IBM System/370 Guide

Page of GC20-1730-0
Added 11/20/70
By TNL GN20-2277
Two-Channel Switch. This optional feature provides switching
functions for tape units similar to those provided by the two-channel
switch for direct access devices. A 3803 control unit with the two-
channel switch installed can be attached to two channels that are in
the same system or in two different sytems. Tape units attached to
the 3803 can then be accessed via either channel. This feature can
be present on a 3803 that also has tape switching features installed.
A 3803 with this switch can be set to allow access to all its tapes
by either channel. one channel at a time. If channel A requests an
operation when the control unit is busy performing an operation on
channel B, then channel A must wait until the control unit becomes
available again. If both channels are on the same system, this
arrangement essentially provides two channel paths to the tape units
on the switched 3803.
A RESERVE CONTROL UNIT command is provided for use with this feature.
It permits a channel, via prograrnmming, to maintain exclusive use of
the control unit and its tapes until the RELEASE CONTROL UNIT command
is issued. These two commands are of benefit when the control unit
is shared between two systems.
Tape units on a 3803 with a two-channel switch can also be
partitioned. Partitioning is the manual assignment of tape units (via
switches) to one channel or the other so that access to each unit is
limited to one of the two channels. This facility can be used for
backup purposes to switch tapes from one system to another or from
one channel to another in the same system.
The two-channel switch for the 3803/3420 subsystem offers
configuration flexibility not generally available to 2400-series tape
unit users. A two-channel switch currently is provided only for a
2803 Modell control unit and can be used only in Model 67 and Model
65 multiprocessing configurations.
Reliability, availability, and serviceability features. The
3803/3420 hardware subsystem has several RAS features, in addition
to the reliability and availability features already discussed for
the tape media itself.
The 3803 control unit embodies a totally new design. The newest
monolithic logic technology is used in the 3803 control unit and,
therefore, it offers greater reliability and more compact physical
design in comparison to the 2803 control unit. (The 3803 is
approximately half the size of a 2803 control unit.) In addition,
both logic circuitry and mechanical components in the control and tape
units are functionally packaged to enable more rapid fault location
and faster replacement.
As a diagnostic aid, additional sense bytes are generated by the
microprogram-controlled 3803 control unit. The 3803 uses ROS for
microprogram residence. Twenty sense bytes are provided, instead of
the six generated by the 2803, certain of which can be used in tracing
control unit microprogram malfunctions. Some of the other additional
sense bytes identify the control unit and tape unit by serial number,
optional features, and engineering change (EC) level.
Two other very significant new serviceability features are
microdiagnostics resident in the 3803 control unit and radial attachment
of 3420 tape units to the 3803.
Resident microdiagnostics in the 3803 enhance test operations for
the 3803/3420 subsystem by relieving the CPU of the execution of most
time-dependent tests. Diagnostics in the 3803 are executed via use
of a diagnostic command issued by a program.
51.7

IBM System/370 Specifications

General

Brand	IBM
Model	System/370
Category	Server
Language	English