PLC control VS hardwired control

Thread Starter


Joined Jan 22, 2015
Anyone has views on the advantages and disadvantages between the 2?
Is there a chance that the PLC will fail eariler then the hardwired ?
Is there a need for maintenance on the PLC control ?
Is it true that when safety is concerned hard wiring is the way to go ?


Joined Jul 18, 2013
1st one: the advantages of hardwired logic is no update delay of the O/I table, akin to real time logic, usually not an issue with machine control logic as devices are as slow if not slower in activation.
2nd: failure rate would be similar when used in the same environment.
3rd: I/O are usually conditioned, some with fused outputs, these require replacement on fault, otherwise external safety measures are used.
4th: Not really, both are dependent on the programmer and integrator to build in safety, in conjunction with external protection devices.
Main advantage with PLC is display of ladder logic to assist maintenance personnel to trouble shoot in a familiar ladder display.
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Glenn Holland

Joined Dec 26, 2014
In the rail mass transit industry, we use a specially configured PLC for safety critical functions.

However it is not a commercial off the shelf version and it's specially made for railroad signal applications.

It has two separate processors each running a separate algorithm. The result is compared and verified by a third processor.