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Old January 16th, 2004, 10:06 PM   #1
gb_sv
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Isolation of input and output modules of PLC

Hi,

I don't have hands on experience on PLC hence i would like to have few clarifications on PLC specifications as indicated below.

One of the client PLC specification indicates the following under Input and Output modules description:

a) All input output modules shall have optioncal isolation from logic system(1500 VDC)

b) The I/O field channels shall be elctrically isolated from the I/O module electronics.

I would like to know what is the difference between these point a & b. Also, why it's required.Please clarify.

c) Also i would like to know whether if a PLC specification says relay output then the relay is in-built in the Output card itself or it's external relay.

d) In-built relays are of always Solid-State type?


Thanks in advance for valuable reply.

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Old January 16th, 2004, 10:38 PM   #2
Eric Nelson
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Re: Isolation of input and output modules of PLC

Quote:
Originally posted by gb_sv
a) All input output modules shall have optioncal isolation from logic system(1500 VDC)

b) The I/O field channels shall be elctrically isolated from the I/O module electronics.

I would like to know what is the difference between these point a & b. Also, why it's required.Please clarify.
I think 'a' just clarifies 'b' by giving a type (optical) and rating (1500VDC) of the isolation. 'b' may additionally mean that the I/O field wiring cannot share the same power source as the I/O module...

Quote:
Originally posted by gb_sv
c) Also i would like to know whether if a PLC specification says relay output then the relay is in-built in the Output card itself or it's external relay.
If you buy a 'relay output' module, the relays are built into the card and may or may not be field replaceable.

Quote:
Originally posted by gb_sv
d) In-built relays are of always Solid-State type?
No. Solid-state outputs are usually called 'transistor' (DC) or 'triac' (AC) outputs.

beerchug

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Old January 17th, 2004, 12:01 AM   #3
ArikBY
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You wrote

b) The I/O field channels shall be elctrically isolated from the I/O module electronics.

The way I use to do it is to put small realy/SSR on the input the signal from the field swtich the relay/SSR who switch the input, in that way you have TOTAL ISOLATION. Same for the output.For the output you can use PLC transistor outputs card which is cheaper.
You can find very small relay 5mm wide instead your terminal block.
AB or Phoenix Contact have this relays.
I work on project with 2 PLCs Omron asd Siemens and Thosiba robot.
I wanted totally isolation between them to take off the responsibility for any damage from me (I made the machine with the Omron and I responsible to handshake).
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Old January 17th, 2004, 07:43 AM   #4
Steve Bailey
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The important thing to remember here is that my interpretation of the specs (or Eric's or Arik's) is nowhere near as important as the client's interpretation. Anything we say here should only be used to help you ask better questions of the client. Of course, it may turn out that the actual user of the equipment didn't write the spec and doesn't have any idea what it's asking for.

To me, a) is specifying that input and output modules use optical isolation between the logic side and the field signals. That is typical design practice and has been for years. This should not be any problem no matter which PLC you decide to use.

b) could be specifying what we usually call 'interposing relays'. You should get clarification from the client on this one, because the answer could significantly affect your cost and the amount of cabinet space you need.
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