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SCO Unix Serial Communications and UUCP

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What are some common settings for my modem?

This is an ancient post with no relevance to modern systems.

For the real answers, read your modem's manual. If you want to discuss modems, see comp.dcom.modems. Having said that, here are some common setup strings for various features on many modems; see your manual to determine which one(s) apply to your modem. Note that not all modems will accept the following commands; even the manual for one modem is too large to fit, let alone all possible modems.

  • Locked DTE rate - this causes your modem to always talk to your serial port at the same rate, regardless of the data transfer rate between modems. Try AT\J0, AT&Qn (n is a number which depends on other settings as well), (Multitech) AT$SB38400 or (USRobotics) AT&B1.
  • Hardware flow control - this uses out-of-band signalling on the RTS and CTS lines to perform flow control and will not interfere with the transmission of binary data, as software flow control will. Try AT&K3 or AT&E4.
  • Software flow control - particularly on interactive (cu) connections, you may wish to use software flow control in some cases. This is usually done with the same command as hardware flow control, but with a different number (e.g. AT&K4). One disadvantage is that if you get garbage characters (due to line noise, modem disconnection, etc.), you will sometimes get a flow control character as part of the garbage; the system doesn't know it's garbage and so it may appear to hang the port.
  • Correct use of carrier detect - the DCD line is used to indicate to the computer that the modem has established carrier. This setting is almost universally done with AT&C1.
  • Error control - this feature, if negotiated between modems, causes the modems to take care of retransmitting data when errors crop up on the phone line. Try AT&Qn (again, n depends on other settings), AT\Nn (ditto), or AT&E1. See the discussion elsewhere in this FAQ to help determine what, if any, error control you should use. Generally, though not always, you will want to enable all error control methods your modem supports, in order to be able to negotiate connections with other modems, but may need adjustment in some circumstances.
  • Data compression - this feature, if negotiated between modems, causes the modems to attempt to compress the data before sending, improving throughput. Try AT&Qn (n depends on other settings) and AT%C1; some modems also use registers such as S36 to fine-tune this. See the discussion elsewhere in this FAQ for more information on when you might or might not want to use data compression.
  • Correct use of DTR - the DTR line, when dropped by the computer, should cause the modem to hang up (and, according to some opinions, reset entirely). This feature is almost universally done with AT&Dn, but the supported values of n may vary from modem to modem. Try AT&D2 to hang up and AT&D3 to hang up and reset.
  • Writing settings - Two cautions first. First, you do NOT want to do this every time you initialize the modem. The EEPROMs used in most modems to store this information have a lifetime of something like 10 000 writes. At 30 reinitializations a day, you'll burn out your EEPROM in around a year. Second, if you use this to do the initial setup of the modem, document your settings somewhere so that if your modem does die, you'll have the settings you used somewhere (I'd recommend in a comment in your /usr/lib/uucp/Dialers file). See AT&W; some modems have several settings available, written to with AT&W0, AT&W1, etc. See AT&Y on some modems to determine which settings are used upon reset, and ATZn for resetting the modem.
  • To Echo or Not To Echo - you can configure your modem to echo commands back, or not to echo commands back, with ATE1 and ATE0, respectively. You can control the sending of result messages with ATQ1 and ATQ0; some modems also use ATQ2 to disable result codes on incoming connections.
  • Enabling busy detection - ATXn controls both the range of result codes the modem sends (such as enabling the display of connect speed rather than simply CONNECT) and what phone company signals the modem detects (e.g. busy, dialtone). Most people use ATX4; check your manual for other options.
  • Auto-answer - use ATS0=n, where n is the number of rings to allow to pass before answering. ATS0=0 disables auto-answer. You may wish to disable auto-answer while calling out or at other times when you don't wish the modem to answer.
  • Escape code - most modems use a pause, followed by three plus signs, followed by a pause, as a signal from the computer that the modem should go into command mode while on-line. This may cause problems in some cases. To disable this escape sequence, use ATS2=128.

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