PLC-5/40 Digital input register will not update

ceilingwalker

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ceilingwalker
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Mar 2010
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Phoenix, AZ
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Good day all. I have a bit of a problem with one of my digital input module, I think. In the attached ladder (Ladder 20, Rung 29) there is an XIO I:047/10 that does not ever change its register to a 1, even when the output is forced. This represents a hard wired circuit that goes across NO auxillary contacts on the motor starter for a cooling blower. I put a meter on the actual contacts and watch the voltage go to 118 VAC and the LED on the front of the input module for this particular bit lights up. However, I open the data file and watch the condition of this bit and, even though the input is lit, the bit register never changes from 0 to 1, which prevents an alarm. I cannot find anything else that is controlling this bit so I'm not sure what is wrong. Thank you for your help.
 
The file you posted is password-protected.

But I'm devious.

I agree that there's nothing else in the program writing to that bit or word.

Do any of the other Input bits from that module work correctly ? Rack 4 is mostly Output modules, with the Input module in the last logical slot.

It is possible, but not very common, or a 1771 I/O module to have a "quiet" backplane communication failure in which the status LED shows up, but the Input data is not updated. I have typically seen this in systems over 20 years old with corrosion on the backplane connectors.

It is not possible for only one bit out of the module word to fail to transfer.

I don't remember how the Forcing function in a PLC-5 works; maybe it doesn't change the data table value, but rather only the action of instructions that reference the bit.
 
Ken Roach said:
The file you posted is password-protected.

But I'm devious.

I agree that there's nothing else in the program writing to that bit or word.

Do any of the other Input bits from that module work correctly ? Rack 4 is mostly Output modules, with the Input module in the last logical slot.

It is possible, but not very common, or a 1771 I/O module to have a "quiet" backplane communication failure in which the status LED shows up, but the Input data is not updated. I have typically seen this in systems over 20 years old with corrosion on the backplane connectors.

It is not possible for only one bit out of the module word to fail to transfer.

I don't remember how the Forcing function in a PLC-5 works; maybe it doesn't change the data table value, but rather only the action of instructions that reference the bit.
Click to expand...

Sorry, the password is 5161. All other inputs on that module do work. Checking it with my meter I get my voltage all the way back to the terminal point on the input module. I haven't had this happen to me before.
 
Have you tried (if you can) power down and unseat the module then reseat it?

You should be able to force the input and see it in the data table.

even when the output is forced
Click to expand...
Forcing an output is not going to make an input do anything.
 
Last edited:
Mickey said:
Have you tried (if you can) power down and unseat the module then reseat it?

You should be able to force the input and see it in the data table.


Forcing an output is not going to make an input do anything.
Click to expand...

I can't reseat the module yet, other active inputs on that module are being used.
I should have been more clear about my forcing the output. What I meant was, I forced the output at the beginning of that rung and watched the LED on the input module light and watched my voltage meter read 118 volts. Only the register is not updating.
 
ceilingwalker said:
I can't reseat the module yet, other active inputs on that module are being used.
I should have been more clear about my forcing the output. What I meant was, I forced the output at the beginning of that rung and watched the LED on the input module light and watched my voltage meter read 118 volts. Only the register is not updating.
Click to expand...

Okay, now it's worse!

You forced an output at the beginning of a rung, while watching the LED on an input module?

Only the register is not updating?

Where did you put those leads?

I think we need pictures...
 
Here is the rung in question.

When you force an output it only causes to actual (physical)output to go on or off not the data table. (bit box as Ron would say).
When you force an input it does cause the table ( bit box) to change to a 0 or 1.

You should be able to force the input address I:047/10 and see it's data table(bit box) value go to a 1 or 0 depending on whether you force it on or off.

forcerung.jpg
 
Last edited:

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