Schneider C921 | Modicon Communications Processor (CP)

The C921 handles serial communication tasks for Modicon PLC systems. This communications processor offloads network traffic from the main CPU. It manages data exchange between the controller and field devices efficiently. Consequently, your processor focuses solely on executing control logic without interruption.

You can configure this module for various industrial protocols as needed. The C921 supports Modbus, ASCII, and character-oriented communication modes. It also provides both master and slave functionality on the network. Therefore, engineers gain flexible connectivity for diverse field devices.

Hardware Architecture and Processing Capability

The module includes a dedicated microcontroller for communication handling. This processor manages all telegram assembly, transmission, and error checking. The C921 incorporates dual-port RAM for sharing data with the CPU. This memory arrangement allows simultaneous access from both processors without conflict.

A separate EEPROM stores all configuration parameters permanently. The C921 retains these settings even after a complete power loss. It contains no internal battery for data preservation purposes. Below is a complete breakdown of its technical characteristics.

Specification	Detail
Product Type	Communications Processor
Communication Modes	Modbus RTU, ASCII, Character
Network Role	Master or Slave (Selectable)
Interface Type	9-pin D-sub male (RS-232/RS-485)
Dual-Port RAM	4 KB (Shared with CPU)
Configuration Memory	8 KB EEPROM (Non-volatile)
Baud Rates	300 to 19200 bit/s (Selectable)
Dimensions (W x H x D)	25 mm x 176 mm x 160 mm
Weight	0.22 kg (220 grams)
Power Consumption (5V)	250 mA
Energy Storage	None (EEPROM only)

Interface Layout and Connection Options

The module features one 9-pin D-sub male connector for network access. The C921 includes a second 9-pin port for programming connections. Two banks of DIP switches set the node address and baud rate. Three LED indicators show RUN, TXD, and RXD activity status.

This communications processor draws its power entirely from the backplane. The C921 does not require any external power supply for operation. It fits directly into any standard Modicon rack slot. Therefore, installation requires only plugging the module into place.

Operational Benefits for Network Integration

This communications processor simplifies adding serial devices to Modicon systems. The C921 handles all protocol conversion and data buffering automatically. It also provides diagnostic information for network troubleshooting. Engineers value this module for connecting barcode readers, printers, and weigh scales.