Emergency Stop Circuit Scenario

sparkie · Mar 20, 2017

g.mccormick said:
There are several safety PLCs that support safety communications links.

Beckhoff has TwinSafe that runs over EtherCat.

AB, simens, etc all have theres. I have not heard of any safety over Modbus safety controllers though.

I am in no way an expert or even a novice on this subject, but I believe that the one thing that all "safety comms" have in common is that they are built on deterministic protocols.

The RS485 spec is pretty old and the com speed is very slow compared to modern communications, so I wouldn't really want to use it for safety.

mk42 · Mar 21, 2017

sparkie said:
The RS485 spec is pretty old and the com speed is very slow compared to modern communications, so I wouldn't really want to use it for safety.

You can do safety over Profibus (which uses RS485) with no problem. Oftentimes safety systems only communicate every 50 or 100ms. It's less about the speed of the network, and more about the devices guaranteeing safe behavior even if the communication stops.

g.mccormick is right; it's all about the determinism.

g.mccormick · Mar 21, 2017

I would think that modbus rtu over 485 could be more deterministic than tcp due to potential queuing in switches etc.

Emergency Stop Circuit Scenario

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