problem with DI PXI-6509

[snostro]

Hello,

I'm using a PXI-6509 connected to SCB-100 to control some DIO. I have connected the pin 51-53-...-65 to some leds and then to the GND (pin 50) of the board and everything is fine.

Instead I have a very strange behaviour trying to use port 0 (I tested also port 3 obtaining same behaviour) to read some DI.

That's what happens looking from the test panel in MAX. (I performed this test with the DO connected to the board and without)

I force all the lines to 0 by connecting GND to each pin (47-45-...-33). (Some lines were to 1)

After some second (15 more or less) line 7 become high, even if I reforce the line to 0 continues to happen.

If I try to force a pin to 1 by connecting the 5V source of the board (pin 49) random lines go to 1. E.g. touching pin 47 (line 0) I see line 0-1-3-5-6-7 going to 1.

The strangest thing is that is not even constant: thouching pin 47 a second time (with all the lines 0) brings to one lines 0-1-5-6-7 or maybe each line to 1.

Same behaviour (random lines going to 1) for all the other lines of port 0, as I said also for port 3. I did not procede with other tests.

I'm getting a bit confused...

Last fact: port 0 is working as DO, if I change the pin mode to output I can measure 5V correctly with the multimiter.

Any idea of what's going on?

[crossrulz]

The digital lines have a built in pull up to 5V.  So when you have nothing connected to the input, it will read a high.  This is done by design to avoid being stuck in the "unknown" voltage range.

[snostro]

If it was like that I would see high state on all the non-connected pins. Actually I have high just on some of them. And the state change (except for line 7) when i try to force to 1 a pin. I have understood that non-connected pins generate random signal (http://digital.ni.com/public.nsf/allkb/4A795F4FCB501E36862562A80080BECD?OpenDocument) without any pullup or pulldown resistor. I would like to understand why this changes happen when I force a pin.