looking for a digital output device that powers up with low or high-Z TTL

[bmihura]

I recently bought an NI USB-6501 that powers up with its digital outputs high. There's a 4.9k pull-up resistor to 5 volts.

Who designs this type of stuff? I guess you get what you pay for at $99.

I'd like a device that powers up with low or high-Z TTL outputs. Does NI or some other company sell such a device?

[WillPwr]

Hello, Bmihura!

If you're interested in hardware with programmable power-up states, I'd suggest taking a look at the following document:

http://digital.ni.com/public.nsf/allkb/A98392F93E54D07786256E6F004DE4C3

At the bottom, you'll see that you can choose from any of the following NI devices:

• 6509
• 651x (6511, 6512, 6513, 6514, 6515)
• 652x (6527, 6528),
• All M Series boards

[bmihura]

That is a handy document, thanks. I found a $28 device that works great though; the Arduino Uno. I got more than I paid for with that. It powers up in the low state, and it's quite easy to control with LabVIEW with standard serial port commands.

You do have to program the thing, as it's a microcontroller development board. My program sits in a permanent loop waiting for one byte from the serial port, then writes to a digital output pin. The Arduino products have plenty of example code, so writing that program was straightforward.

[acct5]

According to the manual, 371456h.pdf, page 16:

"At system startup and reset, the hardware sets all DIO lines to high-impedance inputs. The DAQ device does not drive the signal high or low. Each line has a weak pull-up resistor connected to it."

So each device pin is powering up as an input but the pull-up resistor makes it look like its being driven high but its not a driven output.  Are you sure this start-up condition is a problem?  What are you connecting to?

[Buechsenschuetz]

There are probably very few cases where a weak pull-up resistor (around 5kOhms) will be disturbing. 

If there is a TTL output from a peripheral device connected to a line with a weak pull up it will sink 1mA (when the peripheral output is in low state) or source almost zero current (if in high state). Both usually will pose no problems.

If the line with a weak pull-up is connected to a peripheral device with TTL inputs, these inputs will see a high state. If the digital line had no pull up and would be in high Z state, the TTL input of the peripheral device will be floating - and this usually is NOT desirable. In most cases, a floating TTL input will be considered to be in high state, but this is not reliable, and the floating (or open) input may pick up noise.

So I think the designers who connected the digital lines with a weak pull up had something in mind which will meet the requirements of most customers. We are using NI boards with similar digital line configurations for years now and never had serious problems.  

[bmihura]

I'm connecting this to a relay board from Flextronics; the pull-up resistors on the NI USB-6501, even though they are weak at 4.9 k, are enough to activate the relays when the computer is rebooted. That causes smoke and fried electronics in my system, whereas the Arduino Uno digital outputs stay low until you tell them to go high.