Classified by structure:
1. Monolithic: It is to install the PLC components together or a few printed circuit boards, and together with the power supply, it is installed in the casing to form a single whole, called the main unit or the basic unit, small and ultra-small PLC. This structure.
2. Modular: It is to make the basic components of PLC into independent modules. Medium and large PLCs are used in this way for easy maintenance.
User program storage capacity: A measure of how much user applications can be stored. Usually in words or K words. The 16-bit binary number is one word, and each 1024 words is 1K words. The PLC stores instructions and data in word units. The general logical operation instructions each occupy 1 word. Timing/counting, shift instructions occupy 2 words. The data operation instruction occupies 2~4 words.