wants it because it easier to integrate in foxboro or something. With a M580 CPU you can do both modbus TCP master and slave or both depending on which end you want to handle the communication from.

[Hennie_Smith]

I would recommend DNP3 rather than MBTCP. If there is any latency in the comms you would need to set the MB timeout very long to prevent false comms failure alarms. Also DNP3 will give you the ability to backfill data if you have historian connected.