Dear All

An instrumentation issue -

A machine is utilising three 4 to 20ma pressure transmitters for control
purposes. They are all on different circuits which have their own supplies.

For certain reasons it has become desirble to have just one transmitter
perform the function of all three (same pressure being measured) - how can
this be acheived? A spliter has been tried and whilst this did indeed
generate three signals from one instrument (the original and two additional
outputs) it could not be connected into the existing installation - for each
circuit a loop was required and it's own voltage was present, whereas the
splitter generated its own power etc.

Is there a method or device that can do this?

Any advice is greatly apprecited.

Thanks in advance

Steve

Steve & Lizzie wrote:
Dear All

An instrumentation issue -

A machine is utilising three 4 to 20ma pressure transmitters for control
purposes. They are all on different circuits which have their own supplies.

For certain reasons it has become desirble to have just one transmitter
perform the function of all three (same pressure being measured) - how
can this be acheived? A spliter has been tried and whilst this did
indeed generate three signals from one instrument (the original and two
additional outputs) it could not be connected into the existing
installation - for each circuit a loop was required and it's own voltage
was present, whereas the splitter generated its own power etc.

Is there a method or device that can do this?

Any advice is greatly apprecited.

Thanks in advance

Steve

It sounds easy in principal, and damned useful. I expect there is, but
I'm not tuned into the industrial process control market.

Try posting the question on sci.engr.control. Someone there ought to
know if there are isolated repeaters out there.

Alternately, call up GE, and Siemens, and Honeywell, and the other usual
suspects, and see if they can sell you something or direct your
attention to someone who can.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

On Mon, 18 Feb 2008 18:38:12 GMT, "Steve & Lizzie"
wrote:

Dear All

An instrumentation issue -

A machine is utilising three 4 to 20ma pressure transmitters for control
purposes. They are all on different circuits which have their own supplies.

For certain reasons it has become desirble to have just one transmitter
perform the function of all three (same pressure being measured) - how can
this be acheived? A spliter has been tried and whilst this did indeed
generate three signals from one instrument (the original and two additional
outputs) it could not be connected into the existing installation - for each
circuit a loop was required and it's own voltage was present, whereas the
splitter generated its own power etc.

Is there a method or device that can do this?

Any advice is greatly apprecited.

Thanks in advance

Steve

You need two "loop isolators." Google that.

John

On Mon, 18 Feb 2008 18:38:12 GMT, the renowned "Steve & Lizzie"
wrote:

Dear All

An instrumentation issue -

A machine is utilising three 4 to 20ma pressure transmitters for control
purposes. They are all on different circuits which have their own supplies.

For certain reasons it has become desirble to have just one transmitter
perform the function of all three (same pressure being measured) - how can
this be acheived? A spliter has been tried and whilst this did indeed
generate three signals from one instrument (the original and two additional
outputs) it could not be connected into the existing installation - for each
circuit a loop was required and it's own voltage was present, whereas the
splitter generated its own power etc.

Is there a method or device that can do this?

Any advice is greatly apprecited.

Thanks in advance

Steve

As John says, isolators will work, but make sure you don't try to use
the loop-powered type as they will probably have too much voltage
drop. You would series the inputs of the two isolators with one load.

In some cases, you can series all three loads and use a single supply,
but you'd have to watch the voltage drop, voltage limit, compliance
and grounding issues.



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On Mon, 18 Feb 2008 18:38:12 GMT, "Steve & Lizzie"
wrote:

Dear All

An instrumentation issue -

A machine is utilising three 4 to 20ma pressure transmitters for control
purposes. They are all on different circuits which have their own supplies.

For certain reasons it has become desirble to have just one transmitter
perform the function of all three (same pressure being measured) - how can
this be acheived? A spliter has been tried and whilst this did indeed
generate three signals from one instrument (the original and two additional
outputs) it could not be connected into the existing installation - for each
circuit a loop was required and it's own voltage was present, whereas the
splitter generated its own power etc.

Is there a method or device that can do this?

Any advice is greatly apprecited.

Thanks in advance

Steve

I'm guessing that you want the three existing outputs from a single
sensor? If so, decode the one sensor and fan out the decoded outputs.

Tom

Tom2000 wrote:
On Mon, 18 Feb 2008 18:38:12 GMT, "Steve & Lizzie"
wrote:

Dear All

An instrumentation issue -

A machine is utilising three 4 to 20ma pressure transmitters for control
purposes. They are all on different circuits which have their own supplies.

For certain reasons it has become desirble to have just one transmitter
perform the function of all three (same pressure being measured) - how can
this be acheived? A spliter has been tried and whilst this did indeed
generate three signals from one instrument (the original and two additional
outputs) it could not be connected into the existing installation - for each
circuit a loop was required and it's own voltage was present, whereas the
splitter generated its own power etc.

Is there a method or device that can do this?

Any advice is greatly apprecited.

Thanks in advance

Steve

I'm guessing that you want the three existing outputs from a single
sensor? If so, decode the one sensor and fan out the decoded outputs.

Tom


I believe Tom is correct in what the OP wants.

So this should help:

http://focus.ti.com/lit/ds/symlink/xtr117.pdf

Each Iloop connection can have its owm driver.

donald

or you could buy a small shoe box plc with 1 4-20ma in and 3 out
"Steve & Lizzie" wrote in message
news
Dear All

An instrumentation issue -

A machine is utilising three 4 to 20ma pressure transmitters for control
purposes. They are all on different circuits which have their own
supplies.

For certain reasons it has become desirble to have just one transmitter
perform the function of all three (same pressure being measured) - how can
this be acheived? A spliter has been tried and whilst this did indeed
generate three signals from one instrument (the original and two
additional outputs) it could not be connected into the existing
installation - for each circuit a loop was required and it's own voltage
was present, whereas the splitter generated its own power etc.

Is there a method or device that can do this?

Any advice is greatly apprecited.

Thanks in advance

Steve