What are the voltages on the IMAQ-D6804 cable's BNC connectors?

[davie]

I am having trouble getting my laser device to trigger (it requires 5V to trigger).  It was actually working before but then suddenly stopped working.  I noticed that the measured voltage on each of the BNC connectors isn't the same. one is 4.83V, two of them are 4.73V and one is 0.22V.  I tried connecting the laser device to all of them but none of them worked.

When I connect the laser device to the bnc connector with the highest voltage, the measured voltage on the bnc connector drops to around 1V.  When a trigger signal is sent, the voltage drops to zero then returns to its original value but the laser itself doesn't actually fire a pulse.

I'd like to know what the measured voltages are suppose to be on IMAQ-D6804 cable bnc trigger connectors when no devices are attached to them as this may help me figure out this problem.

Thanks,

[J.Harv]

davie--

     The output specifications (and what you should see without anything connected) are as follows:

V range--             0-5 V (TTL)
V output high--     2.4 V at 15mA source
V output low--      0.55 V at 10 mA sink

These spec. follow the standard for a TTL signal where the signal will vary between high and low.  So in comparison to a low, where the signal is below 0.55 V,  to be considered high the voltage simply must be above the 2.4 V level, which is high in comparison.  You can read more about TTL specs at the following link:

http://ae.natinst.com/operations/ae/public.nsf/web/searchinternal/acb4bd7550c4374c86256bfb0067a4bd?OpenDocument

The reason that the signal drops to 1 V when connected to the laser is that the laser seems to be non-standard and is drawing more current.

Since, your laser seems to have a non-standard trigger requirement (non-TTL) your options are to use some kind of external circuitry to ensure that the laser gets the 5V it requires.  Some options could be to use a relay or an operational amplifier (comparitor config) I think the latter would be the way to go.  Although you will have to find out what exactly your laser is requiring in terms of volage and current and then make sure whatever you go with can supply the requirements.

If you have any questions, let us know.  Thanks

John H.
Applications Engineer
National Instruments
http://www.ni.com/support

[J.Harv]

davie--

     I apologize, that link was to an internal document.  The exact same document can be found externally at the following link:

http://digital.ni.com/public.nsf/websearch/ACB4BD7550C4374C86256BFB0067A4BD?OpenDocument

Regards,

John H.

[davie]

Thanks for the reply,

Just to clarify.

What you're saying is that when a device is connected to a bnc external trigger that it's high voltage at that point should measure around 2.4V or above (and at low voltage below .55V) in order for the device to trigger properly assuming it takes a standard TTL signal.

I have another device, a flash lamp that does trigger properly when sending the same TTL pulse using Generate Pulse.VI and it's high voltage is also only 1.5-1.8V when connected to the Imaq board (oddly enough it only works on one of the four external triggers, it requires a 5V trigger).  It still works now but I'm guessing that maybe this is just a fluke, right?

With no device connected, one of the external bnc trigger connectors always measures .22V while the others three are all above 4.5V.  Is there a reason for this?  Is it possible to change the .22V to the higher voltages measured on the other external triggers?  I'm using a PCI 1428 IMAQ board.

Message Edited by davie on 01-19-2007 10:09 AM

[J.Harv]

Davie-

       Sounds like you understand the specifications of what a TTL signal is. Everything you said was correct.  Now it is just a matter of figuring out what is going on with these devices.  It might be possible that we have a ground loop or one/both devices are sourcing/sinking too much current.  Without knowing more about these devices it is really hard to tell.  Do you think that you could post specification sheets (or circuit diagrams) for the laser and the flash lamp, specifically the laser?  Just let us know if there is anything else that we can help with.  Thanks.

John H.



    

[davie]

I'll try and find some more info on the devices.  I just checked it again and I noticed that the low voltage when triggered is always 0.8V so with the high voltage only at 1.5V-1.8V it would seem that the flash like the laser shouldn't trigger at all.  I also noticed that regardless of whether the laser or flash is turned on or not, the voltage still drops to 1.5V when connected.  At the moment I've only tested each device separately (my intended setup will have both of them connected at the same time).

Someone suggested to me that for the external trigger that measures 0.22V that I should check the trigger polarity in Measurement and Automation Explorer but I don't know how to do this and don't see any options that would allow me to do this.

What is a ground loop?

Message Edited by davie on 01-22-2007 11:21 PM

Message Edited by davie on 01-22-2007 11:26 PM

[J.Harv]

Davie-

     You can change the polarity of the trigger that you are driving by using the "TriggerDrive 2.vi"  (depnding on your version) and setting whether hi or low is associated with true.

      A ground loop is when you have two sides of a signal connected together and each side is connected to ground and the grounds are at different potential...causing current to flow between the grounds.  More information can be found by searching Google.  Also, Wikipedia has a pretty good description

http://en.wikipedia.org/wiki/Ground_loop_(electricity)

    Anyway, let us know whenever you can get more information about your devices.  Also, feel free to let us know about any other questions

Regards,

John H