IBM System/370 145 Manual page 124

Tape mode setting is band1ed as fo11ows. For write operations on
nine-track tape units with the Dua1 Density feature, a mode set command
must be issued to establish 1600-BPI PE or 800-BPI NRZI recording mode
prior to the first write. Tapes written in PE mode have a format
identification burst recorded at load point that differentiates them
from NRZI mode tapes. During reading, sensing of this burst
automatically puts the tape unit in PE mode. Failure to sense the burst
estab1ishes NRZI mode if both the tape unit and control unit have the
Dual Density feature. If an attempt is made to read NRZI-mode tape on a
unit without the Dua1 Density feature, an error indication results.
Once PE or NRZI mode is established for read operations, it is retained
until the tape returns to load point.
For seven-track read and write operations, NRZI mode, density,
parity, and use of the data converter or translator are established by
issuing a single MODE SET command.
Table 20.25.1. 3803 control unit configurations and capabilities with
Single -Density, Dual Density and Seven Track features
3803 with Single
Density Feature
1. Nine-track, 1600-
BPI PE tape on 3420
Models 3, 5, and
7
with Single Density
feature
3803 with Dual
Density Feature
1. Nine-Track. 1600-
BPI PE tape on 3420
Models 3, 5. and 7
with Single Density
feature
2. Nine-track, 800-BPI
NRZI tapes and
nine-track. 1600-
BPI PE tapes on
3420 Models 3. 5, and
7 with the Dual
Density feature.
3803 with Seven Track Feature
(includes data convert and
translate>
1. Nine-Track, 1600-
BPI PE tape on 3420
Models 3, 5, and
7
with Single Density
feature
2.
Seven-track, 556/800-
BPI, NRZI BCD and
binary tapes on 3420
Models 3, 5, and
7
with
the Seven Track
feature,.
Note: The Single Density, Dual Density. and Seven Track features are
mutually exclusive on the same control unit or the same tape
unit.
Tape-switching features. Tape subsystem configuration flexibility is
provided by field installable tape-switching options that permit up to
four control units to
be
switched among up to 16 tape units. While this
capability is provided for 2400-series tape units via the 2816 switching
Unit, tape switching for the 3803/3420 subsystem offers the advantages
of compact design. reduced cost, and enhanced subsystem availability.
The switching features are built into the 3803 control unit itself so
that space for stand-alone switching units is not required,. The fact
that tape-switching features are contained in the 3803 control units
being switched (rather than in one unit) also enhances tape subsystem
availability. When a switch failure occurs in one control unit, that
unit can be switChed offline. eliminating the necessity of removing the
entire tape-switching subsystem from the operative system configuration.
Using combinations of the Communicator and the Two-Control Switch,
the Three-Control Switch, or the Four-Control Switch optional features,
two, three. or four control units can
be
configured to be switched among
up to 8 or up to 16 tape units. The Communicator must be present in all
control units that are
to be
switched. It allows the control unit in
which it is installed to address tape units that are attached to an
interconnected control unit. Figure 20.25.1 shows the switching feature
114 A Guide to the IBM System/370 Model 145