Fax Answering; Fax Control Transmission; Fax Control Reception - AOpen FM56 Manual

CONNECT message to the DTE when FSK flags are detected from the remote.

4.1.5 Fax Answering

Answering of fax calls is identical to answering of data calls with the exception that the
modem enters the fax handshaking mode instead of the data handshaking mode after
going off-hook. If +FAE=0, the modem, after sending answer tone, automatically enters
fax mode (+FTH=3), sends the CONNECT message to the DTE, and starts sending FSK
flags. If +FAE=1, the modem determines whether the caller is a data modem or fax
modem and sends the DATA or FAX result code, respectively, to the DTE.

4.1.6 Fax Control Transmission

Fax control transmission is initiated by the +FTH=<mod> command. After this command
is issued, the modem generates the CONNECT message and transmits carrier in the
modulation scheme specified by the parameter n. The modem then transmits HDLC
flags for a minimum of 1 second. The modem continues to transmit the HDLC flags until
it receives a character from the DTE.
When characters are received from the DTE, the modem adds start and end flags,
performs zero-bit insertion, generates FCS, and deletes <DLE><chr> character pairs
before transmitting the characters to the remote fax machine. Each <DLE><DLE>
sequence is transmitted as a single <DLE>. <DLE><ETX> is considered as the end of
frame marker and is not transmitted. All data received from the DTE after <DLE><ETX>
is ignored by the modem until the modem generates either the CONNECT, OK, or
ERROR result code.
If no more data is in the transmit buffer and the final bit was a 1 (bit 4 of the second byte
received from the DTE), the modem generates the OK result code and returns to the
command mode. If the final bit was a 0, the modem generates the CONNECT message
and waits for further data from the DTE while transmitting HDLC flags. If no more data is
received before 5 seconds elapse, the modem drops carrier, goes on-hook, and
generates the ERROR result code.

4.1.7 Fax Control Reception

Fax control reception is initiated using the AT+FRH=<mod> command. After this
command is issued, the modem looks for carrier in the modulation scheme specified by
the parameter n. If no carrier is detected before the period of time specified by register
S7 expires, the modem generates the NO CARRIER message and returns to command
mode. If a carrier is detected that is not the one specified by the parameter n, the
modem generates the +FCERROR message and returns to the command mode. If the
specified carrier is detected, the modem generates the CONNECT message and enters
the HDLC receive mode.
In HDLC receive mode, the modem receives HDLC frames from the remote fax machine,
strips the flags, performs zero-bit deletion, performs error checking, and handles
<DLE><chr> character pairs before passing the data to the DTE. The modem prefixes
each <DLE> character with another <DLE> character before sending it to the DTE. After
the last byte in the frame, the modem sends <DLE><ETX> to the DTE marking the end
of the frame. The modem then generates either the OK message if no errors were
detected or the ERROR message if errors were detected (FCS is incorrect), and returns
to
command mode.
While in command mode, the modem continues to receive data in the selected
modulation scheme, and sends the data after sending the CONNECT message to the
DTE when the DTE reissues the +FRH command with the same parameter. If the DTE
issues the +FRH command with a different parameter, the modem clears all buffers and