10 Hostmode
142
This command is only used by the hostmode program. The user cannot enter this
command.
10.4.22
@F
The hostmode command @F allows to setup FAX reception under hostmode control.
Receveiving FAX images through the hostmode interface means that the data transfer
between modem and PC is highly buffered and error-corrected (CRC hostmode). Thus
“Skew” or“ “jumps” within FAX images caused by data loss on the interface side (high
PC operating system latency, etc.) can be completely avoided. Besides that, the @F
command eases incorporating FAX reception to application software that mainly builds
on the hostmode.
Default setting: none
Parameter: @F
(no parameter) Clears the FAX data buffer, see below.
@F0 Disables hostmode FAX reception and switches back to
normal PACTOR Standby operation.
@F1 Activates hostmode FAX reception in FM-FAX, sampling
rate = baudrate/32.
@F17 Activates hostmode FAX reception in FM-FAX, sampling
rate = baudrate/16.
@F2 Activates hostmode FAX reception in AM-FAX, sampling
rate = baudrate/32.
@F18 Activates hostmode FAX reception in AM-FAX, sampling
rate = baudrate/16.
If hostmode fax reception is activated, 8 bit per sample (grey scale) FAX data is available
on hostmode channel 252. The data field length of the hostmode packets on channel 252
is always 256. The sampling rate (number of 8 bit samples, i.e. pixels, per second) is
calculated as baudrate on the serial user interface (COM port) divided by 32 or 16. The
baudrate on the serial user interface of the modem should be chosen as high as possible
(higher than 57600 Bd) to achieve a good FAX resolution. For example, 115200 Bd on
the serial port yield a FAX sampling rate of 3600 or 7200 pixel/sec, respectively. If the
baudrate is equal or less than 2400, the sampling rate is set to a constant value of 75
samples/sec, however FAX reception at baudrates below 19200 Bd generally does not
make much sense because the image signal then suffers from heavy undersampling.
The hostmode FAX demodulators (AM/FM) have the same properties as the
corresponding FAX demodulators in terminal mode (
fax:-
menu). All parameters
accessible through the
fax:-
menu which have an effect on the FAX demodulator remain
also valid for hostmode FAX reception. Any change of the FAX parameters from the
default settings should be performed unter terminal mode control (fax: menu) prior to
starting the hostmode.
All FAX receive data is buffered through the 4096 samples long FAX data buffer (see
@F command without argument). If a buffer overrun occurs (frequency of data polls by
the PC program too low), the buffer is automatically cleared again (4096 samples are
lost). Besides that, the FAX buffer is always cleared automatically when hostmode FAX
reception is activated.
