Abstract: A method of calibrating a process computer, of the kind including an analogue to digital converter for converting analogue input signals representative of measured process parameters into digital signals for processing within the computer, and a digital to analogue converter for converting the processed signals into analogue output signals, comprises applying a known analogue signal to the analogue to digital converter, comparing the resulting digital signal with its expected value, and computing and storing a first calibrating correction value in dependence on the difference. A known digital signal is then applied from within the computer to the digital to analogue converter, whose output is coupled back to the input of the analogue to digital converter for conversion thereby and correction in accordance with the first calibrating correction value.