First, thanks for taking time helping.
OK but what is it. Is it a battery like you showed or is it some form of power supply derived from the mains. If it is then is the output voltage floating or is it tied to ground.
The power source is not a battery, and I have to apologize that the diagram given in this case is misleading. It's a power supply from a main module which delivers voltage output through its own circuit. I will assume the output voltage is likely not floating,
Output of 7805 to uno input pin,
Is this
a) An arduino input pin?
b) The +5V pin
c) The Vcc pin
d) The power input jack?
It's to one of the digital input pin on arduino
uno output pin
Is this
a) A signal output
b) the +5V pin
It is one of the digital output pin.
to a BNC connector that will be hooked up to the downstream device.
So how is this device powered? Is the ground of this tied to any other ground, a BNC plug implies it is. So that when you plug the BNC into your device you will be shorting out the arduino ground with mysterious downstream device.
This downstream device is actually a ADR laser driver, it is powered by an independent 110 VDC, and can take TTL modulation switch (which is what the BNC for). The driver should have its own ground.
Is the arduino connected to a computer through the USB at this point. This will act as another connection to the common ground.
The arduino is powered by a regular AC-DC adapter.
It is my guess that this is what is going wrong, that the grounds are all not floating and therefore can't be connected together
Thanks for pointing out this issue, I had a little concern initially about this grounding scheme, but since the arduino seems worked fine in the absence of downstream laser driver I didn't think too much about it.
I did another test today by removing the BNC from the ground and it didn't help either.
So in this case how will you ground the entire setting??
Again, much appreciated for any suggestion.