Previous in Forum: Vintage (?) Generator Tester and Regulator   Next in Forum: The Gradual Demise of the CR4 Instrumentation Forum
Close
Close
Close
8 comments
Participant

Join Date: Feb 2017
Posts: 1

Three Signals

11/06/2019 8:13 AM

we use in instrumentation 1 pair cable for positioner..how travle our 3 signals (4 to 20ma,24vdc and feedback control valve?

Login to Reply
Interested in this discussion?
You can "subscribe" to this discussion to be notified of new comments.
Click on the Subscribe menu at the top of the page.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 29966
Good Answers: 809
#1

Re: 3 signals

11/06/2019 8:43 AM
__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Login to Reply
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 29966
Good Answers: 809
#2

Re: Three Signals

11/06/2019 12:14 PM

...or an additional multicore cable.

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Login to Reply
Guru
Hobbies - DIY Welding - Wannabeabettawelda

Join Date: May 2007
Location: Annapolis, Maryland
Posts: 6919
Good Answers: 401
#3

Re: Three Signals

11/06/2019 1:41 PM

If you want to use a single pair cable for this application, you will need to transmit your 24VDC as a baseband signal on the cable, then modulate the 4-20mA and feedback signals onto the cable and de-modulate those signals at the remote end. I'd recommend wave division multiplexing. All of this will be a lot more expensive than simply running a 3-pair cable like PWSlack said in #2.

Login to Reply
Guru

Join Date: Jul 2010
Posts: 664
Good Answers: 175
#4

Re: Three Signals

11/06/2019 9:03 PM

A HART analog output could drive the positioner (95% of positioners are 2 wire that derive their power from the 4-20mA control signal) AND provide position feedback in a HART enabled system. I assume Profibus PA, mentioned above, can do the same.

Screen shots from Fisher Fieldvue HART positioner manual, showing two wire 4-20mA HART signal supports reading the "travel" variable that equates to position.

Login to Reply Score 1 for Good Answer
Guru

Join Date: Jul 2016
Posts: 1000
Good Answers: 67
#5
In reply to #4

Re: Three Signals

11/06/2019 10:48 PM

You really have only two functions over the wires.

The 24V is not power for the positioner, it moves a small pneumatic pilot valve which then moves the valve trim pneumatically. The hardware interprets 20mA as a command for one extreme of trim movement, 4 mA the opposite. The voltage drop at currents above 3mA powers the HART signal processing hardware (modem), providing 2 way communication to the valve positioner. Open commands and position feedback, alarms, maintenance mode, calibration routines can be sent across the HART signal pair, though most engineers prefer the 4-20mA signal directly operate the valve position. HART modem signals in heavy industrial environments can have reliability problems if raceway design is sub par.

Login to Reply
Active Contributor

Join Date: May 2013
Posts: 23
#6

Re: Three Signals

11/07/2019 9:00 AM

I think he is asking about an existing process... like he does not understand how 3 different signals can be transmitted across just 2 wires. But I could be wrong.

Login to Reply
Guru

Join Date: Jul 2016
Posts: 1000
Good Answers: 67
#7
In reply to #6

Re: Three Signals

11/07/2019 4:51 PM

Yes, there are very few possible options that fit the limited information provided. HART is the more popular solution that fits with only two wires connected. I think Profibus might require a power source in addition to the communication media, but that technology could have advanced since I’ve checked in on it.

Login to Reply
Guru

Join Date: Jul 2010
Posts: 664
Good Answers: 175
#8

Re: Three Signals

11/08/2019 9:35 PM

Another possibility:

Foundation Fieldbus (FF) is like HART or Profibus PA - DC power is carried on the same two conductors in twisted shielded pair cable as the signal. Multiple variables are available through FF just like HART or Profibus PA.

Login to Reply Score 1 for Good Answer
Login to Reply 8 comments
Interested in this discussion?
You can "subscribe" to this discussion to be notified of new comments.
Click on the Subscribe menu at the top of the page.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

Brave Sir Robin (1); dougefresh (1); Iris (2); PWSlack (2); rwilliams (2)

Previous in Forum: Vintage (?) Generator Tester and Regulator   Next in Forum: The Gradual Demise of the CR4 Instrumentation Forum

Advertisement