Output modules allow the PLC to do (or actuate) things.
Digital outputs generally switch things ON or OFF.
They may be relay outputs - generally single-pole, normally open and with a mains current rating of 3A or so non-inductive. This means that it is often best to use such outputs to energize a relay appropriate to the load - such as a motor contactor.
Other possibilities are transistor and Triac outputs.
Transistor outputs are best for rapid ON/OFF waveforms such as PWM (pulse width modulation).
Analogue outputs produce usually a current - such as 4 to 20 mA, but can also output a voltage in a range - such as 0 to 10 volts which will then be an input to something eg a valve actuator or a speed reference to a VSD (variable speed drive) so that the valve can be positioned 0 to 100% open or the motor run between 30 and 50 Hz.
Author - Nigel Feilden