A.P. Lawrence

This article is from a FAQ concerning SCO operating systems. While some of the information may be applicable to any OS, or any Unix or Linux OS, it may be specific to SCO Xenix, Open Desktop or Openserver.

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Serial Communications and UUCP

I can transfer small files via UUCP but large files won't go

I can transfer small files via UUCP but large ones go really slowly

This may indicate a flow control problem. uucico, the program that actually performs UUCP transfers, turns off all hardware and software flow control on the port it is using (prior to 3.2v4.0, which allows you to specify what it should use). If your modem's buffers are too small, then the stream of data involved in a large file transfer may overflow them, causing transmission errors. This may just cause your throughput to go way down, or it may result in a total inability to transfer larger files. Run uucico with debugging (you may find /usr/lib/uucp/uutry to be useful) and watch for "alarm" messages. If you see these, it's an indication that some characters are likely being dropped during transmission.

If you are using a serial port on a multiport serial card such as one from Equinox or Digi, your board may have shipped with a utility that lets you permanently set flow control on your port. Another alternative is to reduce uucico's window size.

Under 3.2v4.0, uucico does not turn off flow control; it is left at whatever setting the dialer used. If you are using a compiled dialer and have the source and the development system, you can write in whatever stty settings you desire. Under 3.2v4.1, if you are using atdialer, you can specify stty settings in /etc/default/atdial*. Note that it has been reported that cu, even under 3.2v4.x, turns on XON/XOFF flow control and, in doing so, disables hardware flow control.

In 3.2v4.x, a new stty setting has been added to perform bidirectional RTS/CTS flow control. This is CRTSFL. CTSFLOW and RTSFLOW do not perform proper bidirectional flow control; they allow the modem to signal the computer to stop sending, but not vice versa. Depending on what modem you have, you may need CRTSFL if your system can't accept characters as quickly as your modem can produce them.

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