Which protocol is designed to allow communications between embedded programmable logic controllers?

• A. CAN
• B. Fieldbus
• C. Ladder Logic
• D. Modbus

Answer: (A)

Communications between embedded programmable logic controllers rely on a bus that supports multiple nodes, real-time behavior, and strong fault tolerance. CAN, the Controller Area Network, is designed exactly for that environment. It is a multi-master, message-based protocol on a shared bus, so any device can transmit when the bus is free, while arbitration ensures higher-priority messages get through first. This makes CAN well suited for real-time control in embedded systems, such as automotive ECUs and industrial controllers, where timely and reliable data exchange is critical. The protocol includes robust error detection (CRC), acknowledgement, and fault confinement, which help maintain network integrity in noisy environments. The physical layer uses differential signaling on a twisted pair, supporting reliable communication over distances and in electrically harsh settings.

Fieldbus refers to a broader family of industrial networking standards for factory automation, and Modbus is a simple master-slave protocol often used for device communication in PLC environments. Ladder Logic is a programming language used to implement control logic, not a communication protocol.