If you are going to buy a PLC, types of output is one of the thing that you need to consider, I hope this article can help you to choose what kind of PLC output you need. There are two types of PLC output, the relay output and the transistor output. Watch: Types Of PLC Descrete Output (Relay and Transistor)
Relay VS Transistor (Polarity)
Relay output = Output Polarity can be Positive or negative depending on the wiring Configuration. Watch: PLC wiring configuration
Transistor output = Output polarity is fixed (sinking for negative output, sourcing for positive output).
Relay VS Transistor (Response)
Relay output = Relay output performs mechanical switching which response is the same as the response of traditional relay.
Transistor output = Transistor output performs electronic switching which is a lot faster than mechanical switching.
Note: For some applications like servo motor pulse control, transistor output is much better choice that relay output, because sometimes servo motor controller requires 100,000 pulse per seconds to control the servo motor and relay output is not capable of performing this requirement.
Relay VS Transistor (Output Voltage)
Relay output = Some of the relay output can be use to produce AC voltage (Check the PLC specifications).
Transistor output = Transistor output can only used to produce 24 Volts DC.
Relay VS Transistor (Stored Energy)
Relay output = Relay output is not affected by stored energy from inductive load.
Transistor output = Transistor output can be damage by stored energy from inductive load.
Note: In case that you need to use PLC transistor Output together with inductive load you can connect diode in parallel with the inductive load in reverse bias to protect the PLC transistor output from stored energy.