Fax Operation
9-4
V.3600
The Class 1 standard assumes a serial asynchronous DTE-DCE
connection 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
(hardware 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 19,200 bps should be used with the
modem. Since flow control is used, this rate should not be changed
during fax operation. 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, T.30 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 modifiers, 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 on-line
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.
