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 modem drops carrier, goes on-hook, and generates the ERROR result code.
4.1.9 Fax Data Reception
Fax data reception is initiated using the AT+FRM=<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
