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From: liber@darth.UUCP (Eric Liber)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Activating COM3 and COM4
Summary: memory patches
Message-ID: <481@darth.UUCP>
Date: 4 May 89 12:32:01 GMT
References: <8036@fluke.COM>
Distribution: na
Organization: Darth Software, Pittsburgh Pa.
Lines: 45

In article <8036@fluke.COM>, lal@tc.fluke.COM (Larry Lohkamp) writes:
> I have an I/O problem. I have an HP plotter, a micom line, an internal
> modem, and a CTX port. I have been playing human switch box because my
.
.
.
> get the machine to recognise anything but COM1 & 2. If it makes any
> difference, my DOS rev. is 3.3. Can I get all 4 ports to work, or am I
> dreaming the impossible?

Well I screwed up slightly in my last response but I found out last night
how simple it can really be.  You must plug the RS232 table AND the equipment
word to tell the system HOW MANY RS232 PORTS THERE ARE.  After you do this 
mode will work fine.  Here is a turbo Pascal (5.0) program that will do the
deed for com3 at 3E8:

program setcom3;

var
   rs232com3 : word absolute $0040:$0004;
   equipword : word absolute $0040:$0010;

begin
    rs232com3 := $03E8;
    equipword := equipword or $0200;
end.

thats all there is to it.  the equipment word at 40:10 and 40:11 (hex) 
has bits 9 - 11 set to indicate the number of com (rs232) ports in the 
system $0200 turns on bit 9 which changes the value to 3 from the startup
value of 2 (xxxx 010x xxxx xxxx) changed to (xxxx 011x xxxx xxxx).

All you have to do is include the command setcom3 in your start up batch and 
then you can initialize the port with a mode command such as:

MODE COM3:9600,N,8,1,P

The P was necessary in my system to keep the port from timing out too soon.

Anyway all this works on a gateway 386 using phoenix bios 3.??? and msdos 3.3.

If you have any more questions I will attempt to answer them.

good luck
Eric (the red baron) Liber