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1990-04-16
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Modification of the MIDI interface on the Atari ST
--------------------------------------------------
When you don't use the MIDI interface for musical instruments, it is possible
to use it as a general-purpose serial interface.
However, the baudrate is nonstandard (selectable between 7812.5 and 31250 baud)
and the interface is an inverted 5mA current-loop.
It is possible to change this, as follows:
1. Modification of the baudrate.
Find the MIDI ACIA on the cicuit board. It is a 24-pin IC marked "6850".
There are 2 of these on the board, one is for the keyboard and the other
for the MIDI. In a 520ST it's U20. Pin 2 and 6 of this chip should be
connected to some 74LS04 logic, NOT to the keyboard connector.
Remove pin 3 and 4 of the MIDI ACIA from the circuit board, connect them
together, and supply a clock frequency on these pins.
The ACIA can be programmed to divide this clock by either 16 or 64, so if
the clock is 614400 Hz you can select between 9600 and 38400 baud. This
is the selection supported by NET.
Lower clock frequencies are also possible, e.g. 153600 Hz to select 2400 or
9600 baud.
The clock can be generated using a CMOS divider (e.g. 4024) connected to
pin 18 of the 68901 (the 48-pin chip). The frequency on that point is
2457600 Hz, so a division of 4 yields 614400Hz, a division of 16 yields
153600 Hz etc.
----------------
1| |11 or 6
68901 p18 ----------|CK 4024 Q|----------- 6850 p3&p4
| |
| R |
----------------
|2
gnd
A switch could be used to change-over between the original situation and
the new clock, or even between 2 or more new clocks available from the
4024.
2. Conversion to RS232 levels
It is probably convenient to convert the levels to RS232, because most
equipment that can be connected conforms to this standard.
To do this, drivers and receivers of the 1488/1489 type are required.
These are also used in the ST for the MODEM port, and there even is a spare
1488 driver available. The 1489 receiver is already fully utilized.
It is possible to solder a 1489 (and also a 1488) on top of the existing
IC's on the board, only soldering the supply pins. The other pins should
be bent sideways and connected to the ACIA as follows:
------------
| |
TxData 6850 p6 ->---| 1/4 1488 |o----------> RS232 TxData
|--| |
------------
------------
| |
RxData 6850 p2 -<--o| 1/4 1489 |-----------< RS232 RxData
| |
------------
Pin 2 of the MIDI ACIA should be disconnected from the board (like 3&4).
There is also an RTS output pin on the 6850 (p5) that could be brought
out similar to the TxData and used for handshaking (NET does not use this)
It would also be possible to convert the MIDI levels to RS232 external to
the ST. The pinout is like this:
MIDI OUT
+5 ---220R---- 1
+5 ---220R---- 4 ------>---- +
gnd----------- 2 photocoupler
TxData buffered -->----220R---- 5 ------>---- -
RxData buffered -->----220R---- 3
|
|
^ MIDI IN
| 1
| + ----220R---- 4 ------<----
RxData photocoupler 2 current source
- ------------ 5 ------<----
3
Circuit in Atari ST | External circuit