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Two-Channel Switch
Device Switch
This feature permits the 3803 Tape Control to be accessed by a second channel.
(See Figure 4.) Alternate path switching between two channels attached to the
same system is under program control. The two-channel switch can also be used
to share a tape control between two systems. Unless the two systems are
software coupled, the tape control should be offline to one of the systems at all
times. Partitioning between channels attached to two different systems is done
manually.
Two-Channel Switch Feature
Channel 1
Channel 2
Figure 4. Two-Channel Switch
Two Systems Sharing a Tape Control
Through the Use of a Two-Channel
Switch
0--------------7
Tape Units
Tape subsystem configuration flexibility is provided by field installable switching
features that allow up to 16 tape units to be switched among two to four tape
controls. 3803 Models 1 and 2 can be mixed in a switching configuration;
however, attempting to access a 3420 Model 4, 6, or 8 through a 3803 Model 1
is not allowed and will produce unpredictable results.
The three device switching features available with the tape subsystem are:
2 Control Switch (2 x 8 and 2 x 16 configurations)
•
3 Control Switch (3 x 8 and 3 x 16 configurations)
•
4 Control Switch ( 4 x 8 and 4 x 16 configurations)
The location of the device switches depends on the configuration desired. For
example: In a 2 x 8, 3 x 8, or 4 x 8 configuration, the switching feature is
required only on Tape Control 1, while in the 2 x 16, 3 x 16, or 4 x 16
configurations the switching feature is required on Tape Controls 1 and 2 (see
Figure 5).
Each device switch feature requires a Communicator. A Communicator sends
tape unit selection and device interface signals to one of two device switches,
depending on whether tape units 0-7 or 8-F are being addressed.
The Communicator replaces the selection logic circuits and associated device
interface cabling in the basic tape control with different logic circuits and cabling
to the device switches.
16
3803-2/3420 Subsystem Description
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