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Latest revision as of 01:42, 4 August 2020


I'm terribly sorry folks but here is another text about MIDI,
but this time it is for people who do not have a Turbo R A1GT
or a MIDI-interface.

Written by H.G. for MSX Club Gouda.

In this text you can read how to make your own very cheap
MIDI-out channel for your favourite computersystem. All you
need is 2 resistors, 2 connectors (DIN 8 for MIDI and DB9 for
the Joystickport) plus some cable with at least 2 wires and 1
shield. This MIDI-out will not be working with already existing
programs but with the included ST2 driver you can use it in
your own programs or you can replace old ST2 drivers with the
one I made for JoyMIDI.

I did this litle project while I was waiting for Arjan to visit
me with some nice software etc. It actually took less than 1
hour to make and test so it should be real easy for you too.

What I used was this:

SVI-738 (running at 3.5MHz), CM-32L, 2 resistors at 220 Ohm
each, 1 DB9 female connector, 1 DIN8 male and 2 meters of
shielded cable with at least 2 wires in it.

How to connect it:
Do the following connections from the Joystick port.

Pin 6 (Trg1)--> 220 Ohm --> Midi pin 5 (MIDI 2)
Pin 5 (+5V) --> 220 Ohm --> Midi pin 4 (MIDI 1)
Pin 9 --------------------> Midi pin 2 (SHIELD)

you can also add a Zener diode to protect your Joystick port
this is done this way:

(might be difficult to show in QUASAR)

  Pin 6
   / \
  ----- DTZ5.6A or other at aprox. 5 Volts + a bit.
 GND pin 9

Inside the joystickport at pin 6, there is a Open Collector
buffer just like in a real MIDIport. This midiout will be
electrically almost identical with the one in the Turbo R.

Now to the teory behind this project.

MIDI is sent with baudrate 31250 bits/sec. This is aprox. 114
clockcycles on a 3.579545 MHz machine, which is plenty of time
for us to software emulate a MIDI serial chip. At first it
might seem a waste of CPU time, since the CPU has to generate
all 10 bits, Startbit, Data bit0 to Data bit7 and finally the
Stopbit. This is not so ! For each note you want to play you
have to send 3 bytes to the midiinstrument, and later 3 more to
release it again. The most used Uart chip (8251, also used in
MSX RS232 interfaces) only has room for 1 byte in its
transmitbuffer, therefore we have to wait for it to finish the
last transmission each time we want send a new one. In
Pseudocode, it would look like this :

        Wait for the Uart to finish the last byte.
        Send Noteon
        Wait for the Uart
        Send Note number
        Wait for the Uart
        Send Velocity
        (here my routine will have to spend some CPUtime.)

In most SoundTracker files, there are often several notes to
play at the same time. In the worst case of 8 notes it will
take 0.01536 sec with Joymidi and 0.01504 sec for real Uarts so
there we will not be able to notice any difference. With fewer
notes the Joymidi will always use 0.00032 sec. more than on a
machine with a real Uart, this time is equal to the last byte
in each transmission.

Now to the software.

I have re-used the SoundTracker driver that I made for the
Turbo R MIDI. With a litle subroutine, just about 1 screen of
code, the program will now send all 10 bits to the
Joystickport. This is a very sensitive routine. Do not run it
at any other speed than 3.579 MHz, else it wont work. If you
have to make it work at 7 Mhz or other speeds, you can adjust
the numbers of NOP's in the GEN file but then it will only work
on your machine.

The ST2 driver will work just like the old one for the Turbo R
or the original one. It plays fully interruptdriven and should
not steal too much CPU time from Basic.
There are some nice extra features in the driver program:

* You can load files to VRAM (any 16k segment) and then move
them to adr. #4000 with a USR statement. Therefore you do not
need to lift the Basicbottom.
* You can load several musicfiles at once and then play them
later. (This is done by using different mapper pages.)
* The computer is not jammed if you load from disk while
playing or if you call the play or stoproutine twice in a row.
(should be foolproof. I hope.)
* You can send single bytes from Basic using "USR9(int)", Usr9
is automaticly set up when you load "ST2JOY1.BIN",r but can be
changed to any other USR#.

You tell the playroutine which mapperpage it shall play simply
by writing the value to #FD before you start the music.

OUT &HFD,5 : A=USR2(0) 'Start playing musicfile in page 5.

This playroutine is using the RAM in a transparent way, so you
can actually use the area from #4000-#BFFF for other purposes
at the 'same' time. The only thing you must not do is to write
over the routine at #D000-D???. (Surprised?)

If you have a MIDI module and want to try OUT JOYMIDI then you
should look for all the "ST2JOY1.*" files on this disk. See
ST2JOY1.BAS if you want to know more about how to use it, you
can also read my text on Quasar #16 for the Turbo R ST2 driver.

If you want to test it fast:

Clear 200,&HCFFF

This will send a note on to channel 1. You should also see the
Kana-led flicker whenever you send some data. (Only if you have
a Japanese machine.) This can be changed in the GEN file, I
used it mostly for testpurposes.

Good luck ! (You do not need it.)