logicport and additional current?

[pb]

I was trying to debug an issue tonight using an Intronix Logicport USB logic analyzer.  I had most of the channel probes connected and all of the ground lines.  By chance I disconnected the IDC connector on the logicport (instead of disconnecting each channel probe) and noticed the current reading on the bench supply dropped from .5A to ~.28A.

Playing around a bit more, I started disconnecting groups of channels to see if one channel in particular was causing the additional load.  Eventually, disconnecting a specific one of the ground probes dropped the current back to .28A.  Other ground probes were connected and  I didn't replace all of the disconnected channel probes to see if it was really just that ground connection but I'm curious if this additional load is normal?  I guess I expected a small additional load as a result of the probes but 2X seems pretty high.

The circuit in question is an 8088, 8284, RAM, 27C020, associated address logic, etc... if that matters.

Update: after thinking about this more, I guess this isn't additional load but reducing the load given the current goes up. no?

thanks,

[allanw]

Doesn't make sense. Logic inputs are supposed to be high impedance.

If it was changing from a ground probe being inserted then it was some kind of grounding issue. You should do some more experimentation with the ground connection. Perhaps connecting the ground probe which is connected your computer ground which goes to mains ground  can affect your power supply or your circuit somehow?

[pb]

Doesn't make sense. Logic inputs are supposed to be high impedance.

If it was changing from a ground probe being inserted then it was some kind of grounding issue. You should do some more experimentation with the ground connection. Perhaps connecting the ground probe which is connected your computer ground which goes to mains ground  can affect your power supply or your circuit somehow?

The additional current occurs when I connect a second ground probe (2 of 4).  Doesn't matter which two and happens even when no non-ground probes are connected (i.e. only two ground probes).  So.. off to investigate more.