Schneider Electric PC-E984-251 Programmable Logic Controllers

The Schneider Electric PC-E984-251 is a programmable logic controller CPU module belonging to the Modicon Compact 984 architecture. Designed for small-to-midsize industrial automation tasks, this processor features 16K words of user logic memory, 2K words of State RAM, and 24K words of extended data register storage (6xxxx). Operating on a 16 MHz processor, it solves standard ladder logic at execution speeds between 2.13 and 3.0 ms per 1,000 nodes, delivering consistent processing for material handling, machine control, and discrete automation processes.

Processor Memory & Architecture

The PC-E984-251 utilizes a dual-memory system architecture to manage executive routines and user control code. The core system executive firmware is maintained in nonvolatile FLASH RAM, allowing straightforward system updates without replacing physical ROM chips. User ladder logic programs, configuration parameters, and application state data reside in CMOS RAM protected by battery backup. In addition to its standard 16K logic words, the unit supports optional EEPROM auxiliary memory cards for offline application backup and restoration.

Integrated Communication Interfaces

Dual Modbus RS-232/RS-485 serial ports come integrated on the front of the CPU module. These independent channels facilitate simultaneous connections to programming workstations running Modsoft, local human-machine interfaces (HMIs), SCADA supervision nodes, or third-party serial field devices. Unlike models equipped with Modbus Plus options (such as the PC-E984-255), the PC-E984-251 focuses exclusively on dual point-to-point and multi-drop serial topology.

I/O Rack Expansion & System Integration

The module acts as the central processor for the Modicon Compact controller family, interfacing directly with local A120 series I/O expansion racks. It processes discrete, analog, and specialty control signals from connected input modules, solves user ladder logic routines, and updates output registers during every scan cycle. The unit's instruction set retains full 984 family architectural compatibility, enabling code portability across different Modicon 984 hardware platforms.

Frequently Asked Questions

Q: How does the PC-E984-251 differ from the PC-E984-255 CPU module?A: While both processors offer 16K logic memory and 24K extended register storage, the PC-E984-251 features two standard Modbus serial ports, whereas the PC-E984-255 includes one Modbus port alongside an integrated Modbus Plus network interface.

Q: What retains the program memory in the event of a total power disruption?A: User ladder programs and state variables in CMOS RAM are preserved using an internal lithium backup battery, while the base system executive firmware is permanently stored in nonvolatile FLASH RAM.