Used to connect a RQDX3 (M7555) QBUS MSCP controller to floppy and hard disk drive (s).
This board (p/n 5015632 or also 5415633) is tucked underneath the QBUS backplane (H9278-A).
Normally in a BA23, installation of only one hard drive was possible, because of power consumption issues.
More of information about this board here.
In order for a RQDX3-connected hard disk drive to be (A) enabled and (B) writeable, a pull-up resistor must be installed on the appropriate pin(s) of J4. Note that +5V is not available on this board, in a BA23 this pull-up functionality is provided by the front panel, where there are provisions for 1 hard drive control (enable, write lock, in a 4-button panel) or 2 drive control (6-button panel)
Pinout of the RQDX3 50-pin connector:
Schematic of the BA23 Signal Distribution Panel:
