Common Hardware Connections for Digital I/O and Counter/Timer Boards

Aperçu

This document is part of a comprehensive tutorial on industrial Digital I/O and Counter/Timer hardware. Learn about National Instruments product offerings for digital and timing I/O, the Industrial Feature Set including watchdog timers and isolation, complementary devices like relays, solenoids and encoders, concepts like sinking and sourcing, and see how these devices can be used in your industrial application.

For more information return to the Complete Industrial Digital I/O and Counter/Timer Tutorial

Contents

Bank Isolated Digital I/O Boards

Connecting Isolated Inputs
The following figure shows signal connections for a supply connected to an isolated input. The power supply can be any DC voltage within the NI 6514/15 device range.




Connecting Sinking Outputs
The following figure shows an example of driving a load with the photocouplers and Darlington arrays (sinking current).



Connecting Sourcing Outputs
The following figure shows an example of driving a load with the photocouplers and Darlington arrays (sourcing current).

Non-isolated Digital I/O Boards


Connecting TTL Signals, LEDs
The following figure shows an example of signal connections for three typical digital I/O applications. Port 0 is configured for digital output, and port 7 is configured for digital input. Digital input applications include receiving TTL signals and sensing external device states such as the state of the switch in the figure. Digital output applications include sending TTL signals and driving external devices such as the LED shown in the figure.





Connecting Relays (< 24 mA)

The following illustrations show examples of connecting the NI 6509 to a relay that does not require more than 24 mA.

Sinking

 

Sourcing

 

 

Connecting Relays (> 24 mA)

The following illustrations show examples of connecting the NI 6509 to a relay that requires more than 24 mA.

Sinking

 

 

Sourcing

Connecting Solid State Relays

Channel to Channel Isolated Digital I/O Boards


Connecting Isolated Input
The following figure shows signal connections for a supply and load connected to an isolated input of a NI 6528 board.


Connecting Sinking Outputs



Connecting Sourcing Outputs

Isolated Counter/Timer Boards

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.