4.
INPUT
jOUTPUT
OPERATIONS
To
accommodate the
variety
and number
of
I/o
devi ces
which may be required for
scientific
and commercial
appli-
cations,
a Xerox 560 computer system may
include
the
fol-
lowing: External Direct
Input/Output
(DIO)
interface,
Multiplexor
Input/Output
Processors (MIOPs), and Rotating
Memory
Processors
(RMPs).
EXTERNAL
DlO
INTERFACE
An
external
DIO
interface
permits standard and
specially
designed
I/O
devices
to
perform
I/O
operations (normally
in a
real-time
environment)
that
are
controlled
directly
by
the
basic processor
(BP).
Appropriate control signals and
up
to
one word
{32
bi
ts}
of
data
may
be
exchanged
between
the
BP
and
an
addressed
I/O
device
for
each
READ
DIRECT
or
WRITE
DIRECT
instruction
executed
by
the
BP.
During a
WRITE
DIRECT
instruction (Mode 2 through F),
the
BP
holds the control
and
data-lines
stable
until
an
acknowledgment signal is
received
from
the addressed
I/O
device.
During a
READ
DIRECT
instruction (Mode 2
through F), the
BP
holds
the
control lines
stable
until
the
addressed
I/o
device
furnishes
the
data
accompanied with
an acknowledgment signa/. Any
delay
encountered
in
receiving
the
acknowledgment
signal,
for
either
READ
DIRECT
or
WRITE
DIRECT
instructions, does not have an
adverse
effect
upon
I/O
operations being performed by
the
MIOP
or
RMP
systems.
Refer to Xerox
publication
90 09
73
{Interface Design
Manual} for further
detai
Is
pertaining
to the external DIO
interface.
Also, refer
to
appropriate
peripheral
reference
manuals for
details
on control and
data
signals.
MULTIPLEXOR
INPUT/OUTPUT
PROCESSOR
(MIOP)
An
MIOP permits standard and commercially
available
I/O
devices
(e.
g.,
card
readers,
card
punches, magnetic
tape
units,
etc.)
to
be
controlled
primari
Iy
by individual
I/O
subchannels within
the
MIOP
and
associated
device
con-
trollers. Depending upon
the
number of
I/o
subchannels
assigned (maximum of
16, as described under
II
Device
Con-
trollers"),
an
equivalent
number of
I/O
operations may be
performed
si
mu
I taneously.
DEVICE
CONTROLLERS
All
I/O
devices
associated with
an
MIOP
are
connected
via
an appropriate
device
controller. Depending upon the
number and type of
I/o
devices to be
connected,
one or
142
Input/Output
Operati
ons
more
of
the following types
of
device
controllers may be
connected
to an MIOP:
1.
Single-unit
device
controller
{internal or external}.
2.
Multi-unit
device
controller
{internal
or
external}."
3.
Unit-record
controller
{internal or external}.
Generally,
an internal
device
controller
is physically
con-
nected
via
the
internal
I/o
interface.
An
external
device
controller
is
located
remotely to the
MIOP and may require one or more
separate
chassis to
ac-
commodate
it.
A
single-unit
device
controller
{internal or external}
is
specifically
designed to control
only
one
I/o
device,
usually a
unit-record
device
such as a card
reader,
a card
punch, or a line printer.
Characteristics
of
a
single-unit
device
controller
are
dependent
upon the
device
controlled.
(Refer to an appropriate peripheral
reference
manual for
further information. )
A multi
-unit
device
controller
(internal or
external)
is
specially
designed to control more than one
I/o
device,
where all the
I/O
devices
are
of
the same type
{e.
g.,
magnetic
tape
units or
RADs}.
However, only one
I/o
device
at
a time may be
actively
involved in a
data
trans-
fer
operation.
Characteristics
of
a
multi-unit
device
con-
troller
are
dependent
upon
the
I/O
devices
controlled.
For
example,
a multi
-unit
device
controller
for magnetic
tape
units may control up to
eight
units. (Refer to an appropriate
peripheral
reference
manual for further
information.)
Unit-record
controllers {internal or external}
are
designed
to control up to
eight
unit record type
of
I/O
devi ces (e. g. ,
card
readers;
card
PlJnches; line printers). AI!
I/o
de-
vices
attached
to a
unit-record
controller need not be
of
the same type. All
I/o
devices
attached
to a
unit-record
controller
may perform
separate
I/O
operations, including
data
transfers,
si
mu
I taneously.
The
number
of
device
controllers, as well as the number of
I/O
devices,
that
may be
connected
to an MIOP
is
depen-
dent
upon the following considerations:
1.
The maximum number
of
I/O
subchannels within an
MIOP
is
16.
2. Each
single-unit
device
controller
{internal or external}
requires one
I/o
subchannel.
3. Each
multi-unit
device
controller
(internal or external)
requires one of the first
eight
subchannels within
the
MIOP.
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