                Interrupts
                ----------

        ( This version is for Pasokon TV Classic. )


What is an IRQ?
---------------

IRQ is an abbreviation for Interrupt Request.

What is an Interrupt?

There are basically two ways the Central Processing Unit
(CPU) of your computer can talk to peripheral devices such 
as printers and serial ports.

1. Polled - CPU constantly asks about the status.

2. Interrupts - Device notifies CPU when it needs attention.

With polled operation, the CPU is constantly asking all the
devices:

        "Serial port, do you have anything for me?"
        "Printer port, are you ready for another character?"
        "Keyboard, was a key pressed?"
        "Floppy drive, did you move to that track yet?

This wastes a lot of time because the answer is usually the
same:  "I don't have anything for you now, CPU."

The alternative is for the device to "interrupt" the CPU when
it needs attention.  The CPU stops what it is doing, remembers
where it was, does something for the I/O device, and goes back
to what it was doing.  Much like answering the telephone.  You 
don't constantly pick it up to see if anyone is there.  When it
rings you answer it, transfer some information, and generally
go back to what you were doing before it rang.   Often you will
write a note about what needs to be done later when you're not
too busy.

The original IBM PC had the ability to use 8 different
interrupts:

        IRQ             Standard Usage
        ---             --------------
        0               Timer
        1               Keyboard
        2               Video display
        3               Serial port 2
        4               Serial port 1
        5               Printer port 2
        6               Floppy disk controller
        7               Printer port

When the IBM PC/AT was introduced, additional interrupts were
made available:

        IRQ             Standard Usage
        ---             --------------
        8               Real-time clock
        9               Replaced IRQ 2 
        10              spare
        11              spare
        12              spare
        13              Numeric Coprocessor
        14              Hard disk controller
        15              spare


Ten of these are available from the expansion slots, others
can be used only by devices on the mother board.

Clearly the hardware and software configuration must agree.
If a serial port generates IRQ 4 but the software thinks
IRQ 4 means the printer is ready for another character, 
the system won't work properly.
 
The design of the old standard ISA bus does not allow two 
different devices to share the same interrupt.
 

IRQ Selection
-------------

The Pasokon TV Classic interface uses an interrupt to notify
the CPU when it needs attention. 

There are 10 different jumpers on the board for maximum
flexibility but many of them would be VERY BAD choices.

        +-----------------------------------------+
        |                                   R16   |
        |                                   C12   |
        |                                         |
        |                                        ###
        |                                        ###
        |                                        ###
        |           IRQ 15 ... 2/9               ###
        |               ##########               ###
        |                                         |
        +----------+       +--+              +----+
                   +-------+  +--------------+
                   Edge connector for PC slot.


Just in case, the marking on the board is not clear, the order
of the jumpers is:

                15  12  11  10  7  6  5  4  3  2/9


These are the jumper positions, address ranges, and likely
conflicts for those ranges:

        Jumper          Conflicts
        ------          ---------
        3               Serial port 2           BAD CHOICE
        4               Serial port 1           BAD CHOICE
        5               Printer port 2          ?
        6               Floppy disk drive       BAD CHOICE
        7               Printer port 1          BAD CHOICE
        9               Video display           ?
        10                                      Good
        11                                      Good
        12              Mouse on PS/2           ?
        15                                      ?


In the early days of Pasokon TV, IRQ 5 was recommended for two
reasons.  Very few people had a second printer port and there is 
less overhead with interrupts lower than 8.  Back when people
were still using slow 386SX machines, the additional overhead of
higher numbered interrupts was sometimes a consideration.

The Sound Blaster uses IRQ 5 as the default and other compatible
models might do the same.

Some VGA display adapters generate IRQ 9 for compatibility with
ancient (pre-VGA) displays.  However, the manuals I've read all
recommend disabling this.  

A second disk controller channel added to my machine decided to
use IRQ 15 and gave me no choice in the matter.   I've seen
other references to CD-ROM drives using IRQ 15.

This leaves 10 and 11 as probably being the best choices if 
there is no other conflict.  Some times, the Microsoft Diagnostic
program (MSD.EXE) will provide some clues.

If you run into conflicts with add-on devices you have, please 
send details and I will put them here to help other people.


Error Message
-------------

It is possible that you will see and error message resembling:

        Interface not generating interrupt x.

Possible causes:
 - Interface is not installed in computer.
 - Jumper position and software configuration do not agree.
 - There is a conflict with another device using same 
   interrupt.


Solution
--------

1. Examine the Pasokon TV interface and set the IRQ jumper 
   to the desired position.  Consult the chart above for the
   best and worst choices.
   When in doubt, try 10.  Write this number down.

2. Put the interface back in the computer and run PKTV.

3. Pick "IRQ" under the "Setup" menu.
   Select the button corresponding to the jumper position.
   Note that a limited selection is available to make it more
   difficult to make a poor choice.  All the possibilities are
   available by manually editing the PKTV.CFG file.

4. Click on the "Recv" button or press Enter key to start 
   receiving.  If you get the "Interface not generating  
   interrupt ..." message, call us for assistance.  
   Don't change the I/O address jumper in this case; you 
   wouldn't get this far if the basic I/O functions were not
   working properly.

