Thread Starter



I would like to know whether the following thing is possible or not.

Let us take a simple case. I have a PLC and DCS. And my PLC wants to communicate with DCS through hardwired communication.

For example, one pressure transmitter is there in the field. The signal is going directly to PLC as analog input(4 - 20 MADC, 24 DC). The transmitter is powered by PLC (24 VDC power supply). The same pressure transmitter signal is required in DCS also. The PLC gives analog output of the pressure transmitter signal which will be taken into DCS as analog input(4-20 MADC, 24 VDC).

My question is when PLC gives as an analog output 4-20MA DC, 24 V DC, whether DCS can power the loop and take the signal as analog input and at the same time PLC will not loop the power. In otherwords, whether it is possible to have analog output from PLC without powering the loop and DCS powers the loop(PLC-DCS loop and not field-PLC /DCS loop). This is similar, not exactly, I.E., potential free contact output in the case of digital output.

In short, instead of source device (PLC) powering the output loop (4-20MA DC,24 VDC) the destination device (DCS) powers the loop and takes the signal as analog input (4-20MA DC,24 V DC).


Anand S


Kevin Totherow

I don't think that it is possible to have a 4-20mA output from an electronic device like a PLC (or DCS) powered from the input device. However, neither is it necessary.

First, you can install the same transmitter input to both the PLC and DCS with the current loop although I would probably install a 4-20/4-20 loop isolator in the current loop to separate PLC from DCS. The transmitter current output can go into both devices without using a current output in the PLC.

Second, every DCS that I have used allows external powered 4-20mA loops for four wire transmitters so if you wanted to use the output from the PLC let the PLC power the 4-20mA and just let the DCS read it.

Kevin Totherow

Blunier, Mark

> LOOP (4-20MA

First, don't use all capital letters. They are harder to read, than
lower case. It's a good get people to ignore your post, and then you
won't get an answer.

You can power the loop from the PLC, from the DCS, or use External power.
Note some PLCs don't have there own 24V instrument power, and have to
use an external source, so it really is externally powered anyway.

Any opinions expressed in this message may not necesarily be those of the
Dear Anand

I believe that analog outputs are generally powered by the PLC. So the
possibility is remote unless you have a special output module.You could use
a signal isolator in between.

In case you want to provide power from the DCS and use the same loop to
connect to the analog input of the PLC, then

1. the DCS should be in 2 wire mode where it powers the transmitter and also
reads the input (which is similar to your present PLC loop). You may have to
either change the configuration in your DCS module or change jumpers.

2. The DCS+ connects to the Transmitter +. The transmitter - connects to the
PLC + and plc - connects to DCS -

3. The PLC is in non-two wire mode where it only accepts the 4 to 20 mA
input. Jumpers or configuration may require change.

Please refer to your DCS and PLC manuals before proceeding with any change.

In case communication is possible between your PLC and DCS, then you could
access the Analog Data from the communication loop.


Bruce Durdle

If you are interconnecting a PLC and a DCS, be very careful about how you deal with the instrument earth connections. You may find you have accidentally introduced ground loops etc.

My preferred practice, if trying to use one transmitter to feed an analogue signal into two different devices, is to make use of the fact that most DCS analogue inputs convert 4 - 20 mA into 1 - 5 V using a 250 ohm resistor. Take a parallel connection from the DCS to the PLC, and use an isolated voltage input at the PLC.

Just be aware that not all DCS inputs are 1 - 5 V - one manufacturer had 0.2 - 1 V at one stage. But 1 - 5 is pretty well standard.

Anand S,

hi it is possible . I am working on this kind of equipment at presently. Many of the reputed suppliers do not agree to supply their equipmnet program controls on the DCS , rather they will supply as a standalone PLC systems.
1. The easiest way is to do you configure your pressure transmitter on the dcs , and connect to the DCS. The field transmitter is powered from the DCS.
2. The DCS will have more provisions and more user friendly rather than PLCs.
3.So you can decide from the DCS output Wheter you want to pump 1 to 5 volts signal, or 4 - 20 millamps source signal, and 4-20 millamps sink signal to understand your PLC.
4. Many plcs input card do not have provision to supply 24 V DC. They may put external 24 V power supply and then connect to the PLC input in series. This is the correct think about 4 - 20 millamps souce siganl inject to PLC.
5. Regarding Isolators . you are incresing the hard ware you have ore problems devis failure / calibrations etc.
6.The PLC incoming supply ( 230 V or 440 v ) or some transformer feeder and DCS Tranformer feeder / power supply must be connected to same PLANT Earth , to avoid any loop interference and earthing problems.
7.if you follow above , you can easily implement