Fax Data Transmission - AOpen FM56 Manual

proceeds as described previously.
If carrier is lost while in command mode and the DTE reissues the +FRH command with
the same parameter, and there is no data in the buffer, the modem sends the ERROR
result code to the DTE and returns to the command mode. If there is data in the buffer,
the modem sends the next frame of buffered data to the DTE, followed by <DLE><ETX>,
and either the ERROR result code if errors were detected or the OK result code if no
errors were detected. The modem then returns to command mode.
The modem concludes an error is detected if carrier is lost for any period of time during
or before the reception of a frame. If carrier is lost for a time period longer than the time
specified by the register S10, the modem finishes delivering the data in the receive
buffer (if any) to the DTE, sends <DLE><ETX>, generates the ERROR message, and
returns to command mode. All subsequent data received from the remote is discarded.
If the modem detects a frame abort sequence (seven consecutive ones with no zero
insertion) while it is waiting for a frame beginning flag (it was receiving HDLC flags), it
will wait for the HDLC flags again until either carrier is lost, or the DTE aborts the
process by sending an abort character or by dropping DTR with &D2 in effect. If the
frame abort sequence is detected while the modem is receiving a frame it finishes
delivering the data in the receive buffer (if any) to the DTE, sends <DLE><ETX>,
generates the ERROR message, and returns to command mode. The modem keeps
looking for HDLC flags
followed by more data from the remote, with the selected modulation scheme.
If the modem detects a receive buffer overflow condition, it concludes that there was an
FCS error in that frame. The modem will receive more frames only if a starting flag is
detected and there is room in the receive buffer. If a starting flag is detected and there is
no room in the buffer, the modem discards all data in that frame.
If the modem receives any character from the DTE after the +FRH command (except
flow control characters if software flow control is in effect), or if the modem detects a
high-to-low transition of the DTR signal while &D1 is in effect, it sends <DLE><ETX> to
the DTE, generates OK result code, and returns to command mode. The receive buffer
is cleared and all data received from the remote is discarded. If the modem detects a
DTR drop while &D2 is in effect, it goes on-hook, sends <DLE><ETX> to the DTE,
generates OK result code, and returns to command mode. If the modem detects a DTR
drop while &D3 is in effect, the modem performs a warm reset.

4.1.8 Fax Data Transmission

Fax data transmission is initiated by the AT+FTM=<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 constant 1
bits for a minimum of one second and continues to transmit constant 1 bits until it
receives a character from the DTE.
When data is received from the DTE, the modem deletes start and stop bits and deletes
all DLE><chr> character pairs before transmitting the data to the remote. Each
<DLE><DLE> sequence is transmitted as a single <DLE>. <DLE><ETX> is considered
as the end of stream marker, and is not transmitted. All data received from the DTE after
the <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
last transmitted character was not an ASCII NULL, the modem generates the OK result
code and returns to the command mode. If the last character transmitted was an ASCII
NULL, the modem generates the CONNECT message to the DTE and waits for further
data from the DTE while transmitting NULLs to the remote. If more data is received
before five seconds elapse, the modem continues to transmit the data as described in
the previous paragraph. If five seconds elapse and no data is received from the DTE, the