Multiplexer In PLC

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jeetu Associate Join Date: Sep 2011 Location: India Posts: 28	Multiplexer In PLC 10/14/2011 2:29 PM
	hi everyone, in my plant we need to provide trip for the pump from the high value of bearing/winding temperature of motor. All interlocks are in plc and we also require indication in dcs. Earlier we required only indication in dcs but for safe engineering practise we are putting trip also. Now my problem is, i suggested usage of multiplexer for temperature signals but my senior told me that as interlocks are in plc so we have to use separate temperature transmitter for each temperature element (12 winding temperature element) , my question is, cant we use mux output to plc ? __________________ I am man of simple taste.
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Shanivas Active Contributor Join Date: Sep 2011 Posts: 11	#1 Re: Multiplexer In PLC 10/14/2011 3:16 PM
	It is always advisable to use hard wire signals for safety systems. Your senior has said it right....Trips and interlocks falls under IPS safeguarding system and it has its own criticality consideration on plant safety. Even you can go for voting systems in case if the trip is so crucial. I would advise to use a serial/modbus through multiplex for the indication to DCS. The simple understanding of this concept is: Even if the multiplexed indication to the DCS fails, the pump would get tripped which is activated through hard wired signal to the PLC. So we ensure the safety requirement is given first preference. You have to go through the Safety Instrumented system (SIS) philosophy studies and SIL studies to understand it to a larger extent. Cheers.,
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tonykuphaldt Power-User Join Date: Sep 2011 Location: Bellingham, WA Posts: 191 Good Answers: 46	#2 Re: Multiplexer In PLC 10/20/2011 9:12 AM
	Multiplexing all temperature signals to the PLC would create a single point of failure in the interlock system: if the mux fails, the PLC loses ALL temperature indications, which might lead to a "fail dangerous" condition. Your senior has the right philosophy. __________________ They call me "lightning" when wielding a hammer, because I never strike twice in the same place
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