The MIDI Physical Layer
MIDI at the hardware level is very similar to RS-232. The key similarities are:
The key differences are:
|Frame Format||1 start bit|
5-8 data bits
1-2 stop bits
optional parity bit
|1 start bit|
8 data bits
1 stop bits
|Baud Rate||up to 20k|
standard rates of 300, 600,...19k2
extended rates of 38k4, 57k6, 115k2
|31250 baud +/- 1%|
Logic 0: +5V to +15V
Logic 1: -5V to -15V
Logic 0: 5mA
Logic 1: 0mA
|Driver (open circuit)||+/- 25V maximum||Unspecified|
|Driver (short circuit)||+/- 100mA maximum||Unspecified|
|Receiver Load||3k to 7k ohm||Opto-isolator|
|Receiver Sensitivity||+/- 3V||< 5 mA to turn 'on'|
|Receiver input range||+/- 3V to +/-15V||Unspecified|
|Signal rise/fall speed |
30V/us (max) slew-rate
2 us (max) rise/fall times
|Maximum cable||2500pF (approx. 15m)||15m|
|Isolation||Unspecified||Opto-isolator on receiver circuit|
|Handshaking signals||Transmitter: RTS, DTR |
Receiver: CTS, DSR, DCD, RI
|Physical connector||D25||5-pin DIN (180 degree)|
MIDI Standard Hardware
This is the Standard MIDI Hardware (as described in the MIDI specification).
There are 3 types of MIDI Port:
Note that if a MIDI device is capable of sending MIDI data, it will appear on the MIDI OUT port, and never on the MIDI THRU port.
Some MIDI devices have a 'Soft THRU' option, which allows the MIDI IN data to be repeated on the MIDI OUT port.
MIDI Cable wiring
All MIDI cables are wired 'straight through' (as per the diagram).
Unused pins (1 and 3) should be left unconnected.
MIDI System Cabling
Here is a cabling diagram of a typical MIDI system.
Note that MIDI OUT connects to MIDI IN.
You should not connect OUT to OUT, or IN to IN, or OUT to THRU