Is it OK to remove the pullup resistors on the NI 6501?

[codemonk]

Hi,

I have a NI 6501 that I am using to control a TTL device that has pull down resistors on the inputs.  On power up, I end up with the NI pullup resistor forming a voltage divider with the pulldown resistor and I get a voltage of 2.6V on the lines.  I would like to remove the 4.7k pullup resistor from the PCB of the 6501.  Has anyone done this?  Did it work?  Is there a good reason not to attempt this?

Pete

[Wayne.C]

codemonk,

The pullup resistor is there to prevent a "floating" input state which can lead to the input buffer latching up and drawing a potentially lethal dose of current.  I don't recomend removing the resistors.

You may want to consider inserting an inverting buffer between your DAQ I/O and the TTL inputs.  

[codemonk]

An inverter still doesn't gain me anything since that will generate all TTL high signals once I remove the power from the 6501.  The other option I am considering is to add an 850ohm external pull-down resistor (for a total effective pull down resistance of 720ohms including my TTL device.  This will give me a voltage of 0.6x V when I first attach the USB cable, and will draw <7mA of current from the 6501 operating in push-pull mode for a logic high output.  I can guarentee in software that no more than 4 output lines are set high at any given time.

Thanks for the info,

Pete

[codemonk]

Actually, I just realized that I could power the inverter from the 5V line of the NI 6501 so it would go dead when the power is removed.  That may also be an option, though I am looking for the simplest solution that will work.

[Wayne.C]

Put separate pullups on the inverter inputs.    Something on the order of 47K would be about right.