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This memo describes the configuration requirements for tapes
controlled by the DX20 in release 4. As a result of work
done for release 4, the limitations for connecting RH20s,
DX20s, TX02s, and TU7xs are the following:
1. There must always be only one path to a particular
tape unit at any time. This can be accomplished
either by not having enough hardware to allow two
paths, or by using the TX02 switches to disable
ports and drives until only one path exists. This
requirement is necessary because the tape units do
not have serial numbers, and the monitor therefore
cannot detect multiple access paths to them.
Failure to prevent multiple paths will result in the
same tape being used by two different users at the
same time. This restriction applies whether or not
the multiple paths occur from the same CPU or
2. Each TX02 connected to a single DX20 must be a
different unit to the DX20. This is settable by
field service. Failure to make each TX02 a distinct
unit number will prevent access to some of the
3. Since the tape system does not generate on-line
interrupts reliably, the operators must use the SET
ONLINE command of the OPR program before a new tape
path becomes useable.
Any configuration which meets the above requirements should
work in release 4. This means that the TX03 and TX05
options are fully supported, and multiple TX02's can be
connected to a single DX20.
Procedures to follow because of the above restrictions:
1. As a consequence of the unique path requirement, the
following rules about the TX02 switches can be
deduced as necessary and sufficient:
A. For the TX03 option, only one of the "A" or
"B" port select switches can be up.
Otherwise two different DX20s can talk to
the same drives.
B. For the TX05 option, for any particular tape
unit, only one of the unit select switches
can be up. Otherwise two different TX02s
can talk to the same drive.
3. The SET ONLINE command is non-trivial to use in a
complicated tape system. The syntax of that command
OPR> SET ONLINE channel-number , unit-number,
channel-number is which channel this tape is
connected to (IE, which RH20).
controller-number is which DX20 the tape is
connected to. (IE, which unit of the RH20
the DX20 is, which is usually unit 0).
unit-number is which tape unit on the DX20
is to be set on-line. This number will
select the path that has been made
available. (See below).
The unit number of a tape on the DX20 is an 8 bit field,
split up into three separate sections. The fields look
! ! use ! !
! TX02 unit number ! TX05 ! tape unit of TX02 !
! ! switch ! !
bits 7 4 3 2 0
"TX02 unit number" is which TX02 of the DX20 is used. Since
most tape systems have only 1 TX02 connected to a particular
DX20, this field is usually zero.
"Use TX05 switch" is set if the tape belongs to the "other"
TX02 when using the TX05 option. If not set, the tape unit
belongs to the TX02 unit as described above. If the TX05
option is not used, this bit will be zero.
"Tape unit of TX02" is which tape drive of the selected TX02
1. The system has the DX20 as the first unit of channel
3, and the DX20 is only running one TX02 as unit 0,
and the TX02 has 3 drives. To set on-line the first
tape drive, you type:
OPR>SET ONLINE 3,0,0
to mean channel 3, unit 0 (TX02 unit 0, and no TX05
switch), and DX20 unit 0.
2. The system has DX20's on channels 3 and 4, and each
DX20 is connected to the same two TX02 units, one
for each port. The TX02 unit numbers are 0 and 1.
Each TX02 has all 8 drives. The first TX02 has both
port "A" and port "B" deselected, so that all tape
accesses must occur through the TX02 unit 1.
Furthermore, port "A" of the second TX02 is also
deselected, so that all accesses can only come
through the second DX20. Finally, all of the unit
select switches on each TX02 are on, so all tapes
are accessable. Then, to set online tape unit 4 on
the first TX02, you type:
OPR>SET ONLINE 4,34,0
which means channel 4 for the second DX20, use TX02
unit 1, use the TX05 switch, and then use unit 4.
Here you see that:
34 = 4 + 1*10 + 1*20
^ ^ ^
! ! !
! ! TX02 unit number
! use TX05 switch
unit of TX02
APPENDIX: Release 3A Requirements
Release 3A of TOPS-20 supported TX02s to a limited extent.
Differences between release 3A and 4 are the following:
1. Each DX20 must have a unit 0 in existance. This
means that a TX02 must exist which is unit 0, and a
tape unit 0 must exist on that TX02. This
requirement exists only for the startup of the DX20
microcode, which will halt if unit 0 does not
2. The SET ONLINE command does not exist. Therefore
the only tapes which can be accessed by a system
are those which are accessable when the system is
started. So when a system is first brought up, the
switches on the TX02 must be set to allow access to
all of the tape drives. Then once the system is
running, the switches can be put back to disallow
3. When a system is restarted or the DX20 microcode is
reloaded, the switches on the proper TX02 must be
set to allow access to tape unit 0. Once the DX20
microcode is running, the switches can be disabled
again. The DX20 microcode will not interfere with
any other use of the tapes when it is restarted.
Note: The above two rules contradict the rule
concerning enabling port "A" and "B" at the same
time, since for a small time there are two
different paths to the tape drives.
4. Only one TX02 can be connected to a DX20, and its
unit number must be 0.
5. The TX05 option can not be used.