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SubjectSerial Port voltage drops with Linux

we have a home-made device connected to the serial port which we are trying
to program with Linux. I have posted here already and got a lot of useful
answers, BitWizard (thanks) even translated a QBasic source which was
supposed to do what we needed for our project into C for Linux.

Thanks to all who answered!

But now we have a somewhat different problem.

The situation is this: When we connect the hardware to the serial port, the
signal amplitude breaks down to lower than +3V (which is miniumum for the
PC to detect a "0"). With DOS, the signal amplitude varies from -10..+10 V
(approx) for 1/0 signals. With Linux, it's -10..<+3V, and that is not

I tried not starting setserial when starting Linux, this did not change.
When gpm was started, the serial port status changed though - although the
mouse is on ttyS0 and our hardware on ttyS1.

The next thing for us will probably be dosemu....

Yes, I know this project is ridiculous. I didn't invent it. And nobody
wanted to learn MS Visual C++ & Co, we'd all be happy getting this to run
under Linux.

Do you have any further ideas about this? Why do the ports behave so much
differently (no matter if we start setserial or not) when using Linux? In
theory (yeah, right) it's hardware that should behave the same under any
OS, at least in respect to voltage levels and power consumption?

We'd all really appreciate if you can give us any further insights (or
RTFMs...) you might have on this.

BTW, we tried three different serial port hardware boards (ISA) so the
hardware should not be at fault.

I have put the QBasic source and the C source at, perhaps someone can shed a little
light on this. There is also a complete DOS floppy boot image including
QBasic and the .bas file for those who do not have a bootable DOS handy

Thanks a lot!

_ciao, Jens_______________________________
"[Microsoft] ... guarantees 99.8% NT uptime for certain hard-/software.
That's exactly the 3 minutes daily that my NT server needs to reboot."
-- ZDnet editorial

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