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COM3N4.TEC
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1990-10-08
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ID:CM COM Ports and DESQview
DESQview Technical Note
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 re-using 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) 1990 by Quarterdeck Office Systems
* * * E N D O F F I L E * * *