     A Special Note about COM ports and Interrupts (IRQ)
                         04/16/97 -med


**** THIS INFORMATION IS FOR MSDOS/WIN31 INSTALLATIONS ONLY ****


The FRYERS.COM driver will select the default interrupt for the 
selected COM port (IRQ4 for COM1, IRQ3 for COM2). You can select
COM3 or COM4 if you wish as well. The default interrupt for the 
selected COM port will be used (IRQ4 for COM3 and IRQ3 for COM4). 

Be warned that multiple RS232 cards that share the same interrupt 
tend to have problems of locking up the computer because of 
interrupt conflicts. It is generally not recommended to have more 
than one serial card attached to a specific interrupt. 

Some RS232 cards will let you select interrupts other than the 
predefined IRQ3 and IRQ4. You can explictly tell the audiometer 
programs to use the interrupt you want by adding the command line 
option Ix where 'x' is the IRQ number to use. Some IRQ numbers 
are not allowed because they are predefined for use by the 
computer already. Selecting IRQ0 will cause the default IRQ for 
the COM port to be used.



Selecting a Non-standard Configuration:

As an example, suppose you wish to use a serial port that has 
its hardware configured as COM4 using IRQ7 with the AGRAM 
program. You would give the command at the DOS prompt like this:

C>AGRAM 4 I7
^ ^       ^ ^
| |       | using IRQ 7 
| |       |
| |       using COM 4   
| |
| program name
|
DOS prompt



If you don't provide an interrupt destination, the standard 
default interrupt for the selected port will be used.  Example:

C>AGRAM 2
^ ^       ^ 
| |       using COM 2   (IRQ 3 is automatically selected)
| |
| program name
|
DOS prompt



If you don't provide the COM port number, COM1 with IRQ4 will 
be used. Example:

C>AGRAM     
^ ^        
| |
| program name   (COM 1 using IRQ 4 is automatically selected)
|
DOS prompt



Default COM Port Interrupts:

     COM port     Interrupt
        1            4
        2            3
        3            4
        4            3



Available Interrupts:

If you select a different interrupt, make sure that it does not
conflict with another interrupt already in use. 


  Available                Unavailable 
  Interrupts               Interrupts

     2   *note2 (video)        0     PC/AT: hardware timer 
     3          (serial)       1     PC/AT: keyboard
     4          (serial)       6     PC/AT: floppy controller
     5   *note1                8     AT: real-time clock 
     7                         13    AT: coprocessor
     9   *note2                14    AT: hard disk
     10
     11
     12
     15


*note1: On the old style XT, IRQ5 was used for the hard disk and
should not be used for anything else on an XT. 

*note2: On the old style PC/XT, interrupts over seven do not 
exist. The interrupts were extended on the AT through IRQ2.
Because of this, you must treat IRQ2 carefully on at AT if you 
decide to use it. 


IRQ2 and IRQ9:

The old IRQ2 hardware line was assigned to IRQ9 on the AT. 
Yet most PC boards still indicate IRQ2 for the selection. 
The AT bios will attempt to reflect the interrupt to IRQ2 to 
support software that expects the interrupt to appear on IRQ2. 
However, if possible, this should be avoided. Since all the 
interrupts higher than seven must be directed through IRQ2, 
nasty things can happen if the program does something nasty 
like disabling the interrupt. This will usually result in the 
computer being locked up.

If you are using IRQ2 on an AT computer, you should tell the
software that you are using IRQ9 so that it will use the proper 
interrupt instead of relying on the unstable activity of 
the bios trying to reflect the IRQ9 hardware signal to IRQ2.


History: 11/10/92 -med original
         04/16/97 -med revised
<eof>

