Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 225)

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4. Issuing ATO to modem will start the V.32bis handshake. The host then waits for the CONNECT message before
sending data. (For an originating modem, the ATO command must be sent as soon as possible so the modem
will be ready when the remote answering modem starts. For an answering modem, the ATO should be delayed
a little to give the remote originating modem a chance to get ready first.)
5. The host disconnects the V.32bis session (using either +++ followed by ATH, or with DTR, if it has previously
been enabled with AT&D2).
6. The host issues AT+VNH=2 to the modem again to maintain off-hook status for next cleardown.
7. AT\N0 removes error-correction, if it was used during the V.32bis period.
8. SIA protocol communication resumes by starting up the transmitter in Bell 103, SIA mode with AT+F0.
8.1.1.3. Considerations when Disconnecting the Session
Because AT+VNH=2 keeps the modem off-hook during a cleardown, the first ATH or DTR desertion will not put
modem back on-hook but only force AT+VNH=0 (per V.253). The host must issue a second ATH to put the modem
back in on-hook idle state. Alternatively the host could issue a AT+VNH=0, then ATH (or AT+VNH=0;H)
Table 142 lists the AT Commands provided to support SIA Level-3 Protocol communication.
Table 142. AT Commands Provided to Support SIA Level-3 Protocol Communication
AT Command Modem Function Remarks
AT+F0 Enable B103 Transmitter for Data Block
transmission. Modem turns on FSK transmitter and starts marking
for the minimum duration required by standard.
Sends CONNECT message to DTE when that is
accomplished.
Modem will then interpret the first character from
DTE as Block Header and use the byte count to
allow transmission of the specified number of char-
acters to the remote modem and then send OK
message to DTE to indicate ready for next AT com-
mand.
Note1: FSK transmitter remains on. DTE is
expected to issue either a +F2 (detect ACK/NACK)
or another +F0 (to send another data block) com-
mand to the modem.
Note2: The “+++” escape sequence can be used to
abort the data block transmission and return the
modem to AT command mode.
Note3: The +F0 command can be used to resume
SIA protocol communication after a voice listen-in or
V.32bis interruption.