14.3.6. Modem Settings

If you have a modem whose parameters may be permanently set in non-volatile RAM, you will need to use a terminal program (such as Telix under PC-DOS or tip under FreeBSD) to set the parameters. Connect to the modem using the same communications speed as the initial speed getty will use and configure the modem's non-volatile RAM to match these requirements:

• CD asserted when connected
• DTR asserted for operation; dropping DTR hangs up line & resets modem
• CTS transmitted data flow control
• Disable XON/XOFF flow control
• RTS received data flow control
• Quiet mode (no result codes)
• No command echo

Please read the documentation for your modem to find out what commands and/or DIP switch settings you need to give it.

For example, to set the above parameters on a USRobotics Sportster 14,400 external modem, one could give these commands to the modem:

ATZ
AT&C1&D2&H1&I0&R2&W

You might also want to take this opportunity to adjust other settings in the modem, such as whether it will use V.42bis and/or MNP5 compression.

The USR Sportster 14,400 external modem also has some DIP switches that need to be set; for other modems, perhaps you can use these settings as an example:

• Switch 1: UP - DTR Normal
• Switch 2: Do not care (Verbal Result Codes/Numeric Result Codes)
• Switch 3: UP - Suppress Result Codes
• Switch 4: DOWN - No echo, offline commands
• Switch 5: UP - Auto Answer
• Switch 6: UP - Carrier Detect Normal
• Switch 7: UP - Load NVRAM Defaults
• Switch 8: Do not care (Smart Mode/Dumb Mode)

Result codes should be disabled/suppressed for dialup modems to avoid problems that can occur if getty mistakenly gives a login: prompt to a modem that is in command mode and the modem echoes the command or returns a result code. I have heard this sequence can result in a extended, silly conversation between getty and the modem.

14.3.6.1. Locked-speed Config

For a locked-speed configuration, you will need to configure the modem to maintain a constant modem-to-computer data rate independent of the communications rate. On a USR Sportster 14,400 external modem, these commands will lock the modem-to-computer data rate at the speed used to issue the commands:

ATZ
AT&B1&W

14.3.6.2. Matching-speed Config

For a variable-speed configuration, you will need to configure your modem to adjust its serial port data rate to match the incoming call rate. On a USR Sportster 14,400 external modem, these commands will lock the modem's error-corrected data rate to the speed used to issue the commands, but allow the serial port rate to vary for non-error-corrected connections:

ATZ
AT&B2&W

14.3.6.3. Checking the Modem's Configuration

Most high-speed modems provide commands to view the modem's current operating parameters in a somewhat human-readable fashion. On the USR Sportster 14,400 external modems, the command ATI5 displays the settings that are stored in the non-volatile RAM. To see the true operating parameters of the modem (as influenced by the USR's DIP switch settings), use the commands ATZ and then ATI4.

If you have a different brand of modem, check your modem's manual to see how to double-check your modem's configuration parameters.