It would be best if you provided more information on this. For example, what type of plant or equipment are these installed in? What functions do they currently perform? What brand and model? What industry are these in? Etc.
You cannot expect a good technical answer if you don't provide sufficient information. This is a common fault of posters to provide minimal information and expect the people on the mail list to some how provide a good technical answer. In this case, several poster have tried to provide an answer but the response has been little or no additional information(more like a riddle than a question). People on this mail list cannot read your mind.
If you want a good technical answer to your question, provide adequate information in your question.
Bill Mostia
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William(Bill) L. Mostia, Jr. P.E.
Partner
exida.com
Worldwide Excellence in Dependable Automation [email protected](b) [email protected](h) www.exida.com 281-334-3169
These opinions are my own and are offered on the basis of Caveat Emptor.