DART 200 CDPD Modem User’s Guide | 8 Application Programming |
CAUTION: Whenever \O is changed, verify that the F, \M, and \T settings are still appropriate for the application.
Hardware flow control operation
The DART 200 communicates with the MAS using the CTS lead (modem controlled) when receiving data, and sensing the RTS lead (MAS controlled) when sending data.
When the modem receives data from the MAS it holds the CTS line in the ON state and only transitions it to the OFF condition to exert flow control (stop data transmission from the MAS).
When the modem sends data to the MAS it monitors the RTS lead from the MAS; sending data only when RTS is ON, and stopping when the MAS exerts flow control (wants to stop data transmission from the modem) by transitioning RTS to the OFF condition. If Hardware Flow Control is specified, the modem does not begin data transfer to the MAS unless RTS is ON.
PAD operating mode
The DART 200 is capable of operating in two modes while transferring data:
∙Packet Assembler - Disassembler (PAD) mode - When in PAD mode the internet protocol support is provided by the modem
∙SLIP mode - When in SLIP mode the internet protocols are provided by a software stack in the attached device, usually a PC
The DART uses PAD mode to provide support for UDP, TCP, and telnet. The form of the dial command (ATDT or ATDP) specifies to the modem whether TCP or UDP protocol should be used. However, telnet is a TCP application; something more than the form of the dial command is required to inform the modem that it should operate in telnet rather than straight TCP mode. This function is provided by the \O parameter.
Telnet mode is specified by the \O1 parameter (default setting). When a telnet session is established the modem will begin a negotiating process with the host to determine the terminal being emulated, whether local or remote echo will be used, as well as other operating rules for the session. In addition, when data transfer starts only printable characters and control characters will be passed. This mode of operation is not suitable for any application other than telnet: it is not compatible with telemetry requirements.
For telemetry the option should be changed to \O0 and saved with the &W command. However, this causes some potential problems because the \O parameter also controls the F, \M, and \T settings. Each \O setting has a preferred setting for F, \M, and \T, which may not be compatible with the
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