IBM System/370 Manual page 66

Page of GC20-1730-0
J\dded 11/20/70
By TNL GN20-2277
Tape mode setting is handled as follows. ~-0r write operations on
nine-track tape units with the Dual Density feature, a mode set command
must be issued to establish 1600-BPI PE or SOO-BPI NRZI recording mode
prior to the first write. Tapes written in PE mode have a format
I
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 establishes 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 Dual 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 c0mmandG
Table 20.25.1. 3803 control unit configurations and capabilities with
Single Density, Dual Density, and Seven-Track features
, r l
-------------------------.------------------------.--~------------------------
3803 with Single
Density Feature
3803 with Dual
Density Feature
1. Nine-track, 1600- 1. Nine-Track, 1600-
BPI, PE tape on 3420 BPI, PE tape on 3420
Models 3, 5, and
7
Models 3, 5, and
7
with Single Density with Single Density
feature feature
2. Nine-track, 800-BPI
NRZI tapes and
nine-track, 1600-
BPI PE tapes on
3420 Models 3, 5,
7
with the Dual
Density feature.
3803 with Seven-Track
I
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-serie~;
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
~ U~
i 1
~
hi 1 i
t-,\1 .
- - - - - - - - - - - J , -
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
~ubsy~tem
availabilit.y. When a switch failure occurs in one control
unit, that. unit
can be switched offline,.
eliminating the necessity
of removing the en·tire tape switching subsystem from the operative
system configuration.
Using corebinat.iortF of the
1~:.:GIrmn.:.rL~cator Clnd the 'l'tvo--control Switch,
the Three-Control. Swi t.chO' or the
f'fJDr-Cont::::ol
Switch optional features,
two, three, or fClUe
COL'.!:'-i1
unit':; can be confiqu:teii to be switched
among 1].1) to 8
OJ:
tiP
~-;-.,
16
t.ape ~~rl:Lls.., ~:rtje (~()lf!l!~.llTlic~~i
LC'L
rH'..1st
be
preserlt
in all control
UE
i
c'"
tha"t~
aret:.o b'2
s~!i
tched.
~
It allows the control
51.5