Grex Hardware Conference

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Grex Hardware Item 50: Need help with COM3/COM4
Entered by davel on Wed Jun 2 20:11:00 UTC 1993:

We're kind of behind the times where I work, I know, just now moving up
into the Real World, but ...
To make it short: we're finally needing to use a mouse for something.
(This is on a 386 running DOS 5 or 6 and, now, Windows.)  The serial ports
COM1 and COM2 are already in use (direct connection to the mini, and
modem, respectively), so we've tried putting the mouse on COM3 or COM4.
It doesn't work.  Apparently, when the system comes up there are messages
indicating that things are OK (this isn't *my* machine, so I don't have
the exact text at my fingertips), but no matter how we switch things
around nothing will accept the fact that COM3 (or COM4) is there.

If you're familiar with the program CHECKIT, *it* shows COM3 (or COM4)
there OK.  But nothing that actually might run off the thing.

I suspect that there's something *very* basic that we're missing.  I can
collect & report more data, if that's not the case; but I really hope
someone can tell me right off what's the likely cause.

Thanks very much!

8 responses total.

#1 of 8 by mju on Thu Jun 3 00:53:22 1993:

Most versions of CheckIt I've seen display all four COM ports
in the "Test serial device" menu, so that isn't necessarily
the best way to find out.

You're probably going to have problems, though.  COM3 usually
is set up to use the same interrupt line as COM1 (IRQ4), and
COM4 is usually set up to use the same interrupt as COM2 (IRQ3).
Since the PC can't share interrupts, that means that you can't
use COM1 and COM3 at the same time, and you can't use COM2 and COM4
at the same time.  You'll have to decide how you want to hook
things up so you're only using two simultaneous serial ports.
Or, you can get a "bus mouse", which comes with its own ISA adapter
card and usually uses interrupt 5 or 2.

#2 of 8 by davel on Thu Jun 3 00:57:43 1993:

Thank you.  In theory I think (!) the cards in question let you select
an interrupt, and that attempts were made to choose different ones.

#3 of 8 by jdg on Thu Jun 3 01:48:47 1993:

Along with Windows 3.1 is a program called MSD.EXE.  This Microsoft diagnosis
aid can tell you which COM ports are using which IRQ.  It will help you.

#4 of 8 by rcurl on Thu Jun 3 05:28:18 1993:

I fought the same battle, and lost. I now use switches. COM1 is shared
by the modem or my home network; COM2 by the serial mouse or the X10
controller interface. I would never want to use the modem/network,
or mouse/X10 simultaneously, so there are no conflicts. Also, I can
prevent the mouse.sys from being installed, by booting in the X10
position.

#5 of 8 by n8nxf on Thu Jun 3 12:26:46 1993:

there is more than just interrupts involved.  The addresses of all the
4 ports are all different.  Make sure you are changing the port interrupt
as well as the port address.
As far as I know, you can share the same interrupt, you just can't use
the two devices at the same instant in time.

#6 of 8 by rcurl on Thu Jun 3 14:25:00 1993:

I tried to use COM1 for modem and COM3 for mouse, sharing the interrupt
(not being able to change it on the board I have), and thought things
were going swimmingly - until I moved the mouse while I was connected
with the modem. Interesting message! Sort of, *squeek!*.

#7 of 8 by tsty on Thu Jun 3 18:05:04 1993:

<<heh>>

#8 of 8 by davel on Fri Jun 4 00:13:53 1993:

That's exactly what Linda complained of when the things were sharing an
interrupt ... or was it the other way around?  The modem was supposed to
be inactive (connected but nothing going on while she went to check on
something for which she needed the mouse), but either static or some
activity turned up, & the mouse cursor went wild.

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