Control and status byte
Control byte for process data exchange
The control byte is transferred from the controller to the terminal. The control byte is not used for KL331x and KL3302.
Status byte for process data exchange
The status byte is transmitted from the controller to the terminal. It contains various status bits of the analog input channel:
Status byte:
Bit 7 = 0bin
Bit 6 = 1bin: Error (general error bit)
Bit 5 to bit 2: reserved
Bit 1 = 1bin: Overrange
Bit 0 = 1bin: Underrange
Compensation
The terminals are compensated when delivered.
In order to compensate tolerances of the external components, gain and offset registers for compensating the thermocouple voltage are implemented for each channel, i.e. R17 (thermocouple voltage offset) and R18 (thermocouple voltage gain). For compensating the cold junction temperature (temperature at the transition between the thermocouple and the terminal contacts), a gain register (R21) is implemented, which is identical for both sets of registers.
Compensation can be carried out as follows:
First of all the offset calibration is carried out with 0 V input voltage, reference temperature deactivated and linearization switched off. 0xF100 is entered in the feature register.
This is followed by gain compensation with a maximum voltage of 125 mV (typical value: 70 mV). With this terminal setting with vendor scaling deactivated, the voltage is output in 4 µV per digit.
Gain and offset compensation of the thermocouple voltage is carried out separately for each channel.
In the next step, the temperature of the cold junction is compensated.
To this end, a thermocouple has to be selected via the feature register, and cold junction temperature compensation must be active (R32 0x1006 type K). With short-circuited inputs (0 V), the temperature of the terminal contacts is determined, and the temperature output by the terminal (measured via an internal temperature sensor) is set accordingly (via R21).
The cold junction temperature only has to be calibrated once for each terminal, i.e. R21 is identical for both channels.