Hcmarc-0ct by B & R Automation

The Hcmarc-0ct is a communication module manufactured by B&R Automation for the HCM Maestro PLC Modules series. Installed beside CPU and I/O cards, it gives a controller rack dedicated ARCNET media access so distributed machines can exchange status and command data without external gateways. By integrating network timing and arbitration into the backplane, the card helps maintain deterministic update cycles needed for coordinated motion, batch sequencing, and alarm handling in industrial automation cells.

The module implements the ARCNET protocol at a fixed throughput of 2.5 MBaud. When coaxial cable is used, the network requires a characteristic impedance of 93 ?, which helps minimize reflections that could corrupt data frames. A single coax segment may span up to 300 m and support as many as eight stations before an active hub or repeater is required. For cabinet-to-cabinet wiring with Category 5 conductors, the twisted-pair option permits cable runs of 120 m, providing flexibility for routing through trays or energy chains.

Network access is controlled by the Token Passing (ISO 802.4) method, allowing every device to transmit without collisions while supporting a logical limit of 255 addressable nodes on one network. Within an unamplified segment, the coax bus can host 8 stations and the twisted-pair bus can support 10. Greater fan-out is achieved with hubs, which can also extend the maximum station distance to 6 km through fiber or active coax amplifiers. This layout allows cable type, segment length, and node count to be matched to the required network topology and real-time performance.