The core part of CP451-51 is a PLC system, which adopts SIEMENS high-performance modular S7-300/400 series PLC to complete the collection and adjustment of process parameters and the start/stop control of process equipment in the production process, and it is placed in the control room of the production site in order to shorten the distance to the primary detection components and actuators on the site. It is placed in the production site control room to shorten the distance to the primary detection elements and actuators on site. A variety of modular components in different ways conveniently combined together and suitable for dense installation, structural form specification, improve the flexibility of the design of the automatic control system and the reliability of the operation, due to the need to deal with a large number of signals, the use of multi-rack mounting.

CP451-51 CPU module has a variety of different properties, for example, some CPU integrated with input / output points, some CPU integrated with PROFI-BUS-DP communication interface, etc., the production line automatic control system CPU is specially upgraded to a higher level, the use of the CPU for the 413 model (later due to lightning strikes one of the reasons for the replacement of the CPU412-2)

CP451-51 for digital and analogue input/output, is the interface between the PLC system and the production process; field equipment start, stop, limit alarm, fault alarm, operation and running status, as well as temperature, pressure (differential pressure), flow, voltage, current, speed and other important parameters are connected to the PLC system through these signal modules. The DI signal module used in this system is SM321, DO signal module is SM322, AI signal module is SM331 and AO signal module is SM332.