3-Wire Proximity Switches in 2-Wire DCS System

Nothing like some details....

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Please post the wiring diagram for your prox and an DI input for the DCS.

It is simply two wire circuit coming from Digital input module. The only confusion is that for 2 wire proximity, one of the wire from DCS gives the return path for positive 24 Vdc but in 3-wire how will we take the return path if there are three wires in the field and the instrument is passive i.e. its -ve should be from DCS power supply.

Best Regards,

very good answer sir.

So, it can't be done without changing something.

So change something.

I hate to sound like a broken record, but...

Nothing like some details....

As per my knowledge, 2 wire DI proximity switches are as per NAMUR standard which need a Input barrier (with driver circuit in it) at the DCS panel side which supply power to switch and receive signal on 2 wire and makes it intrinisically safe.

In 3 wire configuration no barrier is needed and power is directly supplied to switch with out any barrier inbetween, and using 3 wire can be hooked up to get a DI signal. But this doesnt make it Intrinsically safe i guess

Inyour case since your DCS is having 2 wire configuration, may be you need to supply power to proximity switches from other local source and use two wires from DI switches to hookup to DCS for sensing.

I have never tried this configuration though, comments from members required

hello kindly see the attached an example we had for a 3 wire prox sensor connected to the PLC may help

Dear Guest,

Where is your attachment?

Best Regards,

hello

the attachment is not appear to me , if you send me your mail I will forward it to you

Dear AA,

I also came accross with the same problem when i was working with these proxy 3 wire switches.

First question that you should ask your self is that whether your Digital input card is VFC (Voltage free contact) type or it can also accept PNP/NPN devices. Normally we use VFC type cards. For these cards we can not connect proxy type switches either 2 wire or three wire. what we need is PNP/NPN Card that converts it to VFC type contact to be connected to DCS or we use switching reay card which also converts it to VFC. I m using both in my DCS. What you need to do on urgent basis is that you send the specs of proxy switch to various vendors (i contacted weidmuller) and ask them to provide you the switching relay for these probes.

If you need further explanation and details of circuit diagrams then send me your email.