Fax Operation
9-4
V.3400
The Class 1 standard assumes a serial asynchronous DTE-DCE connec-
tion using EIA-232D (or CCITT V.24) circuits. EIA-232D circuits AB
(signal ground), BA (transmitted data), and BB (received data) are
required for fax operation, while additional circuits are optional. Refer
to Chapter 2 for hardware installation and EIA-232D data interface
information.
The modem uses inband unidirectional DC1/DC3 (XON/XOFF) flow
control to match the DTE-DCE data rate to the line signaling rate (hard-
ware RTS/CTS is optional). Since 1200 bps is the minimum rate needed
to support T.30 Phase C data transfer using V.29 9600 bps, a serial port
data rate of at least 19200 bps should be used with the modem. Since
flow control is used, this rate should not be changed during fax opera-
tion. The modem detects the DTE-DCE data rate from the AT command
prefix as described in Chapter 5.
CLASS 1 COMMANDS
The commands in this section are normally issued by the fax software
and not by the user. They are provided here for reference. While only
the lower order 7 bits of each character are used for commands and
parameters, Phase C data transmission or reception requires all 8 data
bits. Therefore, a data format which uses 8 data bits should be used for
fax mode.
Dial Command D
The dial command D, described in Chapter 5, tells the modem to go off
hook, dial, and begin call origination. For fax operation, set the modem
for Service Class 1 operation before the dial command is issued; use the
+FCLASS=1 command.
The dial modifier, except R, can also be used in fax mode. Depending
on the X option selected, the modem will return the BUSY and NO
DIALTONE result codes as appropriate. If the modem is in the online
command state when a dial command is issued (e.g., a connection has
already been established), the modem returns an ERROR result code.
The NO CARRIER result code indicates that a connection could not be
established (no carrier detected) within the number of seconds specified
in status register S7, or the dial command has been aborted due to a
character sent from the DTE.
