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1989-09-15
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DESQview Technical Note
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, DESQview allows you to define the
COM port that is to be used by the program. The choices are 1
and 2. 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 so 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 here that is important to
understand when trying to use ports beyond 2 in a multi-tasking
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 while reusing IRQ3 or IRQ4. This works fine,
when using the ports from free standing programs, one at a time,
but in multi-tasking systems, these setups are generally
unsatisfactory as users of multi-tasking 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 free IRQ, other than IRQ3 or IRQ4
that is free on your machine, say IRQ2 or IRQ5, then that 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 out there that are
now accessing multiple ports through the use of FOSSIL drivers
and special 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 and setups up communications with
the hardware, which may contain 4, 8, or even more ports. The
hardware usually uses ONLY one standard hardware IRQ, usually
IRQ3 (normally COM2). The driver then is addressed 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
which physical port on the board the data is intended for. 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 that are
specifically written to use the FOSSIL driver.
For more information about FOSSIL drivers, check out a file
called FOSSIL.ARC, available on many bulletin board systems
around the country.
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