The GPIO connectors are wired as follows;

1 I/P 8+ve O/P 8+ve
2 0v 0v
3 I/P 7 –ve O/P 7 –ve
4 I/P 6 +ve O/P 6 +ve
5 0v 0v
6 I/P 5 –ve O/P 5 –ve
7 I/P 4 +ve O/P 4 +ve
8 0v 0v
9 I/P 3 –ve O/P 3 –ve
10 I/P 2 +ve O/P 2 +ve
11 0v 0v
12 I/P 1 –ve O/P 1 –ve
13 +5v +5v
14 I/P 8 –ve O/P 8 –ve
15 I/P 7 +ve O/P 7 +ve
16 0v 0v
17 I/P 6 –ve O/P 6 –ve
18 I/P 5 +ve O/P 5 +ve
19 0v 0v
20 I/P 4 –ve O/P 4 –ve
21 I/P 3 +ve O/P 3 +ve
22 0v 0v
23 I/P 2 –ve O/P 2 –ve
24 I/P 1 +ve O/P 1 +ve
25 0v 0v

Both the GPIO inputs and outputs are optically isolated.

GPIO Inputs
To trigger an input detection, a GPIO input pair would need to have a voltage difference applied to them of between 5v and 24v (across the GPIO +ve and –ve pair).
5v Differential Max Current 1mA
24v Differential Max Current 5mA
There is a resistor on the board for the I/Ps so the Voltages can be applied directly across the hot and cold pins

GPIO Outputs
For the GPIO outputs, these are the collector and emitter junctions of a transistor and the safe max voltage across them would be;
Max Collector / Emitter junction 24V
Max Current 100mA.

Each Input and Output also gives a +5v rail which is protected by a 500mA poly fuse, and a good 0v ground.