Telenetics 56 - Manual (Page 147)

AT Commands
Page 139
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 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