U6

An example of an unpowered isolated signal would be a photocell where the sensor leads are not shorted to any external voltages.  Such a sensor typically has two leads, where the positive lead connects to an AIN terminal and the negative lead connects to a GND terminal.

One example of measuring a signal from the LabJack itself, is with an analog output.  If you are new to our products and need to write code to measure or output analog voltages, this is one of the first things that you should try to do.  All I/O on the LabJack share a common ground, so the voltage on an analog output (DAC) can be measured by simply connecting a single wire from that terminal to an analog input compatible terminal.  The analog output must be set to a voltage within the range of the analog input.  

Example Connection:

Simply connect a DAC channel to an AIN channel.  Try writing a value to a DAC channel and then reading the AIN channel.

A common question is "can this sensor/signal be measured with a LabJack".  Unless the signal has a 
voltage (referred to the devices ground) beyond the devices limits, it can be connected without 
damaging the LabJack.  However more thought is required to determine what is necessary to make useful 
measurements with a LabJack or any measurement device.

A few important considerations must be considered before connecting an analog sensor to a LabJack must 
be made to be sure that you don't harm the device and get useful information.  

• Signal Voltage
• Signal's Source Impedance
• A Devices Resolution (and Accuracy)
• Speed

Signal Voltage:

Whether you are trying to make single-ended analog readings or differential analog readings (if your 
device permits) you need to make sure that the voltage on each channel with respect to ground is still 
within the common mode limits.  When measuring parameters other than voltage, or voltages too big or 
too small for a particular LabJack, some sort of sensor or transducer is required to produce the proper 
voltage signal.  Examples are a temperature sensor, amplifier, resistive voltage divider, or perhaps a 
combination of such things.

Notable piece of information: Our U6 devices feature Fully-Differential Inputs that output calibrated 
voltage readings.  

Controlling relays using a labjack requires a little more attention to details, but is still very easy.  The important detail to note is that all of the Digital I/O lines on a labjack have series resistance that restrict the amount of current they can sink or source.  This being known, most solid-state relays (SSRs) can still be controlled directly by the digital I/O.  A more robust way of controlling relays is to connect the positive terminal of the SSR to the LabJack’s VS (~5 volts) terminal and connect the negative terminal to a Digital I/O pin.  This is known as “sinking configuration” and essentially inverts the logic necessary to control the relay.

Figure 1. Relay Connections (Sinking Control, High-Side Load Switching)

The two important states to control the SSR are:

• Output-Low, causes the control current to flow which turns on the relay
• Input mode, causes the control current to stop flowing which turns off the relay

The third available digital I/O state, which is not recommended, is:

• Output-High mode, which often puts the SSR control voltage in a region that is not defined by most manufacturers.  

For this example, the Series 1 (D12/D24) or Series T (TD12/TD24) relays from Crydom were used.  They are specified to have a max turn-on of 3.0 volts, a minimum turn-off of 1.0 volts, and a nominal input impedance of 1.5kΩ.  The following is how a LabJack performs in each of the three modes when modeling the relay as a simple resistor:

To detect whether a mechanical switch is open or closed, connect one side of the switch to the LabJack's ground and the other side to a digital input.  The behavior is very similar to the open-collector described above.  

Figure 1. Basic Mechanical Switch Connection To Digital Input

When the switch is open, the internal 100 kΩ pull-up resistor will pull the digital input to about 3.3 volts (logic high).  When the switch is closed, the ground connection will overpower the pull-up resistor and pull the digital input to 0 volts (logic low).  Since the mechanical switch does not have any electrical connections, besides to the LabJack, it can safely be connected directly to GND, without using a series resistor or SGND.

When the mechanical switch is closed (and even perhaps when opened), it will bounce briefly and produce multiple electrical edges rather than a single high/low transition. For many basic digital input applications, this is not a problem as the software can simply poll the input a few times in succession to make sure the measured state is the steady state and not a bounce.  For applications using timers or counters, however, this usually is a problem.  The hardware counters, for instance, are very fast and will increment on all bounces.  Some solutions to this issue are:

Open-collector (also called open-drain or PNP) is a very common type of digital signal.  Rather than providing 5 volts and ground, like the push-pull signal, an open-collector signal provides ground and high-impedance.  This type of signal can be thought of as a switch connected to ground.  Since the LabJack digital inputs have a 100kΩ  internal pull-up resistor, an open-collector signal can generally be connected directly to the input.  When the signal is inactive, it is not driving any voltage and the pull-up resistor pulls the digital input to logic high.  When the signal is active, it drives 0 volts which overpowers the pull-up and pulls the digital input to logic low.  Sometimes, an external pull-up (e.g. 4.7kΩ from Vs to digital input) will be installed to increase the strength and speed of the logic high condition.

Figure 1. Open-Collector (NPN) Connection To Digital Input

Most LabJack devices have digital I/O lines providing the ability to control external devices and receive input.  In general, digital I/O lines have three states to be aware of:

• Output-High also known as Active High
• Output-Low also known as Active Low
• Input also known as High Impedance or Tri-State

These three states can be sub-classified into two more general modes:

• Output mode (contains the state Output-High and Output-Low)
• Input mode

The most basic connection to a LabJack digital input is a driven signal, often called push-pull.  With a push-pull signal the source is typically providing a high voltage for logic high and zero volts for logic low.  This signal is generally connected directly to one of the LabJack's digital I/O pins.  If the signal is at a voltage higher than the devices maximum tolerated voltage an additional series resistor can be used to drop the voltage to a tolerable level.  LabJack devices have protective devices that clamp the voltage at GND and VS, the additional series resistor is ued to limit the current through these protective devices.  For instance, if a 24 volt signal is connected through a 22K ohm series resistor about 19 volts will be dropped across the resistor, resulting in a current of 0.9mA, which is no problem for LabJack devices.  The series resistor should be 22K ohm or less to make sure the voltage on the I/O line when low is pulled below the 0.8 volt logic threshold of LabJack devices.

The other possible consideration with the basic push-pull signal is the ground connection.  If the signal is known to already have a common ground with the LabJack, then no additional ground connection is used.  If the signal is known to not have a common ground with the LabJack, then the signal ground can simply be connected to GND.  If there is uncertainty about the relationship between signal ground and the LabJack's ground (e.g. possible common ground through AC mains), then a ground connection with a ~10 ohm series resistor is generally recommended.  

Figure 1. Driven Signal Connection To Digital Input

AppNote explains the operation and use of the SPI protocol with our LabJack products and the SCA3000 3-axis accelerometer sensor. 

Compatability

• UE9
• U6
• U3

Overview of SPI

The basis of SPI technology is a 4-wire serial communication protocol.  One of the wires is chip select, driven low for the sensor that you wish to communicate with; another is clock, determines the speed at which data is transferred to and from the device; MISO (Master In Slave Out), the line that allows the slave device to give the master device information; and MOSI (Master Out Slave In), the line that allows the slave device to receive information from the master.  Our labjack devices are full duplex compatible so they transfer information out of the SPI register at the same time as information is put into the register (information that is read from the sensor).  

Using a LabJack as the Master

 Before attempting to connect a sensor to a LabJack device, make sure that the following conditions are met:

This AppNote explains the operation and use of the I2C functionality of our LabJack devices with special regards to the Melexis HMC6352 IR temperature sensor.  All example VI's created in this example were created in 6.0.2 and use our labview library which can be found here.

Compatability