1 The measuring devices are regularly monitored for their ability to measure, without a detected error, 10 analog values between 4 and 20 mA. The linearity of the measuring stages is verified at the same time.

2 Each input value is acquired by 2 identical circuits. The measuring values are compared by the safety processor. If values are different, that channel is determined to be not valid. A maximum discrepancy of 0.35% of the 20 mA full scale range is tolerable between the two values.

3 The companion chip supplies the safety processor, continuously diagnoses the safety processor, and monitors backplane voltage.

4 The supply voltage from the backplane is monitored to detect if an over voltage or under voltage condition occurs.