Sierra Wireless DART 200 CDPD Modem manual Flow control and the new end-user

DART 200 CDPD Modem User’s Guide 8 Application Programming

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.

PN1197-00 Revision 1.0 8-13