ID:CM COM Ports and DESQview
DESQview Technical Note #145
by Stan Young

Q:  Why doesn't DESQview support more than COM1 and COM2?

Q:  How can I run more COM ports on my machine in DESQview?

In DESQview's Change a Program menus, DESQview allows you to 
define the COM port that is to be used by the program.  The 
choices are 1, 2, Y and N.  This does not mean, however, that 
DESQview does not support the use of COM3 and COM4.  It does.  

DESQview allows you to specify whether the program uses COM1 or
COM2 so that it can attempt to make the system appear to have only
the port (1 or 2) that you have specified.  This is done so that
ill-behaved programs that clear both ports on startup, making the
assumption that they are the only program running on the machine,
will not interfere with a program that is already using the other
port.  These programs do not typically interfere with COM3 or COM4,
so if you are using one of these ports, you can set "Uses serial
ports" to either N or Y.  It makes no difference.

However, there is a key issue that is important to understand 
when trying to use ports beyond 2 in a multitasking system.  
There are two parts to serial communication:  

1.  The software side, which is the BIOS COM port identification.
2.  The hardware side, which is the hardware interrupt (IRQ) that 
    is being used.

A good analogy for understanding the importance of this is to think
of serial port communications as a telephone system.  The COM port
represents the actual telephone unit (the extension, if you will). 
The hardware interrupt, or IRQ, represents the line out that
connects to the phone company.  COM1 and COM2 have their own 
dedicated lines (IRQ4 and IRQ3, respectively).  This is pretty much
a standard, defined configuration in the PC environment. 

But when you add a COM3 or COM4, they must be assigned an IRQ.  
You have two options: You can assign them their own separate IRQ,
or you can assign them to one of the IRQ's that are already in 
use (3 or 4).  However, if you assign a new COM port to an IRQ 
that is already assigned to another port, it is like adding 
another telephone extension to an existing line.  You will be able
to call out from either extension, but you will not be able to call
out from both extensions at the same time -- for that, you need a
dedicated line.

Most of the hardware devices (modems, mice, plotters, scanners or 
whatever), that allow you to define them as a port above 2 
unfortunately do so by reusing IRQ3 or IRQ4.  This works well when
using the ports from free-standing programs, one at a time, but in
multitasking systems these setups are generally  unsatisfactory, as
users of multitasking systems want to be able to use the ports
simultaneously.

If the hardware that supplies your additional COM ports allows you 
to configure the port to a IRQ, other than IRQ3 or IRQ4, that is 
free on your machine -- say IRQ2 or IRQ5 -- then that situation
would be like a phone with another line out, and you would be able
to use it simultaneously with COM1 and COM2.  Otherwise, you are 
limited to using these ports serially, not simultaneously.

Use of FOSSIL drivers:  There are some systems that are now 
accessing multiple ports through the use of FOSSIL drivers and 
special multi-port hardware (sometimes with its own on-board 
processor). A full discussion of FOSSIL drivers is beyond the 
scope of this technical note, but, briefly, this is how they work: 


A driver is loaded in the CONFIG.SYS file which sets up
communications with the hardware supplying the ports.  The hardware
may contain 4, 8, or even more ports.  The hardware usually uses
ONLY one standard hardware IRQ, usually IRQ3 (used by COM2 in
standard configurations).  The driver is then addressed by the
communications software (which must know how to address the FOSSIL
driver) as though it were a series of COM ports (again, 4, 8, or
more).  When communications to one of the ports is received, the
driver multiplexes through the single IRQ to communicate with the
board which, since it knows how to talk to the driver, can
determine to which physical port on the board the data is to be
directed.

We have had a number of users who have used systems with FOSSIL 
drivers to run multiple ports on electronic bulletin board systems
and for CAM process control.  To implement a multi-port system
using FOSSIL drivers, however, you need three things:  The
hardware, the FOSSIL driver and communications programs that are 
specifically written to use the FOSSIL driver.

For more information about FOSSIL drivers, obtain a file called 
FOSSIL.ARC, available on many bulletin board systems around the 
country.

        Copyright (C) 1991 by Quarterdeck Office Systems
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