How to prevent using a current loop isolator?

Theboss

Member
Join Date
Jun 2010
Location
Durban
Posts
159
hi guys, im having difficulty with this issue, in trying to understand why this happens. ok let me begin....

im using a 4 wire instrument, current signal is being connected back to the plc analog i/p card. but no mA signal at the plc, so this obviously forces me to use a loop isolator.

so why does this happen? and how do i refrain from using a loop isolator, pls help
 
Hi!
Please tell PLC card type. Sure it can do 4-wire transmitters?

If you connect the mA meter to the instrument, what do you get?

Have you tried to connect the minus-leader from current loop to the OV reference of the AI card? you could start using a resistor in case you are allergic to blue smoke;).

Kalle
 
hi guys, im having difficulty with this issue, in trying to understand why this happens. ok let me begin....

im using a 4 wire instrument, current signal is being connected back to the plc analog i/p card. but no mA signal at the plc, so this obviously forces me to use a loop isolator.

so why does this happen? and how do i refrain from using a loop isolator, pls help

I have a car, and it won't start. Please tell me what to do to fix it?

Good information results in good answers.

Stu....
 
its a siemens s7 400 plc, configured for a 4wire instrument. the instrument works fine tested mA o/p and tested input to the card with current source, so there nothing wrong with the input channel and the instrument.
 
its a siemens s7 400 plc, configured for a 4wire instrument. the instrument works fine tested mA o/p and tested input to the card with current source, so there nothing wrong with the input channel and the instrument.

I have a Ford car that wont' start. Please tell me how to fix it.

Come on!

Exact Model of the Analog Input Card.

Exact Model of 4 wire Analog Device. How about a Link to the Manual.

Wiring Diagram as to how you have them wired to each other.


Stu....
 
its simple i just matched the polarities from the instrument to the card itself. the analog card(AI8*12BIT 6ES7 331-7K02-0AB0. the instrument is route 2700 uses a loadcells and 4-20mA O/P to analog card
 
hi. im not with you on this, because its a 4-20mA that im transmitting to the plc, where is the 0V or do u mean the -ve lead of the 4-20mA?

AND
can u be kind enough to elaborate more about this resistor, and what does it do, and how i go about connecting it, just out of interest.
 
You have quoted the analog card as 6ES7 331-7K02-0AB0 (an S7-300 card?) but stated you are using S7-400 plc so am I correct in assuming the analog card is remote to the plc main rack? Is that part# correct?

You have verified that the 4-20mA from the instrument is oparational.

You have verified with a current sim. that the AI card channel is operational (assuming via STEP 7 software looking at the PIW value).

Are you using the signal isolator now and sure it's correctly configured?

On the side of the AI card will be 4 module blocks. What letter is at the arrow?

What PIW value are you seeing in the STEP 7 software? 32,768?

What signal isolator are you using?

What cable distances are between instrument, signal isolator and AI card?

Are all connections correct and secure?

.
 
@silva.foxxx - that is the correct serial number for the card, sorry i was mistaken, its for a s7-300 plc.

I think u are mistaken, the loop works fine only when connected up to the loop isolator.
all connections are correct and secure...
 
is this caused because the instrument earth and plc earth are at different potentials? and how do u prevent this?
 
@Theboss... it really would be appreciated if you would help us help you by answering the questions posted by the various posters. :nodi:

There are two sets of people on this thread: one who has the problem and another who doesn't, that set can't see what you can so have to imagine what you have.

Again, one set has the problem and needs it resolving whereas the other set can give up on the issue once they become bored with not recieving the responses to their help. 🤾

;)

What is the part number of your signal isolator?

Is the signal isolator powered?

Is the AI card correctly set for 2-wire mA input?

The squares on the side of the AI card... what are they set to? A, B, C, D...?

We await (no doubt) a partial response :whistle:
 

Similar Topics

We am planning to build an electrical enclosure on the manufacturing line. The only concern is to prevent dust from getting collected inside the...
Replies
3
Views
2,037
Hi All, I am using a Micrologix 1100 to load various Jobs on a cognex camera. The way it works is that I send a command to load the file and I...
Replies
1
Views
1,516
We have L72 controller of Rockwell make with 4 nos. analog inputs from pressure transmitters. Due to break in cable connection OR transmitter...
Replies
7
Views
2,026
Hello, We have 5 floating licenses for Studio 5000. Despite training people to disconnect from the PLC and close the software, we still have...
Replies
6
Views
2,678
Hello, A few years back I was having a SLC 5/05 hard faulting, losing program, and requiring a download to get going again. The processor was...
Replies
7
Views
2,883
Back
Top Bottom