The lack of keyboard and other input-output devices is very noticeable on a PLC. On the front of the PLC there are normally limited status lights. Common lights indicate;
1. Power on – this will be on whenever the PLC has power.
2. Program running – this will often indicate if a program is running, or if no program is running.
3. Fault – this will indicate when the PLC has experienced a major hardware or software problem.
These lights are normally used for debugging. Limited buttons will also be provided for PLC hardware. The most common will be a run/program switch that will be switched to program when maintenance is being conducted, and back to run when in production.
This switch normally requires a key to keep unauthorized personnel from altering the PLC program or stopping execution. A PLC will almost never have an on-off switch or reset button on the front. This needs to be designed into the remainder of the system.