I received this email from a client:

"We yet to receive both instrument and IO list prior to our DCS system configuration. For the IO list, please mention the method of power supply. And please use the word as powered by DCS side or powered by field, because the wet/dry contact can have different meanings for different sides."

My understanding is that he could referring to AO - to differentiate between 2-wire (DCS powered) or 4-wire (field powered). But why is he talking about wet/dry contact? I thought AO does not involve contacts? TIA.

 

There must have been times in our career when an Analogue doesn't work because 2 power supplies are providing loop power, or even better when neither 'end' uses their power supply with the best of intentions that the other end is [supplying power].
The same also applies to digital signals, so better to make clear who is doing what in the early stages of any contract.

Even the suggestion of an analogue '4-wire' loop can lead to confusion. From a designer's point of view it is much easier to power each loop from the DCS, as the analogue card must be the last in the loop to have it's i/p close to or at ground/-ve. Many old Analogue cards wouldn't accept anything else, so it was convenient to send out +ve supply {around 24vDC), 'loop' round one or occasionally two field instruments then return signal to +ve card i/p. But this is still only 2 wire.
Digital signals are likewise explained in recent article:
https://control.com/technical-articles/sourcing-and-sinking-for-control-system-modules-explained/

 

Even though he is talking about wet/dry contacts, he does not want you to have it in your IO list as it creates confusion. Only have “powered by DCS” or “powered by field”. This could apply to Analog or digital, inputs or outputs.

 

What Supplies Voltage to your Pressure Switches, Limit Switches, Proximity Switches, Relay Contacts, Motor Starter Contacts.??? Most DCS Systems can power those with a Card that has 24VDC on it, 48VDC on it. That is the DCS powering the contacts.(Wetting them). If you use Motor Starter 120VAC powering the Contact, that is the Field wetting the Voltage and the DCS sinking it. Takes a different card. Hope this Helps. Have a Good day..