2.8 - Digital I/O
The LabJack U3 has up to 20 digital I/O channels. 16 are available from the flexible I/O lines, and 4 dedicated digital I/O (CIO0-CIO3) are available on the DB15 connector. The first 4 lines, FIO0-FIO3, are unavailable on the U3-HV. Each digital line can be individually configured as input, output-high, or output-low. The digital I/O use 3.3 volt logic and are 5 volt tolerant.
The LabJackUD driver uses the following bit numbers to specify all the digital lines:
0-7 FIO0-FIO7 (0-3 unavailable on U3-HV) 8-15 EIO0-EIO7 16-19 CIO0-CIO3
The 8 FIO lines appear on the built-in screw-terminals, while the 8 EIO and 4 CIO lines appear only on the DB15 connector. See the DB15 Section of this User’s Guide for more information.
All the digital I/O include an internal series resistor that provides overvoltage/short-circuit protection. These series resistors also limit the ability of these lines to sink or source current. Refer to the specifications in Appendix A.
All digital I/O on the U3 have 3 possible states: input, output-high, or output-low. Each bit of I/O can be configured individually. When configured as an input, a bit has a ~100 kΩ pull-up resistor to 3.3 volts (all digital I/O are 5 volt tolerant). When configured as output-high, a bit is connected to the internal 3.3 volt supply (through a series resistor). When configured as output-low, a bit is connected to GND (through a series resistor).
The fact that the digital I/O are specified as 5-volt tolerant means that 5 volts can be connected to a digital input without problems (see the actual limits in the specifications in Appendix A). If 5 volts is needed from a digital output, consider the following solutions:
- In some cases, an open-collector style output can be used to get a 5V signal. To get a low set the line to output-low, and to get a high set the line to input. When the line is set to input, the voltage on the line is determined by a pull-up resistor. The U3 has an internal ~100k resistor to 3.3V, but an external resistor can be added to a different voltage. Whether this will work depends on how much current the load is going to draw and what the required logic thresholds are. Say for example a 10k resistor is added from EIO0 to VS. EIO0 has an internal 100k pull-up to 3.3 volts and a series output resistance of about 180 ohms. Assume the load draws just a few microamps or less and thus is negligible. When EIO0 is set to input, there will be 100k to 3.3 volts in parallel with 10k to 5 volts, and thus the line will sit at about 4.85 volts. When the line is set to output-low, there will be 180 ohms in series with the 10k, so the line will be pulled down to about 0.1 volts.
- The surefire way to get 5 volts from a digital output is to add a simple logic buffer IC that is powered by 5 volts and recognizes 3.3 volts as a high input. Consider the CD74ACT541E from TI (or the inverting CD74ACT540E). All that is needed is a few wires to bring VS, GND, and the signal from the LabJack to the chip. This chip can level shift up to eight 0/3.3 volt signals to 0/5 volt signals and provides high output drive current (+/-24 mA).
- Note that the 2 DAC channels on the U3 can be set to 5 volts, providing 2 output lines with such capability.
The power-up condition of the digital I/O can be configured by the user with the "Config Defaults" option in LJControlPanel. 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 5 ms at power-up where all digital I/O are in the factory default condition.
If you want a floating digital input to read low, an external pull-down resistor can be added to overpower the internal 100k pull-up. 4.7k to 22k would be a typical range for this pull-down, with 10k being a solid choice for most applications.
The low-level Feedback function (Section 5.2.5) writes and reads all digital I/O. For information about using digital I/O under the Windows LabJackUD driver, see Section 4.3.5. See Section 3.1 for timing information.
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 low-level function ConfigU3, the parameter FIODirection is a single byte (8 bits) that writes/reads the power-up direction of each of the 8 FIO lines:
- if FIODirection is 0, all FIO lines are input,
- if FIODirection is 1 (20), FIO0 is output, FIO1-FIO7 are input,
- if FIODirection is 5 (20 + 22), FIO0 and FIO2 are output, all other FIO lines are input,
- if FIODirection is 255 (20 + … + 27), FIO0-FIO7 are output.
- UD Series (U3, U6, UE9)
- User's Guide
- 1 - Installation
- 2 - Hardware Description
- 3 - Operation
- 4 - LabJackUD High-Level Driver
- 5 - Low-level Function Reference
- Appendix A - Specifications
- Appendix B - Enclosure and PCB Drawings
- U3 Firmware Revision History
- User's Guide
- T Series
Thanks for all of your help.—Ian, USA