
                    Tips for Selecting an Interrupt Level


One of the most ticklish areas for new owners of the MIDI Starter System has
been installing the MIDI Co-processor Card at a usable interrupt level.
A distinct advantage the MIDI Co-processor Card provides over the MPU-401 is
its ability to be configured on different interrupt levels.  Four switches
allow you to select interrupt level 2, 3, 5 or 7 (referred to a IRQ2, IRQ3,
IRQ5, and IRQ7).  With the MPU-401 and associated interface card, you must use
a soldering iron to change interrupt levels.  Furthermore, the "fixed" nature
of the existing MPU-401 has lead to the development of software that only
works when the MIDI interface is at interrupt level 2.  As of this writing,
only the MIDI Starter System and Cakewalk(tm) allow you to change interrupt
levels.

The question under consideration is how do you choose an interrupt level?
Based on the previous paragraph, there are obvious advantages to installing
the MIDI Co-processor Card at IRQ2.  Unfortunately, on many machines, this can
not be done.  The first step to choosing an interrupt level is understanding
what kind of machine you have, and what other boards you have in your machine.

We begin with the PCs and PC/XT type machines.  The following table summarizes
how interrupts are generally used in XT type machines:

    IRQ       Use

     0        Timer
     1        Keyboard
     2        Reserved
     3        Secondary asynch adapter (COM2)
     4        Primary asynch adapter (COM1)
     5        Hard disk controller
     6        Floppy disk controller
     7        Printer (not used in most machines)

Using this table, we can formulate these rules:

    1.   If you have a hard disk, you can not use IRQ5.

    2.   If you have more than one asynch port (ie., you have COM1 and COM2),
         then you can not use IRQ3.  If you have a serial mouse and a COM
         port, you are most likely in this situation.  Note, that if you have
         a bus mouse, and a single COM port, you probably have IRQ3 available.

    3.   If you have no other adapter boards in your machine, then IRQ2 and
         IRQ7 are available.  However, if you have a multi-function I/O board,
         you should be aware that some of these boards make IRQ2 unusable.

    4.   While IRQ7 is assigned for printer use, there seem to be very, very
         few printer adapters that really use it.  In fact, the DOS print
         spooler does not support it.  Therefore, IRQ7 is usually a good
         alternative to IRQ2 (providing you do not have something else there).

Given these rules, the best approach is to try IRQ2.  If that does not seem to
work, then try IRQ7.

This brings up the question how do you know it works?  The usual symptom of
IRQ conflict is the observation that the Easy-8 sequencer works some of the
time, but not all of the time.  When it fails, it hangs.  It is a frustrating
way to get your board working, but unless you are really fluent with PCs, this
organized "trial-and-error" method is about as good as you can do.

The following table summarizes the interrupt level usage for PC/AT type
machines.

    IRQ       Use

     0        Timer
     1        Keyboard
     2        Orred summary of IRQ8-IRQ15
     3        Asynch port 2 (COM2)
     4        Asynch port 1 (COM1)
     5        Parallel port 2 (LPT2)
     6        Floppy disk controller
     7        Parallel port 1 (LPT1)


From this table, we can define the following rules for AT compatibles.

    1.   On a true AT compatible, IRQ2 is not available.  However, we are
         aware that some AT compatibles do not use IRQ2 as the original IBM
         PC/AT did.  If you have such an AT compatible, you may find that IRQ2
         is usable.  As a general rule, we recommend against using IRQ2 on an
         AT compatible.

    2.   If you have more than one asynch port (ie., you have COM1 and COM2),
         then you can not use IRQ3.  If you have a serial mouse and a COM
         port, you are most likely in this situation.  Note, that if you have
         a bus mouse, and a single COM port, you probably have IRQ3 available.

    3.   If you have no other adapter boards in your machine, then IRQ5 and
         IRQ7 are available.

    4.   While IRQ5 and IRQ7 are assigned for printer use, there seem to be
         very, very few printer adapters that really use interrupts.  In fact,
         the DOS print spooler does not support it.  Therefore, IRQ5 or IRQ7
         is usually a good alternative (providing you do not have something
         else there).

In summary, you must know your computer and what you have in it in order to
select an interrupt level for the MIDI Starter System.  Hopefully, this
discussion will get you up and running if you are having "IRQ problems".

