Using Allen Bradley's 1794 IT8 Analog input module


Thread Starter


I use AB's 1794 IT8 Flex IO Analog Input Module with J type thermocouple for temp measurement. Initially when heater is off it show 1 0r 2 degree C instead of room tempertaure. Later on as the heater is turned on, it measures the temperature properly. Please advise...
I think the problem is not with the module or temperature. What are you using to display the temperature? Check the parameters or the logic which you are using to move values to the HMI.
A scaling operation gone wrong is the most probable cause for your problem.
Hope this helps. Detailing your problem will help to understand it more clearly.

Eddie Willers

It sounds to me like you didn't hook up the Cold Junction Compensator (CJC). The module needs that CJC installed in order to sense ambient temperature at the module itself, otherwise it has to assume that the module is at 0 degreees C.
In order to use cold junction compensation technique a seperate component 1794-CJC/2A is required which contains two cold junctions. Since i haven't ordered this, is there still a way i can get the correct temperature?
I use Devicenet Network at 500 KBPS & Panel-View display as one of the nodes to show the temperature.The 1794 Analog i/p is a flex input connected via 1794-Devicenet Adapter to 1756-Devicenet scanner in Control Logix PLC slot 2.

I already selected J-Type Thermocouple as my i/p type in the devicenet network,so that should automatically do the scaling.