Section III. PRINCIPLES OF OPERATION
This section provides a brief explanation of disk unit operating principles.
The disk unit operates under the control of programs running in a host computer. An adapter card, plugged into an I/O
slot of the host computer, translates commands from the host computer into the format required by the disk units
controller PCB. The adapter transfers data between the disk unit and the host computer memory using the data channel
or the burst multiplexer channel of the host computer.
If additional disk units are slaved to the master disk unit to increase the overall storage capacity, they are connected in a
serial fashion using standard cables (fig. 1-4). The total length of cabling allowed in the serial hookup is 10 feet. Model
3840 terminator plug assemblies are required to electrically terminate critical signals.
The slave disk units are identical to the master disk unit except they do not contain a controller PCB. Up to three slave
units can be connected in series to the master disk unit. The controller in the master disk unit, using commands from the
computer I/O adapter card, can transfer data to or from any of the four disk drives. Since the master disk unit has a
storage capacity of 35.6 Mbytes, the addition of three slave disk units increases the total system storage capacity to
The asterisk that is used with signal names
(WSTR*) in this chapter refers to an inverted
The following paragraphs describe the operating principles of the six major assemblies that make up the disk unit. The
descriptions also apply to any slave disk units that may be employed, except that the slave units would have no Disk
Disk Controller PCB. The Disk Controller PCB occupies slot A3 in the card cage of the disk unit. The
controller communicates with the I/O adapter PCB in the host computer and with the Device Electronics PCB in slot A2
of the disk unit's card cage. A one-byte, bidirectional bus and nine control signals handle all communications with the I/O
adapter while the signals to the electronics PCB consist of a one-byte, out-bus and control signals.
The controller accepts commands from the adapter and controls all disk operations and all transfers to or from the I/O
adapter. It provides all the necessary control for the master disk unit and up to three slave disk units. All data,
commands, and status are transferred between the I/O adapter and the controller PCB over the one-byte, bidirectional
bus (IBUS 0-7). The byte on the bus is loaded into the output buffer of the controller by signal WSTR*