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At the terminal, automatic answering and disconnecting
automatically originated calls depends upon the terminal's
-ability to control the 'data terminal ready' interface line.
Such control is available on the second interface of termi·
nals with dual communications interface facilities (a 2780
with Feature #3401, a 2020 sub-model 2 or 4 with
RPQ M58081, or a 2020 sub-model 5 with RPQ E71851),
except that disconnection is manual. Full switched net-
work capability is available for terminals with reloadable
program-controlled communications adapters (a 5406,
a 5410, or a 2020 sub-model 2, 4, 5, or 6). Other terminals
must answer and be disconnected manually; they cannot
answer automatically originated calls.
Program Considerations and Program-Operator
Communication
Switched network backup operation may require additional
communication between the operator and the OS/DOS BT AM
program. The development of clear, detailed sets of instruc-
tions for individual CPU and terminal operators will make
apparent the need for additional operator-program com-
munication. See "Standby Changeover and Operation",
following, for guidance in developing instructions and
procedures.
Messages between the operator and the program may be
implemented by using WTOR (OS) or PUTs and GETs to the
CPU console (DOS).
Note: When using GET, operator response must be prompt
to avoid placing the entire "TP partition" in a "wait"
for a console reply.
The CPU program should facilitate:
• Recording any error statistics suggesting that SNBU
operation might be advantageous. Threshold counters in
the LERB macro, user error counters with branching to
an operator alert, or a high retry count with the hex
'41' return code as the operator alert, may be used.
• Closing and reopening a line that has excessive errors.
• Opening a switched network backup line and rerouting
the data.
• Changing the polling or addressing list and providing a
delay between the polling or addressing cycles when the
list has been reduced to one terminal. The operator must
be notified when communications with a terminal have
been completed, so that he can place a different terminal
on the list.
STANDBY CHANGEOVER AND OPERATION
System planning and implementation (see "Standby Config-
uration Planning", previously discussed) may require certain
changes in equipment and programming to prepare for an
SNBU operation. Aside from these changes, connecting and
disconnecting the switched network backup call are the main
C-4
IBM 3872 Modem User's Guide
operating considerations. Any call placed through a telephone
operator should be identified as a data call, to preclude in-
tervention or unexpected disconnection. Connecting and
disconnecting procedures for each valid configuration of
switched network backup operation are described in the
following paragraphs.
Note:
If SNBU data transfer appears to be unsatisfactory,
the operators should disconnect and try again. The 1200
switched mode may be used as an alternative; if it is, both
3872 Modems must be set to 1200 SWITCHED.
Point-to-Point Backup
In addition to making planned cable or program changes,
using the 3872 Modem SNBU feature requires setting both
3872 Modems to 2400 SWITCHED or both to 1200
SWITCHED.
Manual Answering (CDT DAA) at Both Ends
To Connect:
1. CPU operator closes the line in the program.
2. CPU operator switches the 3872 to TALK and calls
the terminal CDT/3872 number, using the telephone
attached to the CDT DAA at the CPU.
3. Terminal operator switches the 3872 to TALK, answers
the telephone, and the operators arrange for SNBU
operation and data transfer. The 3872 Modems are both
set to 2400 SWITCHED.
4. Terminal operator pulls up the data key on the CDT
telephone cradle, makes the terminal ready, and
switches the 3872 to DATA.
5. CPU operator pulls up the data key on the CDT tele-
phone cradle, switches the 3872 to DATA, and opens
the line in the program.
6. CPU operator enters any CPU console message neces-
sary to initiate data transfer.
Note:
If the terminal is to transmit first, the sequence
of the foregoing steps is: 1, 2, 3, 5, 6, 4.
To Disconnect:
1. By prearranged recognition of completed data transfer
or by separate phone call, the operators agree to dis-
connect.
2. CPU operator closes the line in the program.
3. Both operators cradle (hang up) the telephones
attached to the CDTs.
Manual Answering (CDT DAA) at Terminal and Automatic
Answering (CBS Data Coupler) at CPU
Control of the 'data terminal ready' interface line must be
effected at the CPU. The program data path may have to be
changed, and the interface cable to the 3872 Modem may
have to be switched (Figure C-3).
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