Tape Switching Configurations For The 3803/3420 Magnetic Tape Subsystem - IBM System/370 145 Manual

requirements for permissible swit,ching combinations. The switch
combinations shown for switching control units among up to 16 tape units
are the same that are required for switching control units among up to
eight tape units.
3803
Basic control
1 X 8 unit with no
switching
3803
Communicator
2 X
16
Two-Control
Switch
3803
Communicator
3 X 16
Three-Control
Switch
3803
Communicator
4 X
16
Four-Control
Switch
3803
3803
Communicator
Two-Control
Switch
3803
Communicator
Three-Contro I
Switch
Communicator
Four-Control
Switch
3803
Communicator
3803
3803
Communicator
Communicator
Figure 20.25.1. Tape-switching configurations for the 3803/3420
Magneti.c Tape Subsystem
A
two-control-unit switching configuration is required to replace the
2804 and 2404 read-while-write control units. The advantage of the
tape-switching approach is that for a small price increment better
performance is possible. This is true because any two tape operations
can be active concurrently in a sli,itched configuration (including two
reads or two writes>, while the d~~gree of simultaneity achieved using a
read-while-write control unit is application dependent. That is, the
application must lend itself to rE!ading, then writing (or vice versa).
Two-Channel switch.
A
3803 control unit with the two-channel switch
installed can be attached to two (:hannels in the same system or in two
A
Guide to the
IBM
'System/370 ModE~l
145
115