Input/Output Operations; External Dio Interface; Multiplexor Input/Output Processor (Miop) - Xerox 550 Reference Manual

4. INPUT/OUTPUT OPERATIONS
To accommodate the variety and number of I/o devices
which may be required for scientific and commercial ap-
plications, a Xerox 550 computer system may include
the following: External Direct Input/Output (DIO) in-
terface, and Multiplexor Input/Output Processors (MIOPs).
EXTERNAL 010 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 tC' one word (32 bits) 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 stabl e unti I the
addressed I/O device furnishes the data accompanied with
an acknowledgment signal. 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.
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 detai Is 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 primarily 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 simultaneously.
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
more of the following types of device controllers may be
connected to (.!n MIOP:
1. Single-unit device controller {internal or external}.
2. Mu I ti -un it dev ice con tro II er (i n terna I or externa I).
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 devices {e. g.,
card readers, card punches, line printers}. All 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, simultaneously.
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.
Input/Output Operations 125