1771-OFE analog output scaling issue

jonney · Nov 17, 2020

My 1771-OFE analog output card was previously configured to Voltage output OFE1 (voltage version), all four channels are 0-10Volts corresponding to 0-4095. All channels are working normally. Recently, some machineries are dismantled, the four channels are free. I changed to 4-20mA OFE2(4-20mA version) and scale to 0-4095. But when N4:68 = 2048, the output got 20mA full scaled output. look like the full scale is 0-2048 instead of 0-4095, what is going on? For my understanding, the current output has nothing with DIPs setting at backplate of this board. Right?

drbitboy · Nov 18, 2020

What is the output (mA) when N4:68 is 1024?

jonney · Nov 18, 2020

drbitboy said:
What is the output (mA) when N4:68 is 1024?

N4:68 is 1024: 12mA

I confirmed 0-2048 corresponding to 4-20mA, N4:68 = 1024 measured 12mA using meter.
2048 to 4095: 20mA-21mA

Bit_Bucket_07 · Nov 18, 2020

Just to be clear, you changed out the module, right?

You didn't simply change the configuration and leave the same module in the slot?

jonney · Nov 18, 2020

Bit_Bucket_07 said:
Just to be clear, you changed out the module, right?

You didn't simply change the configuration and leave the same module in the slot?

I took out the module and do nothing then put back. then i changed the configuration from voltage version to current version in program mode.

Ron Beaufort · Nov 18, 2020

you might already know this - but just in case ...

if you are using a module 1771-OFE1 then it is only physically capable of putting out voltage ... (notice the ONE) ...

if you are using a module 1771-OFE2 then it is only physically capable of putting out current ... (notice the TWO) ...

the setting that you are changing in the software will ONLY tell the software what type of module that you have actually installed ... specifically, changing the software selection will NOT change whether you get a voltage or a current output ...

newer types of modules (ControlLogix, etc.) can be set up to put out either voltage or current - but the older PLC-5 modules (bless their little hearts) weren't designed to do that ...

drbitboy · Nov 18, 2020

[Update: 2minutes behind an expert!]

How are you measuring mA?

I think Bit_Bucket's point is that since the module was originally 0-10V, it sounds like this is an -OFE1 module, which only does voltage. To me it does not seem explicit in the manual, but it seems to say that there are three types of 1771-OFE module sub-types: the -OFE1 is a voltage module; the current modules are -OFE2 (4-20mA) and -OFE3 (0-50mA), i.e. either physically different modules, or perhaps the placement of jumpers determines the module sub-type. Can a picture be posted? How are the jumpers set?

If I had to guess it may be that 2048 (half of the 0-4095 scale) has an -OFE1 sub-type putting 5V (half of the 0-10V scale) across a 250Ω resistor to give a reading of 20mA, but that does not explain 1024 => 12ma when it should be 1024/4095 => 2.5V/10V, and 2.5V across 250Ω => 10ma.

Mickey · Nov 18, 2020

drbitboy said:
[Update: modules are -OFE2 (4-20mA) and -OFE3 (0-50mA), i.e. either physically different modules, or perhaps the placement of jumpers determines the module sub-type.

As Ron explained they ARE physically different modules.

There are jumpers to control (configure ) the hold last state values.

jonney · Nov 18, 2020

drbitboy said:
What is the output (mA) when N4:68 is 1024?

drbitboy said:
[Update: 2minutes behind an expert!]

How are you measuring mA?

I think Bit_Bucket's point is that since the module was originally 0-10V, it sounds like this is an -OFE1 module, which only does voltage. To me it does not seem explicit in the manual, but it seems to say that there are three types of 1771-OFE module sub-types: the -OFE1 is a voltage module; the current modules are -OFE2 (4-20mA) and -OFE3 (0-50mA), i.e. either physically different modules, or perhaps the placement of jumpers determines the module sub-type. Can a picture be posted? How are the jumpers set?

If I had to guess it may be that 2048 (half of the 0-4095 scale) has an -OFE1 sub-type putting 5V (half of the 0-10V scale) across a 250Ω resistor to give a reading of 20mA, but that does not explain 1024 => 12ma when it should be 1024/4095 => 2.5V/10V, and 2.5V across 250Ω => 10ma.

I need to reconfirm whether 1024 is 12mA, probably as you said 10mA, because I am rush to measure and meter has accuracy problem.
I attach the on board DIP settings for Last State and Channel 1.

JohnCalderwood · Nov 19, 2020

attached is the page from the manual regarding the links at the top of the module

JohnCalderwood · Nov 19, 2020

and the config links - one set per channel

drbitboy · Nov 19, 2020

As mentioned by others, that board can only output voltage; it cannot output a current signal.

Marborg PLC · Nov 19, 2020

Ditto, one card for voltage, another for current. Grew up with PLC 5, just the way it is.

jonney · Nov 19, 2020

JohnCalderwood said:
attached is the page from the manual regarding the links at the top of the module

I see. So I have to move the Last State jumpers to either "Hold Last State" or "Max"?

drbitboy · Nov 20, 2020

jonney said:
I see. So I have to move the Last State jumpers to either "Hold Last State" or "Max"?

No, that -OFE1 card is designed to provide a voltage output, not a current output. To send a 4-20ma current signal will require the -OFE2 card or similar. A Rube Goldberg alternative would be a voltage-to current converter driven by the output of the current card with the software configured for voltage output e.g. 1-5V.

1771-OFE analog output scaling issue

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