COMMON_MODE_TC_VALUE


If, during a library call to the mcc134.t_in_read(), the call returns -7777.0.
This means "Return value for thermocouple input outside the common-mode range."

This can be caused by the thermocouples touching surfaces having a voltage potential greater than 0.8 VDC.
As stated in the published spec's,  "Between any CHx+ or – input and any other input."

Though not stated, an added possibility is when using one thermcouple and seeing the call return the -7777.0.
The problem here is the common mode spec is being violated when the single thermocouple has a potential greater than 0.8V DC, but where is the other measurement point?
The answer is there can be a voltage potential on the GPIO bus of the Raspberry Pi.  
The solution is to make a connection to the device you are measuring and any GPIO ground terminal.
The GPIO ground pins (terminals) are: 6, 9, 14, 20, 25, 30, 34, and 39.

Posted 8/15/2019 9:10:42 AM by Jeff
https://kb.mccdaq.com/KnowledgebaseArticle50785.aspx