The power-up condition of the digital I/O can be configured by the user. From the factory, all digital I/O are configured to power-up
as inputs. Note that even if the power-up default for a line is changed to output-high or output-low, there is a delay of about 100 ms
at power-up where all digital I/O are in the factory default condition.
The low-level Feedback function (Section 5.3.3) writes and reads all digital I/O. See Section 3.1 for timing information. For
information about using the digital I/O under the Windows LabJackUD driver, see Section 4.3.5.
Many function parameters contain specific bits within a single integer parameter to write/read specific information. In particular,
most digital I/O parameters contain the information for each bit of I/O in one integer, where each bit of I/O corresponds to the same
bit in the parameter (e.g. the direction of FIO0 is set in bit 0 of parameter FIODir). For instance, in the function ControlConfig, the
parameter FIODir is a single byte (8 bits) that writes/reads the power-up direction of each of the 8 FIO lines:
if FIODir is 0, all FIO lines are input,
if FIODir is 1 (2
0
), FIO0 is output, FIO1-FIO7 are input,
if FIODir is 5 (2
0
+ 2
2
), FIO0 and FIO2 are output, all other FIO lines are input,
if FIODir is 255 (2
0
+ … + 2
7
), FIO0-FIO7 are output.
2.9.1 - Typical Digital I/O Connections
2.9.1.1 - Input: Driven Signals
The most basic connection to a UE9 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 UE9
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 about 0.9 mA, which is no problem for the UE9. The series
resistor should be 22 kΩ or less, to make sure the voltage on the I/O line when low is pulled below 1.0 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 UE9, then no additional ground connection is used. If the signal is known to not have a common ground
with the UE9, then the signal ground can simply be connected to UE9 GND. If there is uncertainty about the relationship between
signal ground and UE9 ground (e.g. possible common ground through AC mains), then a ground connection with a 100 Ω series
resistor is generally recommended (see Section 2.7.3.4).
Figure 2-9. Driven Signal Connection To Digital Input
Figure 2-9 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 UE9 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.
2.9.1.2 - Input: Open-Collector Signals
Open-collector (also called open-drain or NPN) 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 UE9 digital inputs have a 100 kΩ 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. 4.7 kΩ from Vs to digital input) will be installed to increase the strength and speed of the
logic high condition.
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Summary of Contents for UE9
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