Sierra Wireless DART 200 CDPD Modem - Manual (Page 107)

DART 200 CDPD Modem User’s Guide 8 Application Programming
PN1197-00 Revision 1.0 8-13
Flow control and the new end-user
Flow control often causes problems when applications are first online.
Flow control is not operational when the modem is in command mode
and not interacting with the CDPD network. Since an end-user’s initial
experience with the DART 200 is normally in command mode, it is not
unusual for them to experience a flow control caused modem lockup
when first using online functions, such as registration or ping.
Since the default for this feature is Hardware Flow Control (\Q2), an
attached device that does not support flow control, or a communications
cable with a missing or broken RTS lead will cause a modem lockup.
The modem cannot transfer data and becomes locked until it sees an RTS
signal from the attached device. It remains in the locked state waiting for
RTS until the modem is power cycled.
To avoid this problem check that the attached device and the modem
have matching flow control settings, and that a communications cable
has all the leads needed by the DART 200 present and in working order.
Otherwise, set the modem for no flow control (\Q0).
Flow control in application (online) mode
The purpose of flow control is to prevent buffer overrun in both the
DART 200 and the MAS. The DART 200 has a 256-byte Interrupt
Service Routine (ISR) buffer and a 6K packet buffer, while the MAS
buffer size is application or device dependent. Most MAS devices are
faster than the modem so flow control is normally only a potential
concern for the DART. If the maximum data transfer from the MAS is
less than the 256 byte ISR buffer size, then flow control is not needed
(\Q0).
Some MAS devices have small buffers or insufficient intelligence to
support hardware or software flow control; for those cases, specify the no
flow control option (\Q0). Also, avoid or detect possible buffer overruns
and request retransmission of the data.
For situations not in the above categories, then hardware (\Q2) or
software
( \Q1) flow control can be used. However, if transferring binary
(non-character) data is anticipated, only use hardware or no flow control.
Binary data transfer is incompatible with software flow control, 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 and unwanted flow control, the data
character is stripped from the data stream, causing unpredictable results.