Relay PLC logic and what is it?

First what is a Mechanical Relay like you would install in a machne?

It has one coil which has power that is applied to a coil. Typicallly on Terminals A1 and A2. When power is applied the coil turns into a magnet and pulls essentially two pieces of metal together which are attached to two terminals that you can wire up to. The two pieces of metal are what we call contacts.

What is a PLC Relay contact referred to in terms of a PLC Ladder istruction?

There are two types of relays used in PLC's.

1. Internal Relays- Relays used for writing software Ladder logic.
2. Output Relays- Physical contacts mounted to PLC.

Instructions or ladder logic is an internal relay built into a plc in the form of a processor that pretends it has lots of relays, almost unlimited.

Relay Logic