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MCC 134 returning error -7777


JRys
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Posted on behalf of a customer

Hello,

We are experiencing an ongoing problem that comes in and out and is somewhat of a headscratcher for us. We ae using the MCC134 Thermocouple Measurement HAT for the Raspberry Pi, and what we experience from time to time is a drop in signal, and get the error code -7777 reading. We know that means return value is outside the common-mode range, and were wondering what advice you can provide to help mitigate this problem. Like I said this error seems to come in and out, and we know the thermocouples are working and isolated the problem to either the thermocouple wire or the mcc134 board. I read online that it is advised to make a connection to the device you are measuring to any pi GPIO ground terminal, is that something you can advise? Are there any other actions we can try to take?

Thank you,
 

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Hello,

 

To measure the thermocouple voltage, the MCC 134 uses a differential input connection. However, it is not known if the thermocouple will be in contact with a ground metal surface or if it will be isolated – perhaps suspended in a tank. Because of this dilemma, we use a reference resistor to the Raspberry Pi ground. The reference resistor allows the converter to make the measurement in either situation – grounded and ungrounded. However, this exposes you to the possibility of a ground loop. A ground loop can happen between two grounded thermocouples or with another system ground connection. Depending on the ground currents, you may not get an error until sometime later in your process. Maybe something nearby is powered on or a process starts.

Debugging this situation is difficult. First, you can test the MCC 134 operation by replacing the thermocouple connections with a wire short across the channel inputs. A wire is a zero volts thermocouple and will produce a stable temperature that is close to ambient. If you have multiple thermocouples, replace all but one with a wire short. If it works, you have a ground loop between the other thermocouples. Another trick is to isolate the thermocouple ground end using a piece of mica or Kapton© high temperature tape.

 

Kind Regards,

John Rys

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