Sierra Wireless DART 200 CDPD Modem - Manual (Page 111)

DART 200 CDPD Modem User’s Guide 8 Application Programming
PN1197-00 Revision 1.0 8-17
Flow control considerations
Software Flow Control cannot be used. Instead, Hardware Flow Control
(\Q2) (default), or no flow control (\Q0) must be used because in a
binary data stream any byte can inadvertently have the same bit
configuration as the XON or XOFF control characters. In addition to the
device receiving unexpected flow control, the data character is stripped
from the data stream, causing unpredictable application results.
Data forwarding considerations
Manual data forwarding cannot be used. This feature is controlled by the
AT\M command; the default is \M0 (disabled). Use automatic data
forwarding for binary data transfers. This feature is controlled by the
AT\T command; the default is \T1 (enabled). In the automatic data
forwarding mode, data is forwarded based on occurrence of an
intercharacter time-out (ICT). The ICT value, found in S-Register 50, is
measured in 1/10 second increments, and has a default value of 20 (2
seconds). Reduce this value to 1 or 2 if speed of operation is an issue.
As with flow control, the reason for not using manual data forwarding is
that in a binary data stream any byte can inadvertently have the same bit
configuration as the data forwarding characters. This event would cause
an unintended transmission of a partial data block, missing the block
ending BCC if one is being used. Avoid this as it can cause unpredictable
and erroneous results.
Parity considerations
The CDPD system assumes parity to be a local issue between the MAS
and the DART 200’s RS-232 interface; parity is not carried over the
airlink. When transmitting 7-bit data with parity, the most significant bit
of each byte in the packet is set to zero and any parity present in the high
order bit is not transmitted. If this data is received by another DART, and
its RS-232 interface is configured for 7-bit data with parity, any required
parity bits are regenerated for compliance with the interface
specification.
In an application with a DART on both ends of the session then, parity is
not an issue. However, if there is a digital interface instead of a modem
at the Fixed End System (F-ES) then parity is not regenerated, possibly
creating a problem. The solution is either to modify the F-ES application
to accept data without parity (preferred), or to develop a bypass at the
MAS end of the session.
? ?
NOTE:
This section only
applies to applications
where the F-ES
requires odd or even
parity data, or the MAS
communicates uses odd
or even parity.