Solid state outputs are entirely electronic and involve no moving parts and so service life is increased.

Relay outputs which are most common type of output. With relay outputs when the relay is energized, the
moving armature is attracted by the coil and so actuates the contacts, which change state.When the relay
is de-energized, both the armature and the contacts revert to their initial position.

Relay outputs provide isolation between the supply and the output.