What's the digital output difference between PCI-DIO-96 and USB-6008?

[Kevin2]

I try to use the digital output to trigger relay module (CIO-ERB48).

After test, i found PCI-DIO-96 has enough power to trigger but not USB-6008.

What's the reason for that? Any way to solve the problem because i have to use USB-6008?

BTW, the attachment is the relay document.

Thanks!

Kevin

[johnsold]

The USB-6008 has a somewhat strange output configuration.  It can be set to operate as open collector (open drain) with 0.6 mA current sourcing capability, up to 8.5 mA with appropriate external pull-up resistor, or 8.5 mA at 3.3 V with active drive.  Read the Digital I/O section and the Specification section of the NI USB-6008/6009 User Guide for details and guidelines for calculating the external resistor.

Note that an external pull-up resistor will cause the relay input to be high from the time power is applied until your program has configured the line as an output and written a low value to the line.

Lynn

[Kevin2]

I’ve tried following the instruction of USB-6008 about selection of the external pull-up resistor for digital I/O port. The current goes up when turn down the value of the pot resistor (10K). The current only reach up to about 2.5ma. The load I used is one of the ctrl channels of the CIO-ERB48. I measure the resistance of the pot and show only 0.7 ohm, then I selects a 1 ohm resistor to connect to the load circuit. When hook up to the PC, the current wont changed and still remain in 0.7ma. Change the state from H to L or L to H still nothing change. The relay won’t trigger. The voltage BTW digital port and GND is about 1.3VDC remain no change. An interested thing is, I did try to hook up the analog output port to instead the digital port. It worked! The relay can be trigged follow the H-L or L-H control. The load current is about 3.5ma (single relay channel). Please advise how I can make the digital port work for the relay module of CIO-ERB48.

Thanks!

Kevin

[johnsold]

Kevin,

It is a little difficult to tell exactly what you were doing.  The smallest value of external pull up resistor which can be used with the USB-6008 DO lines is 680 ohms.  That in parallel with the internal 4700 ohm pullup will result in 8.5 mA flowing though the output transistor when it is pulling low.  Using a resistor of that size does not leave any current for the load device.  Your value of 1 ohm could have destroyed the device if it does not have internal current limiting.

Your CIO-ERB device has a 2200 ohm resistor to ground and the input must be pulled high (value not specified) to activate the relay.  If we assume that the voltage at the input must exceed the typical TTL Vinh >= 2.0 V, then the pullup to +5 V needs to be less than 3300 ohms. Recalling the internal 4700 ohm pullup, the external pullup resistor can be any value less than 10000 ohms and greater than 680 ohms.  I do not like to push the limits at either end, so I would probably choose a value between 1000 and 3000 ohms.

Remember: This will turn on the relay from the time the power is applied unitl the software is running, has configured the USB-6008 DIO lines as outputs, and has written a Low value to the lines.

Lynn

[Kevin2]

Lynn,

I use 3.3K and 3.3K / 2 (parellel another 3.3K) pull-up resistor, but the current is still about 0.75mA.

But the good news is to connect an external 5V (not the 5V that DAQ card provide) to the pull-up resistor, i can trigger the relay and the current is about 2.45mA......

I wonder if the DAQ card 5V can provide the enough power or not, but it should be reach 8.5mA if follow the manual.....

Thanks!

Kevin

[johnsold]

Kevin,

The +5 V output on the USB-6008 can provide up to 200 mA.

How are you measuring the current?  You cannot get access to the current through the internal pullup resistor so you will never see 8.5 mA.

The key issues are (1) does the voltage go high enough to reliably activate the relay when the digital output is High? and (2) is the total calculated current through the digital output sink transistor less than 8.5 mA?

Can you post a diagram showing exactly how you have everything wired up?

Lynn

[Kevin2]

Lynn,

I attach a pic for you. The pic is for the test circuit. The way we connect the current meter is show in the pic.

(1) does the voltage go high enough to reliably activate the relay when the digital output is High?  A: Yes

(2) is the total calculated current through the digital output sink transistor less than 8.5 mA? A: Yes

Thanks!

Kevin

[Joshua-B]

Hi Kevin,

I took a look at your diagram, but am having a little bit of difficulty understanding it.  Do you have the relay wired in parallel with the pull-up resistor?  Would it be possible for you to draw a diagram showing just the components you are using and the pins on the USB-6008 without anything internal to the device?