Classified by structure:
1. Integral type: It is to install the various components of the PLC together or a few printed circuit boards, and install them together with the power supply in the chassis to form a single whole, which is called the host or basic unit, and is used by small and ultra-small PLCs. This structure.
2. Modular: The basic components of PLC are made into independent modules. Medium and large PLCs adopt this method, which is easy to maintain.