Controllogix analog output OF8H not reaching 20mA

bit3399

Member
Join Date
Nov 2012
Location
Africa
Posts
16
Hello,

Please i need some help on how to get acccurate values current (4-20mA) at the analog output terminal of controllogix analog output module 1756-OF8H. I need to use PID block later but want to be sure the output module will give me accurate current value from 4-20mA as expected by forcing in the tag monitor window online.

Here is what i have done to date

Slot 0-L61 processor
Slot 1-ENBT ethernet module
Slot 2-IF16H analog input module
Slot 3-OF8H analog output module

Configuration tab of OF8H has
high signal=20mA
low signal =4mA

high engineering=100
low engineering=0
output range=4-20mA

HART check box not checked.

Any time i go online and enter a 100 in the Local:3:O.Ch0Data field in the controller tag monitor window, it gives me 4ma and if i enter 0 it gives me 0.8mA.

It seems to be holding 4mA as highest value. I tried same on all channels with same result.

Please help, its driving me crazy!
 
Can you provide details on your output wiring, the type of load and your method of measurement of the current. I hope you don't have just the meter as the load.
 
Thanks Bernie for the prompt response.

Here is what i have;

A current (0-200mA) meter is connected to the current output channel 0 of the 1756-0F8H analog output module that is terminal 3 & 5 as in below

Terminal 1 - VOUT-0
Terminal 3 - IOUT-0
Terminal 5 - RTN
Terminal 7 - VOUT-1
Terminal 9 - IOUT-1
Terminal 11 - VOUT-2
Terminal 13 - IOUT-2
Terminal 15 - RTN

-There are no load attached aside from the multimeter (current meter) used to measure the output when i force value
 
@Hardwerk918
I have tried this and the highest output i get is 3.9~4mA.

Anytime i use
high engineering=100
low engineering=0

if i enter 100, i get 4mA,

Anytime i use
high engineering=32767
low engineering=0

if i enter 32767, i still get 4mA,
 
There is another tab in the module configuration farther to the right that allows the clamping of each analog output to a minimum and maximum.
 
@gmferg

Are you refering to the limits tab?

I have earlier before put the max=100 and min=100 to be in tandem with the low/high=0/100 engineering value in the configuration tab.
 
Hi Bernie,

I curious as regards what ur response will be based on ur earlier comment that if I was using only meter as my load. That is exactly the situation. I don't have a valve or load available but I believe if I force value to the output, I should get corresponding current value.

Uptill now,my settings are as below

Low EU=0
High EU=100
Low signal=4mA
High Signal=20mA

Low limit=0
High limit=100

If force 0, I get 0.8mA, if I force 100, I get 4mA.

I'm stock!
 
I re-thought my comment regarding the meter. But I was thinking of a minimum resistance load for a voltage output, not a current output. Sorry to mislead you. I have no experience with this card so I don't have any meaningful help.
 
@gmferg

Are you refering to the limits tab?

I have earlier before put the max=100 and min=100 to be in tandem with the low/high=0/100 engineering value in the configuration tab.


If that is not a typo, then it might be why the module is misbehaving...

You should set the hi-limit to same as Engineering Units max. (100), and the lo-limit to Engineering Units min (0).

The hi-limit sets the maximum EU and the low-limit sets the minimum EU that the card will convert.... setting them the same value is just pointless, and I believe the card just stops converting, because it can't do anything.


EDIT: I just tested this, and you cannot set the High Clamp Limit and the Low Clamp Limit to the same value, so I assume your post was a typo.

The only other things I can think of is that the processor is not communicating with the OF8H module....
Do you have any I/O errors (I/O light flashing on the processor) ??
Is the module inhibited ??
Is the controller in RUN or REM RUN mode ??
 
Last edited:
@Daba,

Its a typo pls. You will see from my earlier mail sent, i put
Low limit=0
High limit=100

I really dont know whats missing and why the card wont give out upto 20mA. It stops at 4mA at EU=100
 
@Daba,

Its a typo pls. You will see from my earlier mail sent, i put
Low limit=0
High limit=100

I really dont know whats missing and why the card wont give out upto 20mA. It stops at 4mA at EU=100

I've edited my post - go back and read the additional info I put
 
Thanks Bernie for the prompt response.

Here is what i have;

A current (0-200mA) meter is connected to the current output channel 0 of the 1756-0F8H analog output module that is terminal 3 & 5 as in below

Terminal 1 - VOUT-0
Terminal 3 - IOUT-0
Terminal 5 - RTN
Terminal 7 - VOUT-1
Terminal 9 - IOUT-1
Terminal 11 - VOUT-2
Terminal 13 - IOUT-2
Terminal 15 - RTN

-There are no load attached aside from the multimeter (current meter) used to measure the output when i force value


You are effectively putting a dead short on the output - a 4-20mA signal expects a minimum load resistance of (typically) 250 ohms.

You can either measure the voltage across the load, or put your meter in series with the load, but a mA-meter on it's own is too low a load resistance.
 
Thanks Daba!

As soon i get to the office later today, i will get the 250ohms resistor and try this out.

I cant wait to try this.

I will revert back with update shortly.
 
Thanks Daba!

As soon i get to the office later today, i will get the 250ohms resistor and try this out.

I cant wait to try this.

I will revert back with update shortly.

I am confident this will be your problem, the analog output is "sitting down" because it has a short-circuit on its output (your millammeter).

If you put a 250 ohm load on the 4-20mA signal, you can measure volts across the load resistance.... V=IR

4mA into 250 ohm = 0.004 * 250 = 1 Volt
20mA into 250 ohm = 0.020 * 250 = 5 Volts

This is usually what 4-20mA Input devices do - they have a fixed resistance through which the transmitter attempts to deliver 4-20mA. The receiving device therefore actually measures the voltage drop across the internal load resistor.

Most inputs use 250 ohm, although it is not mandatory.

Receivers can be fed in series, up to the maximum load resistance the transmitter can allow. That's dictated by the design specification of the 4-20mA transmitter.
 
Last edited:

Similar Topics

Hi I'm trying to change the scale in a analog Input module in the module while online. I got a warning saying something like Module changes could...
Replies
4
Views
2,032
Good Afternoon, I should know the answer to this . I have a 1756- 4-20 ma input card with spare inputs .I would like to add a new Analog...
Replies
3
Views
1,804
Hi Guys, I've been doing some experimenting in the shop and im hoping someone will be able to point me in the right direction. I was able to...
Replies
4
Views
1,924
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
Hi everybody, I inherited a redundant 1756-L71 ControlLogix system with 2 racks with some DI, DO, and AI redundant modules (1715 series). The...
Replies
6
Views
2,361
Back
Top Bottom