Modbus for Safety Systems


Thread Starter


I understand that Modbus tends not to be accepted in safety applications as a protocol.

With the advent of safe protocols such as Profisafe is the scope to re-assess Modbus for this type of application?

What are the main issues with this protocol and can it be changed /assessed for SIL type applictions??


Jose Gonzalez-Conde

Profisafe is an extension for Profibus in order to enhance his safety integrity level up to SIL3. This has been done by protecting its data contents with aditional CRC32, sequential message numbering and some more specials.

Maybe the same technique can be applied to some modbus messages like holding register data.

Lynn A Linse

I doubt the Modbus/RTU CRC16 is strong enough to rated for SIL (i.e.: you have a 1-in-65535 odds that a corrupt packet has a valid CRC)

For a long time one could NOT use "networks" for safety work. Now it has been proven safe - as long as the slave have a kind of fail-safe behavior when the master goes offline. So the master needs to actively stobe all slaves to keep the protected machine operating - if the Master stops or the network fails, the machine shuts down. So I think your second problem is that few (if any) Modbus/RTU slaves will have a robust enough "fail-safe" to guarentee such a shutdown.

ProfiSafe and CIPSafe all include custom protocol features to make them SIL ratable - no doubt a ModbusSafe could be defined and SIL rated, but it wouldn't be traditional Modbus/RTU as we know it today.

- LynnL,