Data com voltage levels

Midi

Sending side

Pin 4 of a female 180 degree Din plug is connected to + 5 Volt via a 220 Ohm resistor.
Pin 5 is connected via a 220 Ohm resistor to;
+ 5 V for a logical 1
0 V for a logical 0

Receiving side

The LED part of an opto coupler is connected to pin 4 and 5 in series with a 220 Ohm resistor; 
 5 >----------*--------+
              |        |
              |        |
            --+--    --+--
            \ | /     /|\   -->
             \|/     / | \  -->
            --+--    --+--
              |        |
      +---+   |        |
 4 >--+   +---*--------+
      +---+
      220 Ohm
This means that during a logical '0' a current of ca 5 mA runs through the cable. A disconnected cable signals a continuous '1', which means no data whatsoever.

RS232 / V24

Limits

Min
- 5 ... 5 Volt
Max
-25 ... 25 V
Most RS232 buffers (1488, 1489) can cope with -15 ... 15 V

Data signals

'1'
< -3 V
'0'
> 3 V

Status signals

ON
> 3 V
OFF
< -3 V

Misc info

PC's use + / - 12 V, most modems + / - 8 V
They consider > 1 V On or '0' and < 1 V Off or '1'
Typical input impedance is 4k7

Balanced standards

General

These use two opposite polarised signals called A and B. 
       +---+   +-
  A    |   |   |
       +   +---+

       +   +---+
  B    |   |   |
       +---+   +-

       +   +---+  <  2 ...  6 V
       |   |   |
 A - B |   |   |
       |   |   |
       +---+   +- < -2 ... -6 V
A - B > 300 mv (US 200 mv) is ON or '0'
A - B < -300 mv (US -200 mv) is OFF or '1'

The Common Mode Voltage ((A + B) / 2) is -7 ... 12 V, so you can drive a line from a single 5 V source.
The line is terminated with 100 Ohms.
RS 422 Buffers (75172, 75173) can be used for most balanced interfaces.
The European version (300 instead of 200 mv) is often referred to as V11 or X27.

V35

V35 drives + / - .55 V into a 100 Ohm load.
The CMV is 0.