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2.8.1.1 - Input: Driven Signals

The most basic connection to a U6 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 the U6 digital input, considering the voltage specifications in Appendix A. If the signal is over 5 volts, it can still be connected with a series resistor. The digital inputs have protective devices that clamp the voltage at GND and VS, so the series resistor is used to limit the current through these protective devices. For instance, if a 24 volt signal is connected through a 22 kΩ series resistor, about 19 volts will be dropped across the resistor, resulting in a current of 0.9 mA, which is no problem for the U6. The series resistor should be 22 kΩ or less, to make sure the voltage on the I/O line when low is pulled below 0.8 volts.

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 U6, then no additional ground connection is used. If the signal is known to not have a common ground with the U6, then the signal ground can simply be connected to U6 GND. If there is uncertainty about the relationship between signal ground and U6 ground (e.g. possible common ground through AC mains), then a ground connection with a 100 Ω series resistor is generally recommended (see Section 2.6.3.4).

Figure 2.8.1.1-1. Driven Signal Connection To Digital Input

Figure 2.8.1.1-1 shows typical connections. Rground is typically 0-100 Ω. Rseries is typically 0 Ω (short-circuit) for 3.3/5 volt logic, or 22 kΩ (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 U6 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 next.

2 comments

I am trying to use a PNP 24VDC laser proximity sensor to count the teeth on a gearbelt pulley as it rotates. The exact sensor I am using is Keyence LR-ZB100P (http://www.keyence.com/products/sensor/laser/lr-z/models/lr-zb100p/index...)

The sensor has 4 wires:

Brown  10-30V DC in  (Plugged into the + terminal of a 24V DC supply)

White  Input  (Not sure what this wire is used for)

Blue  0V      (Plugged into the - terminal of a 24V DC supply)

Black  Output 30V  (Plugged into the U6 FIO0 terminal with a 22kΩ resistor)

This doesn't seem to be working. I also tried plugging the Blue wire into the GND of the Labjack with a 100Ω resistor but now the sensor won't turn on. What am I missing here?

The link you sent says the output is PNP Open Collector.  That (weakly) suggests that the signal changes between HIGH (24VDC driven) and OPEN (high-impedance ... floating).  If that is the case you would need a pull-down resistor to pull the line low when the sensor output is OPEN.  See forum topic #5475.

Keyence.com seems to require registration to see the datasheet.  Please start a topic on our forum and attach the datasheet so we can look at it.