NI 6509 PXI TTL Input Impdeance

[JLAnderson]

I have a requirement to measure a TTL logic signal with a 5.1K ohm load.  The specification for a PXI version of the board has high input impedance as a default condition.  What is the actual impedance and can I just put a 5.1K ohm resistor across the inputs?  Are there pull up/down resistors like the PCIe version?  Can the default be changed?

John Anderson

[Buechsenschuetz]

I am not expert for this board. But the manual states that the inputs are in high impedance state after reset only (as far as the "non-e" version is concerned). This means that the input does not present a load to a signal line. In this state, however, you cannot receive any input signals.

After reset, you may configure the inputs as pull-up or pull-down. In a pull-up configuration, the pull-up resistor will be 4.7kOhms typical, in the pull-down configuration it will be 47kOhm typical. Concerning line impedance you have to imagine that the impedance between Vcc (where a pull-up resistor is tied to) and GND (where the pull-down resistor is tied to) is supposed zero, so the signal source will "see" an input impedance of 4.7kOhm or 47kOhm respectively. The choice of pull-down or pull-up resistor depends on the output configuration of the signal source. If you have an open-collector output, the pull-up resistor will be a better choice.

However, all this relates to a pure transfer of the state of the signal source (high or low), and the resistor values are specified as "typical" on purpose, there is no tolerance field indicated. If you have to make a measurement where the impedance of the signal line plays an important role, you should use some analog high-input circuitry and add a resistor of the required value between the signal line and GND.

This answer contains a lot of analog electronics stuff, but an understanding of basic analog circuitry is crucial if you are dealing with line impedances or terminating resistors.

[JLAnderson]

The NI 6509 User Guide and Specifications (372117C) does not state anything about a reset affecting the input impedance.  The only reference to reset is for a watchdog timer.  It also states only the PCIe has programmable pull up/down resistors.  We simply use DAQmxCreateTask, DAQmxCreateDIChan, and DAQmxReadDigitalLines that works just fine.  I assume the high input impedance is set at power up and remains that way.  I don't know if 'default' implies the input impedance is changeable.  One way to find out the resistance, is to measure the current flowing for a logic 1 and calculate the resistance.  The circuit being measured is a 74ABT244A with the enable input grounded.  There is also a tap with 10K ohm resistor going to a high impedance monitor device that should not affect the measurement.

John Anderson

[johnsold]

Look at page 15 of the NI 6509 User Guide and Specifications. It shows a diagram of the DIO circuitry. The 47 kohm pull down is always there. The 4.7 kohm pull up is software selectable.

If you want to make a measurement with a controlled load impedance, consider using an analog input device with a high input impedance and connect your 5100 ohm load. This will also allow you to measure the actual voltages as opposed to the high/low interpretation of the voltage that you get from a digital device.

Lynn

[Buechsenschuetz]

On p.25 of the Users guide, in the dgital I/O specifications, it is stated that the power-on state of the input lines of the NI PXI/PCI 6509 is high Z (default setting) and the inputs can pe pulled up or down (software selectable).

Also, the diagram on p.15 refers to the PCIe version only, and the input specifications for this version (on p.25) differ from those of the PXI version to which the original post was referred.

I stand corrected that only the inputs of the PXIe version have selectable input pull up/down resistors.

[johnsold]

I did not read the manual carefully enough to notice the difference in the versions.

I stand by my suggestion to use an analog input device in parallel with an external load resistor to characterize the device response to s specific load.

Lynn