The key difference is that PLCs are blinded for extreme conditions (for instance dusts, humidity, heat, cold) and have a comprehensive input / output (I / O) facility. The input / output arrangements can be integrated into a single PLC, or external I / O modules can be connected to a network of computers connecting to PLC. Electrical design is necessary and the design problem is based upon the desired operating order of the lever (or function chart), and the control and input systems consistent with industrial pilot equipment and control are also included in the PLCs. PLC implementations are also highly optimized systems, which reduce the expenses of a PLC package in comparison to the cost of a custom controller.

The production costs (design of electrical supplies as well as input / output hardware), which could spread over multiple sales, and where the end user does not have to alter the monitoring system, will be acceptable when hundreds of thousands of units are made. Automotive applications are an example; million units are installed every year and very few end users change their programming.

The additional input / output, analog, positioning and communication modules can be used for expansion of several PLCs. PLCs typically have 9-pin RS232 and optionally RS485 and Ethernet built-in communication ports. Modbus, BACnet or DF1 are normally part of a communication protocol. Other fieldbuses, including System Net or Profibus, may also be included. Most advanced PLCs can connect to other systems through a network, such as a device that runs a SCADA system or a web browser.