Synxcom SM9234D, User Manual

Page: 152 / 216

AT Commands
Page 152
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 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