V.25bis Options

Since ITU-T V.25bis does not support autobaud recognition, the Hayes AT Command Set includes a feature to select the interface speed, S93. This register is stored in non-volatile memory; its numerical values are the same as S37. The factory default for this S-register is the highest modulation speed of the modem. To change this parameter the user must switch back to AT command mode.

If using sync V.25bis, and stored profile has &X0 or &X2, the modem generates transmit and receive clock according to S93 setting (in sync mode, the modem cannot operate slower than 110 bits/s; if 75 bits/s selected, actual speed is 150 bits/s). If &X1, then transmit and receive clock are supplied by the user on pin 24.

Carrier speed negotiation (S37 and the N command) in V.25bis mode. The same rules are followed as in AT command mode, except that the setting of S93 holds the "last AT autobaud speed."

Since V.25bis does not provide parameter-setting commands (other than storing telephone numbers), the usual stored settings are used. This applies to behaviors which are not specified in V.25bis: if a stored setting conflicts with V.25bis, V.25bis wins. As with the interface speed S-register, the user must switch back to AT command mode to change any of the stored settings.

The &Qn, &Dn, &Sn, and S0 settings are ignored in V.25bis mode which does not affect their values stored in non-volatile memory. It also does not affect the values of the "active profile" visible in the "hidden" AT command state (DTR low).

In async V.25bis, the modem behaves as it currently does in &Q5 mode (regardless of the stored &Q setting). That is, it attempts feature negotiation and, if successful, brings up error-control protocol. If feature negotiation fails, the modem observes the fallback options set in S36. If S36 selects fallback to non-buffered async mode, and the connect speed is different than the DTE interface speed programmed in S93, the call is disconnected. The user can disable feature negotiation and/or error-control by switching back to AT command mode and changing the appropriate S-registers. For example, S48=128 and S36=3 force ASB operation (just like &Q6).

In sync V.25bis, the modem behaves as it does in &Q1 mode, regardless of the stored &Q setting, with the synchronous V.25bis command interface replacing usual AT command state.

Note: There is no Hayes Escape Sequence with Guard Time in V.25bis mode. In this mode, one must use DTR to change states as described above.

Note: AutoSync is not available in V.25bis mode.

Additional bits in S94 select ASCII or EBCDIC character set for V.25bis commands and responses, and NRZ or NRZI encoding. Here are the bit definitions:

Add this to S94:     To get:        Otherwise it will be:       

8                    EBCDIC         ASCII                       

16                   NRZI           NRZ

These two bits in S94 are ALWAYS operative when the modem is in V.25bis mode, even when the lower 3 bits have been overridden by the DIP switches. Note that NRZ/NRZI encoding only pertains to synchronous mode. The character set option (ASCII/EBCDIC) pertains to both synchronous and asynchronous modes.

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