6501 digital input seems to be soucing too much current for my driving circuit to handle.

[vanderkin]

The NI-6501 digital input seems to be soucing too much current for my driving circuit to handle. 

The driving circuit is an open collector with a 68k pull up to 5V. 

The digital output schematic of the the 6501 shows that it has a 4.7k pull-up. 

Is this pull-up connected even when the pin is configured as an input? 

Is there a way to configure the pull up to be removed?

Or can I remove the 4.7k pull-up physically from the 6501 for the 4 inputs that are not able to sink much current?

[Brian F.]

Hello vanderkin.

Thank you for contacting National Instruments.

Because the 6501 has an internal pull-up resistor, the resistor is connected even when the pin is configured as an input.  There is no way to configure the pull up to be removed and National Instruments does not recommend that you physically remove the pull-up resistor.  Doing so would void any type of warranty on the device. 

In order to further assist you, could you tell me a bit more about your application.  Additionally, what current value do you desire to be output and how much current is being output. 

Have a great day!

Brian F

Applications Engineer

National Instruments

[vanderkin]

I am trying to interface the 6501 to an existing circuit.  The circuit uses an infrared LED and a sensor.  The sensor is pulled high through a 68K ohm resistor which passes about 72uA when the sensor is ON.  Without the 6501 connected this results in a voltage of .12V.  When I connect the 6501 the voltage goes up to 3.6V so the current is about 300uA.  I would desire that since I want to use the 6501 as an input that it not supply any current to the existing circuit.

[Brian F.]

Hello vanderkin.

The USB-6501 can be configured to either sink or source current if you are generating a Digital Output Task.  This functionality can be configured in a DAQmx channel property node by selecting Digital Output>>Output Drive Type.  If you change the property node to write, you can control whether or not the device is in "Open Collector" or "Active Drive" mode which effectively configures the device to sink or source current. 

If you have any other questions, we will be happy to answer them!

Brian F

Applications Engineer

National Instruments

[vanderkin]

I am using the USB-6501 as an input, so I am not generating a Digital Output Task.  If I am using the 6501 as an input, can I still perform this configuration so that it sinks current rather than sourcing it as it does now?  Can you send me a schematic of one input for this device in all of its possible input configurations so that I can understand?  Thank you.

[Brian F.]

Hi Vanderkin. 

Sorry for that mixup.  It seems that the 4.7kohm resistor in the 6501 is providing a path of less resistance for your current and thus is being routed back into your circuit after the digital input is measured.  In order to stop this, I would recommend routing the signal to be input into the 6501 to a buffer amplifier first.  The buffer amplifier, with unity gain, will not affect the level of your signal, but it will restrict the current flow into the USB-6501.  For more information on buffer amplifiers, please refer to the following website:

Buffer Amplifier

http://en.wikipedia.org/wiki/Buffer_amplifier

Another way to improve the performance of your application is to upgrade your DIO card to one of our M series cards which offer more input impedance than the 6501.  This higher input impedance will reduce the current and voltage that are being routed back through the circuit. 

Good luck on your application!

Brian F

Applications Engineer

National Instruments

[vanderkin]

Thank you for your help.  I understand your comment on adding a buffer.  It would be much easier for me to remove the 4.7Kohm pull up resistor on one input and see if it works.  I realize that this will void the warrantee.  This is not a problem.  However, I would like to get a schematic of the board (or at least 4 of the input lines) so that I can remove the correct pull up resistor(s).  Can you send me a circuit diagram (with reference designators)?  Thank you.

[Brian F.]

Hello vanderkin. 

I am currently talking to other engineers to explore where those pull up resistors reside as well as verifying the safe operation of the rest of the board without those pull up resistors.  I will let you know when I have an update for you.  However, I would still recommend that you try using a buffered amplifier since this is certainly safer for your board than removing onboard components. 

Have a great day!

Brian F

Applications Engineer

National Instruments

[DSTUD]

Hello,

I too am interested in removing the pull-up resistor for testing purposes.  I am finding this so frustrating that there is no way around this pull-up configuration.  I wish to test one of my company board's digital outputs using the NI digital Input on a 6008/9 or 6501 device.

However, How can I ever test my UUT's output when high if the Digital input is pulled high?  And the solution of inverting the input solves nothing.  It just is changing the perspective, not the physical hardware.

If you can give me the actual pull-up resistor numbers it would be greatly appreciated.

thanks,

Daivd

[MikhailK]

Hi David,

Here is a paragraph from the NI 6501 User Manual that explains how to determine the maximum value of the external pull-up resistor:

----------------

The default configuration of the NI USB-6501 DIO ports is open collector,
allowing 5 V operation, with an onboard 4.7 kΩ pull-up resistor. An
external, user-provided pull-up resistor can be added to increase the source
current drive up to a 8.5 mA limit per line, as shown in Figure 6.

Figure 6. Example of Connecting External User-Provided Resistor

Complete the following steps to determine the value of the user-provided
pull-up resistor:
1. Place an ammeter in series with the load.
2. Place a variable resistor between the digital output line and the +5 V.
3. Adjust the variable resistor until the ammeter current reads as the
intended current. The intended current must be less than 8.5 mA.

4. Remove the ammeter and variable resistor from your circuit.
5. Measure the resistance of the variable resistor. The measured
resistance is the ideal value of the pull-up resistor.
6. Select a static resistor value for your pull-up resistor that is greater than
or equal to the ideal resistance.
7. Re-connect the load circuit and the pull-up resistor.
Additionally, you can configure the NI USB-6501 DIO ports as active
drive. When configured as active drive, the total current sourced by all
DO lines simultaneously should not exceed 65 mA.

 --------------------