DART 200 CDPD Modem User’s Guide | 8 Application Programming |
Bypassing character parity relies on the fact that the DART passes all 8-bits if operating in 8N1 format, and does not autobaud or check character formats in online mode. However, it requires that the MAS be able to dynamically change the character format used; between 7-bit with parity and 8-bit with no parity.
If the DART 200 operates in server mode (responding to calls):
1.The DART 200 must be pre-initialized for 8N1 data format, or (if possible) dynamically changed to 8N1 format with an AT&L command.
2.The modem waits for incoming data using the 8N1 data format. When the modem answers an incoming call (TCP), or receives a packet (UDP server), it goes online and the format of the data to and from the MAS is not checked.
3.The MAS is then able to send or receive 7-bit data with odd or even parity, which is handled by the modem as 8N1 data so the parity bits are preserved. However, there is a potential problem if or when, the MAS returns to command mode to check for errors or to hang up. Unless it can change its parity to 8N1 it cannot communicate with the modem. The RS-232 interface leads can sense status, to escape, or to escape and hang up but other functions cannot be performed.
If the DART 200 operates in client mode (placing calls for service), the situation is different:
1.The MAS communicates with the DART using 8N1 data in command mode, up to and including the dial command.
2.The MAS then switches to the required 7-bit data plus parity for communicating with the F-ES.
3.At the completion of the session the MAS must switch back to 8N1 data format for command mode communication with the modem. If the MAS cannot switch data formats, and must use odd or even parity at all times, it can only function as a server device.
PAD keep-alive considerations
Keep-alives provide the means for a TCP application in the MAS to deal gracefully with time-out conditions or software crashes.
∙In transmit mode, it allows the DART 200 to generate session keep-alive characters if the MAS is slow to provide data to transmit
∙In receive mode, it provides a time-out to allow the application to terminate a session in an orderly fashion, if the F-ES does not send data or a keep-alive character within a end-user specified time-out period
