The MIDI transmission based at a negtive 5V-logic, serial transmission.
Normaly the TTL (5V) logic is measured against the ground (-pol). So
there is a high level (higher than 3V) and a low level (lower than 1V).
This is in the language of the computer 1 or 0.
Serial means that the stream of data are "in a line" - one after one.
If you are interested in further information about MIDI, have a look at:
MIDI-signals are measured against the power (+5V).
If normal (TTL)signal is high (against ground) it is low against power (+5V).
The MIDI transmission is standardly based at a transmission via current
and not voltage. 0 means no current and 1 current flowes. This is
realised with a optocoupler at MIDI-In. The normal current of an
optocoupler is between 5 and 25mA. To guarantee this current (not more),
in the lines are resistors of 220 Ohm. There is one at the OUT and one at
the IN. Often there is also one at the second line PIN 4. So is a current
of maximal 12mA guaranteed and a security against short-circuit.
You will wonder, because in my projekt is no optocoupler .... ??!!
I've tested this kind of circuitry. I'd made a projekt sometimes ago to
connect a DB-50XG-wavetable-card with the AMIGA. This work's fine without
a optocoupler for a long time. The main advantage of this kind of
circuitry is, that there is near no current at the cable. And so you can
use long cables.
And it is also compatible with TTL-inputs of other MIDI-hardware (like
the DB-50XG-wavetable-card from YAMAHA).
The transmission is done via 5pol-DIN sockets and plugs. MIDI uses PIN
4&5 and PIN 2 for the shield. PIN 4 is MIDI-High (power +5V), PIN 5
is MIDI-Low (this is the "working line").
If we will connect MIDI to AMIGA's serial-port we must know, that the
logic of this port is based at RS232.
This logic works with a voltage between +12V and -12V. So is between High
and Low a difference of 24V (!!!). We must translate it in both
directions. Another way is not possible.
The reason: The TTL-logic circuits will be destroyed by RS232-voltages
and RS232-logic won't work with TTL-voltages.
For this reason there are some IC at the market. For our projekt I use
the MAX238 from MAXIM.