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Anonymous Poster #1

Voltage Collides for DCS Signal

02/07/2017 4:53 AM

Dear All,

Installed H2S analyzer sending 4-20 mA signal to Honeywell DCS and DCS also sending 24 volt to the analyzer to get the readings. Due to both 24 volts colliding the analyzer is not able to communicate with DCS. anybody can provide the solution to solve this issue? Much appreciated to share your thoughts.

Thanks.

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#1

Re: Voltage collides for DCS signal

02/07/2017 8:12 AM

<...anybody can provide the solution to solve this issue?...>

Yes, indeed: <...anybody...> with the appropriate training and experience, sight of the cable(s) and terminations at both ends of each cable, sight of the "for construction" termination drawings for both ends and sight of the manufacturer's technical information for the equipment at both ends will be able to do this.

There is insufficient information in the original posting to determine the problem, which cannot be seen from here.

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#2

Re: Voltage collides for DCS signal

02/07/2017 8:26 AM

Are you sure that instruments has a communication match? Read I/O specs on manuals or instrument spec sheet. What's the label said about it?

This question tells us your not qualified to trouble shoot instrumentation system. Just keep out of it. It might get worst.

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#3

Re: Voltage collides for DCS signal

02/07/2017 8:36 AM

Can you explain what the phrase "Due to both 24 volts colliding" means? I've spent the last 25 years as a control engineer and cannot say I've ever encountered a collision of voltages. Maybe you are missing a common connection point?

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#4
In reply to #3

Re: Voltage collides for DCS signal

02/07/2017 9:00 AM

Quite. It's just wired-up incorrectly.

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#5

Re: Voltage collides for DCS signal

02/07/2017 9:06 AM

Reading (heavily) between the lines: It sounds like you have a self powered analyzer supplying the 24V to drive the 4-20 ma loop and the DCS is set as loop powered, so that it is trying to supply the 24V to drive the 4-20 ma loop. Is that correct?

If that is the case, set the DCS to 4-wire/field powered.

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#6
In reply to #5

Re: Voltage collides for DCS signal

02/07/2017 1:34 PM

Yes that would be my guess and answer too.

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#8
In reply to #5

Re: Voltage collides for DCS signal

02/08/2017 9:01 AM

Yes - you probably have nailed the issue. First thing I do when I get a drawing of a signal panel supplied by the customer, not built by us here at work, is check the analog signal circuits to the PLC I/O cards. That mistake occurs on at least 50% of the jobs like this I review. I've even had three different control engineers with degrees argue that I am wrong, until I rewire and show them that now the circuit works. Biggest problem is when the 4-20ma instruments are a mixture of self powered and loop powered. Got to pay attention here control engineers.

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#7

Re: Voltage collides for DCS signal

02/08/2017 5:40 AM

The cheap and cheerful way I do such things is to route each + voltage via a diode and connect the other side to each other. Make sure that the diodes are connected as shown and not reversed, also, that they can handle the current you want to pass....

Thats stops any feedback from one power supply to the other. Something like this for example.

If you experience any noise, then connect the 24 Volt out (top connection), to say a 1MFD cap to ground (Minus!)

There are better ways to do this, but none simpler I feel.....

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#9

Re: Voltage collides for DCS signal

02/08/2017 2:18 PM

Not just anyone, but someone in particular can solve your problem for you. You could solve this problem by (1) reading the OEM manual, and especially reviewing the part about signal outputs from the analyzer/transmitter. Answer such questions as: (a) is this a loop-powered device?, (b) does this output want to power its own 4-20mA loop?, (c) why is the DCS "colliding" 24 VDC with XMTR? (d) what is the source output resistance at the XMTR, and what is the sink input resistance at the DCS?

(2) Once you have read the manual and are thoroughly confused, you can call, text chat, or email the supplier and get contact information back to the OEM. They can elucidate precisely what is wrong with your incorrect setup.

(3) Throw the entire XMTR out and start over if you still cannot make it function. This is even if you put a loop monitor on the XMTR and see the correct loop current being sent.

(4) Are you sure this is an analog output loop, or is it a digital signal output? How sure are you?

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