logic low on 9476 seems high

[Ryan_D.]

I'm supplying 12 V to my 9476 (and have verified this by measuring at the pins). When I toggle any one of my DO channels I see the 12 V when I'm at logic high, but when I set it to low I'm seeing about 7.75 V. I wouldn't be surprised, I suppose, to see some amount of voltage when low, but that's nearly 2/3 or the supplied voltage. I have some level shifters that some of these channels are connected to and that 8V is preventing the shifted voltage from being anything but logic high.

So my questions:

• When I output a zero on one of my 9476's lines is it actually supposed to output a low level or is it instead floating the line (and therefore requires a pulldown resistor at the output)?
• If it is supposed to output an actual voltage, why on earth is it so high relative to the Vsup?

Thanks!

[QueenSelene · Accepted solution]

Hi Ryan,

It looks like this is actually expected behavior, as explained in this KB: http://digital.ni.com/public.nsf/allkb/6D6DDEA68D41E94E86257168005B948D

So to have DO low be zero you would need a pull-down resistor, as you mentioned. The process of doing this is explained here: http://digital.ni.com/public.nsf/allkb/C9019BC9F6544005862572BF0059B532

Hope this helps!

[Ryan_D.]

Okay, I realize with this being an industrial IO module I probably should have been more aware of the output states, that said, maybe this should be called out a little better in the operating instrucstions and specifications for the module? I looked again and still don't see anything in there that would suggest the output state is not (at least close to) zero volts. A KB is nice, but this is definitely the type of information that needs to be in the spec so it's not missed when configuring a system. Or it could be that I just missed it (both times)!