PLC Disadvantage

(a)           Newer Technology: It is difficult to change some personnel’s thinking from ladders and relays to the PLC computer concepts.

(b)           Fixed Program Applications: Some programs are single function application.  It does not pay to use a PLC that includes multiple programming capabilities if they are not needed.  One example is in the use if drum controller/ sequencers.

(c)           Environmental Considerations: Certain process environment, such as high heat and vibration, interfere with the electronic devices in PLC’s which limits their use.

(d)           Fixed-Circuit Operation: If the circuit in operation is never altered, a fixed control system (such as a mechanical drum) might be less costly than a PLC.  The PLC is most effective when periodic changes in operation are made.

(e)           Fail-Safe Operation: In relay systems, the stop button electrically disconnects the circuit; if the power fails, the system stops.  Furthermore the relay system does not automatically restart when power is restored.  This, of course, can be programmed into the PLC; however, in some PLC programs, you may have to apply an input voltage to cause a device to stop.  These systems are not fail-safe.  This disadvantage can be overcome by adding safety relays to a PLC system.