Transmitter in loop with PLC and Display

Ken Roach · Dec 9, 2013

New Forum member ianavina posted a question that I am moving to this new thread and edited for clarity.

[I am] using a Micrologix 1100 with expansion module.

I am using an ultrasonic transmitter to tell me the fluid level in a tank and depending on that [level], I have numerous pumps turn. Using SCP, LEQ and GEQ [instructions], I have all that working.

My problem is that my boss also wants the tank level in gallons displayed on a little LED readout. I have a Red Lion Cub 5P process meter.

I tried just putting it in the series loop with the PLC and transmitter, but no luck. Then I used a copy command to an [analog] output to run that way, but not luck.

My transmitter is 2 wire 4-20 ma and my readout is also 4-20 ma but can run off 0-10vdc if need be.

Mickey · Dec 9, 2013

Post the model number or link to the manual of your transmitter.

Some transmitters have limitations on how much load they can drive thru., check the specification.

Check and double check your polarity.

Check and double check your meter input jumper and programming settings.

ianavina · Dec 9, 2013

A vegason61

Mickey · Dec 9, 2013

ianavina said:
A vegason61

With 24 vdc supply you should be able to drive thru about 500 ohms. It should work. See page 45 in manual link below.

The Red Lion meter appears to have only 10 ohms load when configured for a current input.

Have you checked the current with a meter, with all connected?

http://www.google.com/url?sa=t&rct=...MTJWvg1B77rbuIHB0LHDVjg&bvm=bv.57799294,d.cGU

danw · Dec 9, 2013

Ken Roach said:
I tried just putting it in the series loop with the PLC and transmitter, but no luck.

What does "no luck" mean?
What exactly is the problem?

ianavina · Dec 9, 2013

By no luck I mean that the vegason61 and the red lion cub5p will have the correct reading but the Plc will show a a number below my input min, but as soon as I remove the red lion my Plc scp starts reading the correct level from the vegason, it's like I can only have one or the other

danw · Dec 9, 2013

Red Lion Cub 5P is 4 wire device, its input is not isolated from its DC power

You're looking at a classic ground loop problem.

Two of three devices work with each other, but all three can't co-exist because of the ground loop.

You may be able to float the DC power and get it to work.

Another approach is to use a loop powered indicator.

A loop powered meter drops a couple volts in the loop but it eliminates the potential for ground loops.
http://www.predig.com/PD683-8/index.php

ianavina · Dec 9, 2013

Yeah I saw that today and thought it might the problem but my inexperience shows lol so then maybe go with a different readout like a red lion cub5?

danw · Dec 10, 2013

It pays to read the instruction manual. Red Lion's Cub 5 has exactly the same warning.

Repeating the same action and expecting different results is the definition of insanity. Be my guest.

ianavina · Dec 10, 2013

Oh man, well thanks for y'all's help, I'll look into that predig link you recommended and try that then

jamesau · Dec 10, 2013

Consider using a loop isolator or loop splitter. Many companies offer these. Here's a sampling: http://www.acromag.com/catalog/35

ianavina · Dec 10, 2013

hey thanks alot i think the splitter might just solve the problem, i read the description and by the splitting and isolating the 4-20 the red lion should no interfere with the plc and vice versa,

1 more question guys whats the difference between a 2 wire and 4 wire? i thought i was dealing with a 2 wire but i was told earlier in the thread i had a 4 wire red lion so now im confused lol

danw · Dec 10, 2013

The Red Lion indicators are 4 wire.
2 wires for the signal
2 wires for power
total is 4 wires.

A loop powered device has only a (+) terminal and a (-) terminal: a total of 2 wires.

Power for a 2 wire device is supplied on same wires as the signal is using.

So a loop powered indicator is a 2 wire device.

ianavina · Dec 10, 2013

Cool ! I was googling the difference but your explanation is the best thank you

Mickey · Dec 10, 2013

Just to add to the confusion there is also a three wire, see picture.

Transmitter in loop with PLC and Display

Ken Roach

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