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66 536EX Chipset Developer’s Manual
Intel Confidential
Fax Class 1 AT Commands
When the modem transfers data to the DTE, the DTE must filter the data stream by removing all
character pairs beginning with
<DLE>. The DTE also must recognize <DLE><ETX> as the stream
terminator. Upon detecting
<DLE><DLE>, the DTE must reinsert a single <DLE> in its place. The
modem buffers up to 512 bytes of fax data received from the remote modem if the DTE cannot
read the data immediately (AT+FRM command).
For the AT+FTH and AT+FRH commands, data is sent and received using HDLC formatting. The
format for HDLC framing is provided below. When sending the HDLC data, the modem
automatically generates the flags and the frame checking sequence (Frame Check Sum). All the
other fields are transmitted to or received from the DTE (that is, these fields are under the control
of the DTE).
After receiving a frame correctly using the AT+FRH command (FCS is OK), the DCE returns an
‘OK’ message to the DTE. If the frame is received in error (FCS is not OK, carrier is lost, or data is
lost due to data overflow), the DCE returns an ‘ERROR’ message to the DTE; the DTE should then
discard the frame.
Note: If the telephone line is on-hook, the modem responds back to the DTE with an ‘ERROR’ message
whenever any of the fax AT commands, except the +FCLASS command, are sent to the modem.
Like the data modem mode, the DTE needs to issue an ATD string or ATA command to the modem
to originate or answer a fax call. Unlike the data modem mode, auto-answering is not supported by
the modem while in fax mode. It is recommended that S-register S0 be set to ‘0’ (that is, use ‘ATA’)
whenever the modem is expected to receive a fax call.
Figure 8. T.30 HDLC Frame Format
FlagFlag
Address Control
Facsimile
Control
Facsimile
Information
Frame
Checking
Sequence
Flag
FieldField
Field Field
(FCS)
Figure 9. CLASS 1 DTE-Generated HDLC Frame Information (AT+FTH=<mod>)
Figure 10. CLASS 1 DTE Reception of HDLC Frame Information (AT+FRH=<mod>)
Address Control
Facsimile
Control
Facsimile
Information
FieldField
Field Field
Address Control
Facsimile
Control
Facsimile
Information
FieldField
Field Field
Frame
Checking
Sequence
(FCS)