Switching Modes via Software
For situations where it is inconvenient to "strap" the modem
for V.25bis operation via the DIP switches, means is provided to switch
back and forth between V.25bis and AT command operation by software command.
The DIP switches must be in the factory default, UP UP position.
V.25bis requires DTR to be high during command dialog. When DTR is low,
the modem does not recognize V.25bis commands. Hayes modems recognize AT
commands when DTR is low, regardless of the setting of S94. Thus software
can switch modes by dropping DTR and resetting the modem via ATZ. (In V.25bis
mode, dropping DTR terminates any call in progress. In AT command mode,
this only happens if &D2 or &D3 is set.)
If the DIP switches are in any of the three V.25bis positions, or if the
"dumb strap" DIP switch is set to "dumb", the modem does
not recognize AT commands when DTR is low.
When the switches are set to UP UP, the operating mode after soft reset
is determined by S94. Thus, before issuing ATZ the user must reprogram S94
for the desired mode, and write the new setting to non-volatile memory.
Alternatively, if the user switches frequently between two modes (e.g.,
AT command and sync V.25bis with HDLC framing), they can set up one stored
profile for each mode and switch back and forth using AT commands ATZ0 and
ATZ1.
Stored profile 0 should always be configured for normal AT command operation.
This ensures that an ATZ command (issued with DTR low) always "recovers"
normal AT command operation.
This "hidden"AT command state (DTR low in V.25bis mode) is intended
for configuration commands such as Snnn, &Wn and Zn. Call-establishment
commands (D, A, O, H, etc.) are forbidden; they cause ERROR result codes.
All result codes are in normal AT command form (as influenced by the current
Q and V settings.) Even though some modem settings are overridden in V.25bis
mode (i.e., &Q, &D and &S), this does not affect the actual settings
of &Q, &D, &S etc. which the user "sees"during the hidden
AT command state.
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