DIGITAL CURRENT ON/OFF SENSE (e.g. 0-20mA) APPLICATIONS > DIGITAL INPUT >
Digital I/O Hardware
The following instruNet hardware provides digital input channels that
can sense on/off current with the help of an external non-isolated shunt resistor, or an
external Isolated Current-to-Digital-Output converter; both of which are available from 3rd parties.
The i4xx cards have two kinds of digital inputs which are referred to as
"digital i/o" and
; the later of which is more advanced.
In the typical case, a logic 0 corresponds to 0 to 0.8Volts,
and a logic 1 corresponds to 2V to 5V.
For details on the number of channels and the characteristics of each,
click on the model numbers below the above pictures.
For details on working with digital i/o channels,
Measurement via non-isolated external shunt resistor
The instruNet digital inputs can be driven with an external
shunt resistor that converts current to a digital voltage. For example,
to convert 20mA to 3.0V, one would use a 3.0V / 0.02A = 150ohm resistor.
This would then dissipate 3V * 0.02A = 0.06 Watts when
current is flowing
Subsequently, in this case, one might use a resistor
rated for 0.25Watt or 0.5Watt to maintain a reasonable resistor temperature when current is flowing.
Notice in the below drawing that the instruNet ground is connected to the external
shunt resistor and is also connected to the external power supply ground. It is
important that one wire in this manner in order to produce the correct voltage at the
digital input pin.
3rd Party Isolated Current-to-Voltage Converter
The instruNet digital inputs can also be driven with an external
isolated current to voltage converter, available from a 3rd party, as illustrated below.
One diffrerence between this technique and that which is described above is the
external end user power supply here does not need to share the same ground as the instruNet hardware,
which is why it is referred to as "isolated".