Driven Signals (App Note) | LabJack
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Driven Signals (App Note)

The most basic connection to a LabJack digital input is a push-pull (also called driven).  With a push-pull signal the source is driving (low-impedance) a high voltage for logic high and zero volts for logic low.  Unless the high is too high, the push-pull signal is 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 used to limit the current through these protective devices.  For instance, if a 24 volt signal is connected through a 15 series resistor about 19 volts will be dropped across the resistor, resulting in a current of 1.3mA, which is no problem for LabJack devices.  The series resistor should be 15 or less to make sure the voltage on the I/O line when low is pulled below the logic-low threshold of LabJack devices.  A 15 resistor is usually a great solution for all voltages up to 48 volts.

Other options to handle higher voltage digital signals are the LJTick-Divider-4 or the RB12 with appropriate modules.

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

Figure 1 shows typical connections.  Rground is typically 0-100 ohms.  Rseries is typically 0 Ohms (short-circuit) for 3.3/5 volt logic, or 22K ohms (max) for high-voltage logic.  Note that an individual ground connection is often not needed for every signal.  Any signals powered by the same external supply, or otherwise referred to the same external ground, should share a single ground connection to the LabJack if possible.

When dealing with a new sensor, a push-pull signal is often incorrectly assumed when in fact the sensor provides an open-collector signal as described in the Open-Collector App Note.  

Various mention is made above to protecting from voltages greater than 5 volts.  The actual limits can be found in Appendix A of each device's datasheet.  The U12 starts clamping at VS+0.3, while the U3, U6, UE9, T4, and T7, start clamping at a fixed 5.8 volts, so an added series resistor should be considered for voltages beyond those levels.