# Programmable Logic Controller

Written by Jerry Ratzlaff on . Posted in Instrumentation & Controls

## Programmable Logic Controller

There are four primary types of signals that the programmable logic controller ( $$PLC$$) generates. On a P&ID, when referencing the type of signal for a particular instrument or installation, it should be in reference to the PLC. The types of signals are analog output ( $$AO$$ ), analog input ( $$AI$$ ), discrete output ( $$DO$$ ), and discrete Input ( $$DI$$ ).