Hello all,

I have to know if MODBUS is suitable for being used in safety loops i.e. emergency shutdown system's loop? If no, kindly mention the standard that forbids using modbus for safety loop?

 

I doubt you'll find anyone willing to certify Modbus, plus your insurance company won't like uncertified solutions, meaning you might not get payments if the shutdown failed or acted too slowly.

I doubt the simple CRC16 is strong enough to insure valid data transfers - 1 in 65K bit errors will be missed.

However, MOST things can be used for safety if you have an aggressive heartbeat & fail-safe behavior, which means I/O and control forces the emergency shutdown if (for example) you miss a Modbus message by a few seconds. Literally, the protocol needs to bang away & repeat 'keep running' & the instant that stops, emergency shutdown occurs.

I know Rockwell Automation has DeviceNet and even wireless safety system which ARE certified - obviously with wireless you might suffer more false shutdowns, but it really related to the cost of the shutdown, plus any damage or loss of life/limb caused by failing to shutdown fast enough.

 

Aman,

You should refer to IEC 61508 for safety related communication. Following paper may help you to get idea for safety network requirements.

http://www.isa.org/Content/Microsit...s/Development_of_Safety_Networks_IEC61508.pdf

Hope this helps

Ikhtiander

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The the undefined modes of failure and rates failure of any distributed system, not just MODBUS, prohibit their exclusive use in safety loops.

It is not just a matter of "if it works", where safety is involved, it has to work.

where hazardous life-endangering systems are involved, triple redundancy may not be enough.