Micrologix faulting problem.....

I have a micrologix 1500 LRP series C processor with three 1769-IT6 thermocouple inputs connected to the base rack. This base unit is connected to a 1769-PA2 power supply which has a 1769-OW8 relay module, two 1769-IF4XOF2 combination analog in/out modules, and a 1769-IF4 analog input module connected to it. All of this it tied to Panel View Plus 700 HMI via a ENI ethernet module.

The system was installed over two years ago and has sat untill Feb. of this year when my company was hired to finish the proggramming and commisioning of the system.

Right away I encountered a problem with both of the 1769-IF4XOF2 cards (the analog outputs were not working on either card). Seeing as though I only neeed two AO's I replaced one card.

Now the processor keeps faulting with a 71h - controller cannot communicate with expansion I/O. Exact module cannot be determined.
Major Hault S1/13 bit is set.

One of the inputs on thermocouple card #3 also is not working correctly. I disabled that input and changed it to a spare with no problem. This problem occured after the faulting began.

I guess the question is how can I tell which module is giving me trouble without 'part swaping' in new cards to troubleshoot. Is it even remotely possible that this fault is in the logic somewhere? How could I check this?

Do you have the 1769-ECR end cap on the right side of the assembly ?

The 1769 I/O bus needs to have the termination resistor array in the -ECR installed in order to function.

yes there is a 1769-ECR end cap.
The Fault is an intermitant problem.
Sometimes sone as I clear the fault.
Sometimes alright for hours.

Double check your expansion I/O conections then check again.

Quote from AB knowledgebase below:


Answer Error code 71h indicates that the MicroLogix had a communications failure between one or more of its expansion I/O modules. Check that the I/O connectors are firmly connected. If using relay output modules, make sure that any inductive loads powered from the module have surge suppressors installed across the load.