The fact that the MASSBUS connects to disk drives and tape 'controllers causes terminological difficulties, since there is also a 'conroller' to connect the MASSBUS to the host computer. Hence, the term 'entity' will be used in this article to refer, generically, to things connected to the MASSBUS> A single MASSBUS can contain up to 8 entities.
A variety of controllers existed to connect MASSBUS devices to various kinds of DEC systems: the RH10 for the PDP-10, the RH20 for the KL10, the RH11 for the UNIBUS (usually on a PDP-11), the RH70 for the PDP-11/70, the RH750 for the VAX-11/75x, and the RH780 for the Synchronous Backplane Interconnect bus found on VAX-11/78x, VAX 86xx, etc.
The MASSBUS contained two separate sections (effectively, separate busses): an asynchronous control section (the 'Control Bus'), and a synchronous data transfer section (the 'Data Bus'). The two sections operate completely independently.
The control section is used to allow the CPU access to device registers implemented in the devices. The data section is 18 (optionally 16) bits wide, to allow use with both DEC's 36-bit and 16-bit (later 32-bit) architectures.
Control Bus lines
The Control Bus contains the following signals:
- Control Lines 0:!5 - used to transfer control data
- Control Lines Parity
- Drive Select 0:2 - selects the entity
- Register Select 0:4
- Controller to Drive - direction of transfer
- Demand - set by the controller to initiate a transfer
- Transfer - acknowledgement from the entity
- Attention - set by an entity to notify the controller
- Initialize - set by the controller
- MASSBUS Fail - set by the controller to indicate power loss