The inputs on the NI 6624 can be driven referenced to either the supply or ground of the external device connected to them. They have a current limiter that protects the optical isolator. The optical isolator transfers the externally connected signals to the TIO. A diode protects against reverse connected signals.
Connecting Isolated Inputs
The following diagram shows a single NI 6624 isolated input.
Inputs can be connected to be referenced to either the supply or ground of the external device, depending on whether or not this device can source the amount of current required by the NI 6624 input circuitry. The minimum amount of current required by the NI 6624 inputs to guarantee a digital HIGH is 2.2 mA. The over-voltage and over-current protector does not allow the amount of current flowing through the input circuitry to exceed 10 mA.
Use the following guidelines for connecting the NI 6624 to be referenced to either the supply or ground.
Connecting Isolated Inputs with NI 6624 Referenced to Ground
Connect the external device to the IN+ pin, and connect the device ground to the IN– pin, as shown in the following diagram.
Connecting Isolated Inputs with NI 6624 Referenced to Supply
Connect the Vdd on the external device to the NI 6624 IN+, and connect the output of the external device to the IN– pin, as shown in the following diagram.
Connecting Isolated Outputs
The outputs on the NI 6624 consist of N-channel MOSFETs that are connected as low-side switches. A Schottky diode blocks reverse connections.
The following illustration shows an example of connecting a single NI 6624 isolated output.
In order for the output circuit to function, you must provide and connect 5 to 48 V between Vdd and Vss. Reverse connections of Vdd and Vss do not damage the circuit as long as these connections do not exceed the 60 VDC, as listed in the specifications.
If the amount of current the MOSFET conducts exceeds a certain level (800 mA, typical), such as when a short occurs in the load, the MOSFET turns off for 250 ms to protect itself and the load. After this period of time, the output tries to switch on. If the short still exists, it will be turned off for another 250 ms. This process continues until the overcurrent or short condition is removed, after which switching automatically resumes.