The Q55B is an Extension Base Unit from Mitsubishi, designed to expand the physical I/O capacity of a Q-series PLC rack. It provides the mechanical mounting and backplane bus connection for additional I/O modules, allowing the control system to scale beyond a single base unit.
As an extension base unit, the Q55B mounts directly to the Q-series backplane using the standard DIN rail or screw-mount pattern. It accepts Q-series I/O modules, special-function modules, and network interface modules in its slots. The backplane bus extends the PLC CPU's internal bus to the modules in this base, so no additional addressing or configuration is needed beyond what the CPU's base already provides.