RTD input to S7-315 PLC - Problem

[onyxgaze]

Hi,

We have RTDs connected to a Siemens PLC S7-315 system. Out of the 15 or so RTDs connected, all show correctly except for 5. For all these 5, the process temperature is displayed correctly until 113 degC. Beyond that the value jumps to 285.67.

The RTDs used are 3-wire, PT-100, simplex E&H type.

There is a MTL 7755ac barrier in between the RTD and the PLC AI module, but even on bypassing the barrier, the situation remains the same.

How do I diagnose this problem and what could be a likely solution?

Thanks in advance.

 

[JRW]

If your move one of the 'bad' 5 RTD's to a location where the others are working does the problem follow?
You have to find out if the problem is with the rtd or the module

 

[wallypiper]

So simple ^.
Even simpler: RTD's are such simple devices. You can connect a DMM to them and check to see if there's anything wrong with them but normally they either are good or bad. Charts of temp vs. ohms are widely available on line for all RTD types.
I can't imagine an RTD itself making a jump like that. Configuration problem maybe?