Sierra Wireless DART 200 CDPD Modem manual Parity considerations

DART 200 CDPD Modem User’s Guide 8 Application Programming

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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.

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.

PN1197-00 Revision 1.0 8-17