What Is the State of the Digital I/O Channels of My Device When My Computer Powers On?

NI 6509, 651X, 652X
These devices have an industrial feature set that includes programmable power-up states. The power-up states for the outputs can be set either in MAX or programmatically using the DAQmx Set Power Up States function. The specified states will be stored in nonvolatile memory on the hardware and will be applied to the outputs the next time the board is powered on. The available states are High, Low, and Tristate (High Impedance).

The isolated DIO families (651X and 652X) do not have bidirectional channels. In other words, each channel on these devices can be used for only input or only output. For these devices, the power-up states of the dedicated input channels can not be programmed.

NI 6503, 6507, 6508:
PCI-6503, DAQCard-DIO-24, PC-DIO-24, (NI 6503)
DAQPad-6507, PC-DIO-96, PXI-6508, DAQPad-6508 (NI 6507/6508)
When the computer is first powered on, all channels are configured for input. Internal pull-up or pull-down resistors determine the logic state of the input channel. If a pull-up resistor is present, it connects the line to +Vcc (+5VDC); if a pull-down resistor is present, it connects the line to digital ground. Each of the NI 650X products (except for certain legacy devices that are no longer advertised) has 100 kW pull-up or pull-down resistors.

The PC-DIO-96 and DAQCard-DIO-24 resistors are pull-up resistors. The PCI-6503, PXI-6508, PC-DIO-96, PC-DIO-96PnP, PC-DIO-24, and PC-DIO-24PnP resistors can be configured for pull-up or pull-down by setting a jumper. The DAQPad-6507 and DAQPad-6508 resistors can be configured for pull--up, pull-down, or neither by setting a switch. When configured for neither, the DAQPad resistors are still present and give a resistive path between the individual data lines, but do not connect to power or ground. Refer to the user manual for your device for instructions on setting the jumper and switch.

When your program first configures a port for output, the initial output value is zero, which may represent a glitch if your resistors are pulling up. Therefore, you may want to select pull-down setup for glitch-free power-up and configuration. Furthermore, whenever you reconfigure any of the three ports of an NI 6503 device, all output ports on the same device go to the initial value of zero. Similarly, whenever you reconfigure any one of the three adjacent ports of an NI 6507 or 6508 device, all adjacent output ports go to the initial value of zero. There are four sets of adjacent ports: 0-1-2, 3-4-5, 6-7-8, 9-10-11.


NI 653X:
PCI-6533 (PCI-DIO-32HS), PCI-6534, PXI-6533, PXI-6534, DAQCard-6533
When the computer is first powered on, all lines are configured for input. Each data line has a user-configurable 100 kW internal resistor that can pull up (connect the DPULL on the I/O connector to +5VDC) or pull down (connect the DPULL on the I/O connector to ground). Each control line has a user-configurable 2.2 kW internal resistor that can pull up or down. By default, the data and control lines are pulled down. To make the internal resistors pull up, connect the CPULL line (for the control lines) or the DPULL line (for the data lines) to the +5VDC pin.

When you first configure a port or group for output, the NI 653X maintains the current state of the port or group as the initial output value, so there is no glitch.

Unlike NI 650X devices, an NI 653X device is generally glitch-free at startup. However, even with the NI 653X, there is no guarantee against glitches. For example, you could get noise-induced glitches at power up because the lines (being configured for input) are held in their states by weak (large value) pull-up or pull-down resistors. In this case, you might avoid glitches by externally adding stronger (that is, smaller-value) resistors of your own. Be careful not to violate output source and sink current specifications (of both the NI 653X device or an external digital device), or your application may not work correctly. If a pull-up resistor is too small, the line can never be pulled low; if a pull-down resistor is too small, the line can never be pulled high.

In summary, NI 653X products offer several advantages over NI 650X products:

• The power-up state for the data and control lines (pull-up or pull-down) can be selected independently and without using jumpers.
• Configuring the direction of one group or port does not affect other groups or ports.
• Configuring a group or port for output does not reset the output value to zero, but keeps the current value as the initial output value.

Related Links:
Digital I/O Help for NI 6509/651x/6528 Devices
Product Manuals: NI 653x User Manual
Product Manuals: NI 6503 User Manuals
Product Manuals: NI 6507/NI 6508 User Manuals

Reader Comments | Submit a comment »

DPULL?
does not define use of DPULL. just for power up or during data acquisition?
- Gary England, PREMIER Sys Int. gary.england@premier-system.com - Jun 1, 2006