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2. FUNCTIONAL DESCRIPTION
DATA PRESENTATION
Data are transferred between the controller and the 1/0
system on a four-byte interface (32-bit data path).
The
four-byte feature must be installed on the IOP. All con-
trollers time-share the interface between the collective
controllers and the IOP.
Please note that throughout this
manual "IOP" refers to the Multiplexor Input/Output Pro-
cessor (MIOP) - the Xerox "A" and "B" Removable Disk
Storage Systems do not interface with the Selector Input/
Output Processor (SIOP). Data are transferred between the
controller and the disk pack unit serially, two bits at a
time (i.e., via a two-bit data path).
DISK SYSTEM STATES
There are four device conditions and two device modes.
The conditions are
1.
"Not operational".
2.
"Ready".
3.
"Busy".
4.
"Not available" (reserved for use with devices that
are accessible to more than one controller).
The two modes are automatic and manual.
Condition
a~d
mode combine to define device state, of which there
are three: one not operational and two operational.
The
latter are
1.
Ready automatic.
2.
Busy automatic.
The initial state of the device depends on its pawer status.
Complete absence of power removes the device from the
controlling system (i.e. , no address recognition). If power
is applied to the controller, address recognition occurs
when an 1/0 instruction is issued.
MODE
For single CPU configurations, the disk pack system is
always in the automatic mode, as long as it is in the oper-
ational state. When dual CPUs can access a device, the
disk system is never in automatic mode; see "Dual Access
Considerations" in Chapter 4.
OPERATIONAL STATES
The disk pack system reports the "operational" state if the
1/0 address is recognized and the device is operational as
defined below.
1.
Disk speed is within operating
I
imits.
2.
Temperature is within operating limits.
3.
All interlocks are closed.
4.
Al
I
power is normal in the controller and disk storage
units.
5.
No fault condition exists within a disk storage unit.
When operationa I, an SIO instruction wi II be accepted if
1.
The addressed controller/device is ready.
2.
The device is "available" - that is, not reserved by
another con troll er.
3.
No channel interrupts are pending.
In response to the SIO, HIO, and TIO I/O instructions,
the condition of the disk pack system at any time is returned
to the central processor.
The TDV and AIO 1/0 instruc-
tions obtain other specific indications of device status
(see "Status Response" in Chapter 3).
READY CONDITION
In the "ready" condition, the controller can accept an SIO
instruction if the addressed device is available and the
system is free of pending channel interrupts.
To become
ready, the device must be operational and there must be
no order execution in progress or pending.
BUSY CONDITION
In the "busy" condition, the controller has accepted an
SIO instruction.
It will not accept a new order until the
current order (or orders) is completed.
UNAVAILABLE CONDITION
In the "unavailable" condition, the device is reserved by
another controller; therefore, it cannot accept an SIO
instruction.
Functional Description
5
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