HOW TO: US Robotics Modem Configuration in Linux

The following configuration applies to commonly used US Robotics modems:

1. Dip Switches

Sportster modems should be set initially for:

  • DIP switches 3,7, and 8 down (The rest UP)

Switch3 Down is display result codes

Switch7 Down is Load factory defaults

Switch8 Down is Smart Mode

Courier modems should be set initially for:

  • DIP switches 3 and 8 ON (The rest OFF)

Switch3 On Display Result Codes

Switch8 On Enable AT Command Set


2. Set the DTE baud rate for the modem by connecting to the modem at the baud rate being used by your application to communicate with the tty port. Use dinc from Linux to connect to the tty and issue the following command sequence for the modem:  

# dinc 19200 /dev/ttyxyz


The at&b1 setting configures a fixed serial port rate on the modem.

3. The port should set for RTS and CTS flow control. In Linux this is command is:

#  ditty-rp rtspace ctspace -ixon -ixoff -ixany /dev/ttyxyz

This command can be placed in /etc/init.d/dgrp_ditty to keep the configuration over reboots.

4. If using 8-wire RJ45 cabling (non-Digi cabling), the altpin setting will need to be applied to the Digi unit.  Telnet to the Digi unit, login as root and issue the following command:

#>  set flow altpin=on range=(port#)


  • Switch 5 Up/Off is Auto Answer onfirst ring (or higher if specified in NVRAM with ATS0=# -- the default isS0=1). Thus ATS0 does not need to be set for most applications.
  • To verify DTE port speed on the modem, use the command:


  • Some 14.4+ Modems will not workreliably if set for speeds below 38400.
Last updated: Jun 23, 2019

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