Programmable Logic Contollers

1. Deigned to operate in industrial environments
2. special input/output interfaces
3. control program languages

1. Everything is in one module
   1. Power supply
   2. CPU
   3. I/O
2. Least common

1. Modifiable - Able to change it
2. Each function is its own module
   1. Power supply
   2. CPU
   3. Each I/O is separate

1. Automotive industry
2. replaced mechanical relays
3. Do NOT require excess wiring that relays need

1. Count
2. Time
3. Math functions
4. Process analog and digital signals

Nota:

• Once a program has been written and tested it can be downloaded to other PLCs

1. Logic is contained in the PLC's memory
2. No chance of making a logic wiring error

Nota:

• Original equipment manufacturers (OEMs) can provide system updates for a process by simply sending out a new program

1. Easier to change a program than rewire a circuit
2. Uers can modify the program in the field

Nota:

• Originally PLCs were designed to replace relay control logic.  The cost savings using PLCs have been so significant that relay control is becoming obsolete, except for power applications.

1. If you require more than 6 control relays, it will be less expensive to install a PLC.

Nota:

• A PLC can communicate with other controllers or computer equipment

1. Networked to perform such functions as...
   1. Supervisory control
   2. Data gathering
   3. Monitoring devices and processes
   4. Downloading/Uploading of programs

Nota:

• PLCs operate in real-time which means that an event taking place in the field will result in an operation or output taking place.

1. Objects spend only a short time in front of a sensor
2. Require quick response capability

Nota:

• PLCs have resident diagnostic and override functions that allow users to easily trace and correct software and hardware problems.

1. Can be watched in real-time as it runs
2. Easier to find and fix problems