USB 6008 Digital output high is only 0.07 V

[VckdVj]

My USB 6008 Digital Output high is limited at 0.07 v and it is consistent.

I tested all digital lines, analog input reads them at same voltage. Compared the voltages on multimeter to see if ai pins are alright. The voltage reading matched measured ai values. If I disconnected ai- from gnd, the voltage floats by 1.4v, as mentioned in the manual. The output sources 5v and 2.5v are also not showing their rated voltage.

I tried disabling "USB selective standby". Observed no change in output.

I want to know if there is anything else I can do to verify if its working... or is it broken?

Thanks

Vijay

[Buechsenschuetz]

I do not know this device in detail. Does it have open-collector outputs? This output configuration does not show significant voltage leves in high state. The output voltage will go "high" (i.e. near 3.3/5VDC) if you connect a pull-up resistor to the supply voltage.

[crossrulz]

Well, let's look at the specification.  On page 3 under Digital I/O, it clearly states that the driver type is Open Collector.  What this means is that when the digital line is turned off, it will turn on a FET inside of the module to pull that line to GND.  In order to get the high voltage you want (up to 5.8V), you have to supply a pull-up resistor.  USB DIO tend to be Open Collector for power consumption reasons.

[VckdVj]

Thanks Guys for your quick reply!

I am not familiar with lot of terms in your answers. I looked up for 'pull-up' resistor, but could didn't figure out where should it be here. Attached it between 5v output and analog input as shown in pictures.

I noticed that current is more or less in range as if 5v is applied, but no measurement shows anything closer to 5v.

[TaranjeetHullon · Accepted solution]

There is an internal 4.7kOhm to 5V pull up on the digital output for this module (as per the manual). You also mentioned that your 5V and 2.5V are off.

Is there anything connected to the connector of the module ? Is the module being detected correctly in NI-Max ? With nothing at all connected and if the module comes up correctly, if the 5V value is still wrong, I suspect something is wrong with the hardware.

[Buechsenschuetz]

I am a bit confused, since you mentioned problems with digital outputs in your first post and now you are talking 5V output and analog inputs....

"Open collector" means that there is a transistor (or equivalent circuit) at the output. The base of the transistor is driven by the control board and the emitter is connected to system ground (GND). The collector is connected directly to the output terminal (or pin) without any additional circuitry, so it is "open". 

When the control board sets the base terminal of the transistor to a higher voltage, the collector of the transistor (or the output pin) is connected to GND and you will not see any voltage. When the control board sets the base to zero control voltage, the output transistor will not be conductive. But since there is no connection to the collector, you will not see anything usable at the output. The output (or collector) must be connected to the supply voltage (+5V, sometimes this voltage is available at the output terminal) via a resistor. The current to this resistor will cause a voltage of (almost) 5V across the transistor. 

This is basic electronics, but I know that many software guys are everything but familiar with this (just as we hardware guys do not know about software). If you still have problems how to get proper levels from an open-collector output, you should ask a senior colleague.