Status of Digiatal output lines

[RT4CY]

Hi Tom,

It depends on whether you want your default state to be logic high or low.

Logic High
Use a pull up resistor.  Wire one end of the resistor to the center pin of the BNC connector.  Wire the other end to a 5V rail.  This will come from an external power supply.

Logic Low
Use a pull down resistor.  Wire one end of the resistor to the center pin of the BNC connector.  Wire the other end to the ground on the BNC connector.

Let me know if you need clarification on this.

Message Edited by RT4CY on 07-09-2008 03:01 PM

[Little_Tee]

And I don't connect anything do the Digital GND on the USB M-Series Board? Just the center pin to the Digital I/O (or trigger or counter) ?

[RT4CY]

Hi Little_Tree,

Actually, you don't need to use resistors at all.  Your M-series device supports programmable power-up (default) states.  See the following KB on how you can set the default sate of the Digital I/O lines on your device.

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

[Little_Tee]

Thanks for that inbelievable quick reply 🙂

Though, I think I do need some kind of proper grounding because I'm experiencing some problems with my triggering. I'm using a counter to watch for a trigger on a line, and I notice it sometimes sees a trigger for no apparent reason. I suspected grounding issues?

[RT4CY]

How have you configured the trigger, and what have you set as the default state of the digital line?  I'm interested in what you have set as the trigger source, and whether you are looking for a rising edge.  If you are looking for a rising edge, make sure that you have set the default state of the trigger source line to 0 (logic low).  If you have taken all these measures then there may be something wrong internally with the hardware, since setting this default state to 0 should pull the line low if it has no input.  Also check the integrity of the trigger signal (TTL logic defines 0.8 Volts and below to be logic low).

[Little_Tee]

I'll check the default state of the digital line tomorrow. There is a chance it is indeed not set to low. I'm indeed looking for a rising edge of a TTL pulse. I remember checking the trigger signal and it's a perfect 5V pulse if I remember right. I'll recheck all those things tomorrow at work and I'll report back.

Just to be sure: I only have to set the default state to 0 and connect the inner pin of the BNC connector to the particular line. I do not connect the outside of the BNC to anything...? 

Thanks and 'till tomorrow.

[RT4CY]

Sorry about that, you are correct.  Please connect your ground wire to the outer shell of the BNC connector.  Ideally you would find a BNC cable which would provide a perfect connection for you (with ground referencing).  What type of connection do you have on the device that is generating the trigger? 

[Little_Tee]

I don't know exactly what you mean with:

Ideally you would find a BNC cable which would provide a perfect connection for you (with ground referencing). 

The connection on the device that is generating the trigger is a BNC connector.

The device I'm triggering (as in controlling with my DAQ card) has also a BNC connector, but that is another story. I suspect I'd apply the same technique to both, so lets just stay with the problem above for now 🙂

[RT4CY]

My apologies.  I thought you were making bare wire connections to the BNC terminal on the USB-6251.  If you simply connect your BNC cable from the device that is generating the TTL pulse (trigger) to one of the PFI BNC inputs of the USB-6251, this should provide proper grounding.  The center pin of the BNC connector will carry your signal, while the outer shell of the BNC is the ground.  Make sure that you set the default state of the line to 0 in MAX (Tri-state should be selected by default if you haven't changed anything). 

[Little_Tee]

No apologies necessary. I never explained it very well , so 

... there is no BNC terminal on the USB-6251 . Or it must be very well disguised 🙂

 http://sine.ni.com/nips/cds/view/p/lang/en/nid/202597

I guess I'll connect the center pin of my BNC connector to the line I'm using to watch for a trigger. The outer shell to the Digital Ground. Correct me if that's not the way to go.