The Allen-Bradley 1785-BCM is a high-speed backup communication module for PLC-5 systems. It enables processor redundancy for DH+ and Remote I/O networks, featuring 1.2 Megabaud switchover speeds and a 4K-word buffer to ensure maximum system uptime.
The Allen-Bradley 1785-BCM serves as a specialized redundancy interface for the PLC-5 control platform. This module facilitates a hot-backup configuration by linking a primary processor with a secondary standby unit. It constantly monitors the health of the active controller to ensure immediate failover capabilities. Consequently, the system prevents costly process interruptions during hardware malfunctions or power losses. The module occupies a single slot within a standard 1771 I/O chassis. Engineers utilize this technology in mission-critical applications where continuous uptime remains a top priority.
This communication expansion module utilizes a High-Speed Serial Link (HSSL) for rapid data transfer. It supports transmission speeds up to 1.2 Megabaud to synchronize memory between partner processors. Moreover, the 1785-BCM manages both Data Highway Plus (DH+) and Remote I/O (RIO) network traffic. An internal 4K-word buffer handles data buffering to maintain information integrity during the switchover process. This architecture ensures that the standby processor possesses the latest runtime data. Thus, the transition between controllers appears seamless to the field equipment and remote I/O stations.
The 1785-BCM monitors several critical triggers to initiate an automatic processor switchover. These triggers include hardware parity errors, power supply failures, and communication timeouts. Additionally, the module detects manual mode changes, such as moving a key switch to the Program position. This proactive monitoring ensures the standby unit takes control before the process reaches an unsafe state. Users can also perform manual switchovers for routine maintenance without stopping the production line. This dual-processor strategy significantly enhances the reliability of legacy automation infrastructures.
Proper deployment requires the 1785-BCM to reside in the same chassis as the local processor. However, all field I/O must remain in remote 1771 racks to allow for independent processor access. The 1785-BCM uses the 1771-WG wiring arm to provide secure and accessible cable terminations. Furthermore, the module works in conjunction with the 1785-BEM expansion module for more complex network topologies. This combination provides a robust framework for large-scale distributed control systems. Despite its legacy status, the 1785-BCM remains a vital component for sustaining high-availability PLC-5 environments.
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