General Purpose Output
All General Purpose Outputs of Allen & Heath use a Relay to switch the output.
NO = Normally open (GPO not activated no contact to the common pin, GPO activated contact to Common pin)
NC = Normally Closed (GPO not activated Contact to Common Pin, GPO Activated no contact to common pin)
C = Common (Switches contact between NC & NO Pin)
If there are only 2 normally open pins, the pins will be contacted on active GPO.
All Allen & Heath GPIO units have a + 10V power supply.
The 10V potential difference is between Ground (Down Arrow) and V+
The device or technical specifications will specify how much current can be drawn from the power supply.
A simple way to use an LED as indicator:
You can either use the Negative or positive pin to be switched.
In this example we are switching the Negative Pin.
The voltage supplied = +10V , we need a resistor to take the voltage to 2V as required by the LED.
To calculate the value of a resistor:
Ohm’s Law states:
R=V/I (R Resistance in Ohms, V= Voltage in Volts, I = Current in Amps).
If we want to calculate the correct resistor for a voltage drop of 8 Volts by 20mA (=0,02A) we get 8V/0,02A=400 Ohms.
The nearest standard resistor is 390 Ohm (Orange, White, Yellow).
Some examples how to connect the LED on different devices:
The graphic below is a diagram for GPO 1 on AHM.
The graphic below is a diagram for GPO 2 on AHM
The graphic below is a diagram for GPO 1 on the GPIO Unit
General Purpose Input
The General Purpose Inputs of Allen & Heath, All switch to ground to become active (Pin not connected is Off state)
When not activated on Allen & Heath devices the GPI Pin is set to +10V using a Pull up resistor.
The threshold for the GPI to become active is 8.6V, Everything lower than 8.6V will activate the GPI to become active.
If you want to use a device that uses TTL (Transistor Transistor Logic), the max Vout of the TTL circuit wil be +5V (The +10V will be pulled down to +5V). You then need a relay or Transistor to release the max +5V to overcome the 8.6V switching threshold.
Tech Insight - Pull-up Resistor - for your information only
Most logic signals act on a connection to Ground (or 0Vcc) as an active state. If the logic port is not connected, the 'floating' voltage can cause false readings. A Pull-up resistor is often used to avoid this. The resistor is there to ensure the logic sees the +Vcc when not connected to the Ground (to pull-up the voltage).
We can use a resistor between the +Vcc and the logic input that uses a minimal amount of current, for example 1mA. To get the correct value we can use Ohm's law R=V/I, where R is the resistor value, V is the voltage and I is the current in Amps.
Lets take a small current of 0.001A and a +10V +Vcc rail.
The formula will be
10/0.001 = 10,000Ohm (10kOhm)
Allen & Heath devices always pull to +Vcc and therefore do not require a Pull-up resistor.