Bus Controller CardThe Bus Controller is not plugged to the Backplane as a normal card but instead placed behind it and connected using a different set of connectors. It serves to different purposes.
Bus Interface
The Bus Controller Card buffers the external bus (U-BUS) providing a bridge to the internal buses.
Arbitrator
The Computer Console is capable of taking over controller cards for different operational modes. The Bus Controller arbitrates between the two, passing control to one or another depending of the current mode. For this to be possible, all internal control signals, as well as the S-BUS, connects directly to the Bus Controller card. The Console Controller connects directly to this card too.
Instruction and Memory Bank registers
The Bus Controller contains the Instruction Register (IR). This is in order to provide the IR output to lines D0-15 in IDS Slots while taking the IR input from the D-BUS wired in Data Slots.
It also contains the Memory Bank register (MB). This is because the input for this register comes directly from the IR and the output only goes to the external U-BUS (A16-23).
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