Some theory
The MIDI-standard
MIDI transmission is based on negative 5V logic, which is transmitted serially.
This means:
Normally TTL logic (5V) is measured against ground (-pole).
Voltages above 3V are interpreted as high, voltages below 1V as low.
The range in between is considered forbidden.
In computer language, high = 1 and low = 0.
Serial means that the data stream is transported bit by bit via the serial port.
If you want to learn more about MIDI, see the page:
nuinfo.nwu.edu/musicschool/links/projects/midi/expmidiindex.html
you will find a very good and detailed explanation, which would go beyond the scope of this article.
MIDI signals, on the other hand, are measured against +5V. This reverses the ratios.
i.e.
TTL High = MIDI Low and
TTL Low = MIDI High
However, there is still an important difference:
MIDI transmission is based on current transmission.
I.e.
Current flows = High = 1
Current does not flow = Low = 0
This is implemented with an optocoupler at MIDI-In. The normal current on the
MIDI cable is in the range of 5-25mA. To guarantee this current
(no more), resistors of 220 Ohm are built into the MIDI lines as standard,
so a maximum current of 12mA is guaranteed at 5V, even with short circuit.
When I started building my own MIDI interfaces, I came across the
MAX232 from MAXIM (which works like the MAX238, only with
fewer transmission channels). After long tests I found out that
MIDI transmission with this circuit works safely and without problems without
optocouplers, and is even simpler.
So the first version of my TPP3 did not include an optocoupler at MIDI-In.
However, I later re-inserted the optocoupler so that my TTP3 complies with MIDI standards.
The MAX238/232 provides the safe conversion of the RS232 signal
(-12V=Low, +12V=High) to TTL (see above) and back, and a relatively high
load capacity of the outputs. This means that you can also use this circuit to operate
MIDI hardware that is actually designed as internal PC devices (e.g. the
(e.g. the DB-50XG from YAMAHA).
The MIDI transmission uses cables with normal 5-pin DIN plugs.
PIN 4 and 5 are used as data lines and PIN 2 as shielding.
PIN 4 is connected to +5V (“MIDI ground”)
PIN 5 is the actual data line
In high-quality MIDI cables, the two data lines are twisted inside the shield.
This means that even longer MIDI connections (up to 15m) can be realized
without amplification.