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13.8 DIO Hardware Characteristics

Overview

This page will go into more detail about the hardware behavior of the DIO

Series Resistance

Each Digital I/O (DIO) line has an internal series resister. This serves as a safety feature, protecting the internal processor from Electrostatic Discharge (ESD) and overvoltage conditions caused by accidental wiring errors or incompatible connections.

Impact on Signal Speed This resistance reacts with the capacitance of your cabling and the connected device to create a low-pass filter effect. This effectively "smooths" the edges of digital pulses, limiting the maximum usable frequency, typically to around 5 MHz.

Choosing the Right Pins The amount of resistance varies by pin group. On the T4 and T7:

  • EIO lines have lower series resistance than FIO lines.

  • Recommendation: Use EIO lines for high-speed applications like hardware SPI, I2C, or high-frequency PWM, as the lower resistance allows for sharper signal edges and better data integrity.

For the specific Ohmic values of your device's pins, please refer to A-2 Digital I/O.


Overvoltage Protection

In addition to series resistance, the Digital I/O (DIO) lines are protected by clamping diodes. These diodes are positioned behind the protection resistor and “clamp” the voltage if it exceeds the power rails.

How Clamping Works The diodes are tied to the internal supply rails: GND (0V) and VS (nominally 5V). Because these diodes require a small "forward bias" voltage (~0.7V) to begin conducting, the protection active thresholds are typically -0.7V and 5.7V.

When a voltage outside this range is applied to a terminal, the diodes begin to conduct, forcing the excess voltage to drop across the series resistor instead of reaching the processor.

The "Sacrificial" Resistor If the applied overvoltage is large enough, the current will eventually exceed the power rating of the series resistor, causing it to burn out. This is a deliberate "failsafe" design:

  • The resistor acts as a fuse, sacrificing itself to save the significantly more difficult to replace processor.

  • If a resistor "blows," that specific I/O line will stop functioning (it will appear as a floating channel), but the rest of the device remains operational.

  • This allows for a relatively simple and inexpensive repair.

The No Damage Limit (found in the specifications) is the maximum voltage the device can withstand before this sacrificial failure occurs.


Interfacing with Voltages Above 3.3V

T-Series Devices use 3.3V logic, discussed here: Logic Levels

For applications that need to interface with larger voltages, there are several strategies that can be used. The optimal strategy will depend on the voltage and whether that voltage will be read or written.

Inputs Greater than 5V

When reading digital signals that exceed 5V, external circuitry is often required to protect the LabJack and ensure that the state of the signal can be detected.

  • LJTick-Divider: This module divides the signal and buffers the output. The buffer allows larger resistors to be used while minimizing the effects of filtering and providing a strong signal to the T-Series device.

  • RB12 (Isolated): The RB12 uses isolated relay modules available in both input and output configurations. The isolation provides protection against large transients and common-mode voltages.

  • Resistive Divider: A simple two-resistor divider can be used to drop the voltage to a safe value. Larger resistors pull less current from the signal being measured but will increase signal filtering.

  • Direct Connection: Because of the internal protection resistors and clamping diodes, the DIO can handle signals up to the Maximum Input Voltage limit (refer to Appendix A-2: Digital I/O).

  • Series Resistor: Adding an external resistor in series with the signal limits current further, allowing the DIO to read voltages beyond the Maximum Input Voltage limit. The resistor should be sized to keep the current below 20 µA. Note that adding large resistors can cause signal filtering effects.

Outputs Greater than 5V

When you need to write (output) a signal to a system using 5V, 12V, or 24V, external circuitry is usually required.

  • DAC: For applications that only need one or two 5V outputs, updated at lower speeds, the onboard DACs (Digital-to-Analog Converters) can be used.

  • LJTick-DigitalOut5V: This module will convert the 3.3V logic signals to 5V. It also supplies up to 50 mA.

  • RB12 (Relay Board): Using isolated solid-state relays allows you to switch large DC or AC voltages safely.

  • LJTick-RelayDriver: This module has two built-in open-collector switches and can handle up to 200 mA. While often used to actuate mechanical relays and solenoids, it can also send digital signals by adding an external pull-up resistor.

    • Note: In this configuration, the logic is reversed; when the DIO is set High, the output of the RelayDriver will be Low.

  • PS12-DC: The PS12 uses high-side switches to supply up to 24V and 750 mA to a load. Similar to an open-collector but pulling the line High, a pull-down resistor is required to send a digital signal.

  • Open-Collector (NPN Switch): This uses a transistor as a simple switch to pull a line to Ground (Logic 0). An external "pull-up" resistor then pulls the line to the higher voltage (e.g., 12V) when the switch is off. The result is a strong pull to GND and a gentle return to the power supply voltage.

    • Note: This method is simple but limited in speed and the amount of current it can supply through the resistor.

    • Note: In this configuration, the logic is reversed; when the DIO is set High, the output of the open-collector circuit will be Low.