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2.8.1.1 - Input: Driven Signals

The most basic connection to a U3 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 U3 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 U3. The series resistor should be 22 kΩ or less, to be strong enough to overpower the internal 100k pull-up and 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 U3, then no additional ground connection is used. If the signal is known to not have a common ground with the U3, then the signal ground can simply be connected to U3 GND. If there is uncertainty about the relationship between signal ground and U3 ground (e.g. possible common ground through AC mains), then a ground connection with a ~10 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 U3 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 need to be able to monitor digital inputs up to 34VDC.  The web page says 0.9ma of input current is no problem but does not specify what the maximum input current can be.  Can you tell me if 29/22 ma (34V through 22k) is OK?

A quick answer is that up to 10mA will certainly not cause damage, but dumping excessive current into digital I/O on the U3 can cause small ground shifts that affect analog input readings, so watch for that if important.  34V through 22k will result in about 1.3mA, which is no problem for sure.

Also, if you add a resistor to ground to make a voltage divider, then you do not use the protection circuitry at all.