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Strange Terminal Emulation Problem

© February 2005 Tony Lawrence

Author: TonyLawrence
Date: Sat Feb 19 16:28:35 2005
Subject: Strange Terminal Emulation Problem

I had a strange one the other day. Using Alphacom over a serial line to a SCO machine and the only way it would work was if the emulation was VT100.

Details: I could attach an old Wyse60 terminal to the same line and it worked fine no matter what emulation was chosen. With the PC, however, even changing emulation after logging in, with or without a tset, would send the screen insane - looked to me like the baud rate had suddenly changed. I found that I could still type commands, and doing an stty -a redirected to some file showed that nothing had changed on the Unix side (as expected). I was blaming a bad download of Alphacom, but then I tried regular old Windows Hyperterminal; this looked like it was having communication trouble: dropping characters, some characters set to high bit, etc. I cut the wiring to its basics (2,3,7) but no change.

So my assumption was and is a bad serial card - but why would vt100 work? I could even do long ls -lR's without a glitch.. this has me puzzled.

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Sat Feb 19 18:09:11 2005: 47   TonyLawrence

Fortunately, vt100 was sufficient for the app that needed to be run here.

Mon Feb 21 16:03:35 2005: 53   BigDumbDinosaur

Maybe the serial card drivers are somehow mangling the non-VT100 escape sequences. Since every VT100 escape sequence begins with a CSI ($1B $5B or <ESC> [), it's possible the card drivers expect to see the left bracket as the next char after <ESC> and consider anything else to be bogus.

Mon Feb 21 16:28:04 2005: 54   TonyLawrence

OK, but what business does a serial card driver have looking at escape sequences?

Mon Feb 21 18:27:41 2005: 55   anonymous

I'd suspect the *unnamed* serial card driver also. You could prove this by connecting to the standard COM1 or COM2 ports and seeing if you can replicate the problem. Perhaps the flow control is set to something non-standard like PC-TERM or hardware (DTR) flow control, and is being mangled by the driver.

I don't understand your remark that you could plug in a Wyse 60 terminal and have it work no matter what the emulation was set to. Are you referring to the emulation the terminal itself is setup to, along with a corresponding change to the TERM in the shell session? Or are you saying that if the terminal is using native wyse60 emulation, you could set the TERM environment variable to anything you'd like and it would still work?

It's also possible you've stumbed upon a minor Alphacom bug. Make sure you're using the latest build of version 5 or version 6.


Mon Feb 21 19:24:48 2005: 56   TonyLawrence

I meant, of course, that I could set the dumb terminal to any of its possibilities, and make the corresponding TERM change at the host, and all worked as expected - in other words, its not the wire or the digi or the host.

My suspecion is that the machine itself is whacked. I did try com1 also, with the noted result - partially garbled characters. Not as bad as on the PCI serial card, but bad stuff. I also used Hyperterm with the same problems, so it's not Alphacom either. Very, very puzzling..


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