Why Can't A PLC Or Programmable Terminal Be Used For The Safety Circuit?

The IEC 60204-1 (JIS B 9960-1) standard specifies that a safety circuit cannot be made using a programmable controller alone. This comes from the general requirement of the same standard that safety related functions must be constructed from proven circuit technology and parts. If the circuit relies on the operation of programmable electronic devices, this requirement may not be satisfied.