IBM System/370 145 Manual page 126

different systems. This featore can be present on a 3803 that also has
tape-switching features installed.
When the two-channel switch is installed on a 3803, the switch can be
set to permit only one channel or the other to access the control unit
and its tapes, or the switch can be set to allow access to the control
unit and its tapes by both channels, one channel at a time. In the
latter case, if channel A requests an operation when the control unit is
busy performing an operation on channel B, channel A must wait until the
control unit becomes available again.
The two-channel switch can be used to connect a 3803 to two channels
in the same system to provide the capabi.lity of switching the tapes from
one channel to another or to provide two channel paths to the tapes
connected to the 3803.
Similarly, the two-channel switch can
be
used to connect a 3803 to
two Channels in two different systems to provide the capability of
switching all the tapes from one system to the other, for backup
purposes. Alternatively, the 3803 can be set to allow access to it
by
both systems. In this case, each tape connected to the 3803 can be
accessed by only one
cpu.
Those that are to be accessed by CPU A must
be varied offline to
cpu
B, and vice versa, by the operator.
Partitioning of
the
tapes in a two-system environment is strictly the
responsibility of the operator. There are no physical controls on a
3803 with a two-channel switch-that provide for partitioning the tapes
between the two CPU's, nor is there any programming systems support that
checks whether or not a tape is enabled to both systems.
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
Model
1
control unit and can
be
used only in Model 67 and in Model 65
mUltiprocessing configurations.
Data security erase command. When a data security erase command is
issued, the tape unit selected erases tape from the point at which the
operation is initiated until the tape indicate (end-of-file) marker is
sensed. This command is not provided for 2400-series tape drives.
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 it
therefore offers greater reliability and more compact physical design in
comparison with 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-four 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
(Ee)
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.
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A Guide to the IBM System/370 Model 145