RC224ATL/224ATLV
6.0 Operation
EmbeddedModem Family
6.3 Fax Enhanced Flow Control
6.3 Fax Enhanced Flow Control
The fax Class 1 standard provides for bidirectional X-on/X-off flow control.
However, in multitasking “windowing” environments, the DTE communication
package can be suspended long enough to miss a 19,200 bps serial asynchronous
character sent by the DCE (520.83 µs). X-on/X-off flow control will not solve this
problem because the DTE software does not always know when it is going to be
suspended. To solve this problem, a new enhanced flow control mechanism is
provided. This mechanism is enabled/disabled by issuing an AT+FF=1/0
command.
6.3.1 Parallel/Serial Interface Selection
For parallel host interface operation, the enhanced flow control takes advantage of
the fact that the DCE hardware knows if the DTE has read the last character. The
DCE will not send a new character until the DTE reads the previous one. The
modem fax receiver has an internal 223-byte buffer. Therefore, the DTE can stop
reading characters for 371 ms (4800 bps receive) before any data is lost.
For serial interface operation, the enhanced flow control mechanism utilizes
the fact that the Fax Class 1 serial interface is only required to operate half
duplex. The scheme involves sending characters to the DTE and waiting for the
DTE to send an acknowledgment (if the DTE reads the character OK) or a request
for re-transmission (if the DTE misses a character and detects an overrun). The
required DTE operation using the enhanced flow control mechanism is defined
below.
6.3.2 Fax V.42 Buffer Sizes
Fax V.42 buffer sizes are shown below.
Buffer Type
Size in Bytes
Fax V.27/V.29 Tx
Fax V.21 (HDLC) Tx
RPI (HDLC) Tx
255 (9600:212 ms)
255 (300:6.8 sec)
238 (2400:793 ms)
223 (4800:371 ms)
255 (300:6.8 sec)
Fax V.27 Rx
Fax V.21 (HDLC) Rx
D224ATLVDSC
Conexant
6-7
CONEXANT [ CONEXANT SYSTEMS, INC ]
