Arduino won't power on with jumper cables patched [Uno + Sparkfun MIDI shield]

I have a Sparkfun MIDI shield and am using kuk's "MIDI in basic 0.2" code from the old Arduino forum: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1187962258

Vin pin from a 9V battery, switched. Pin2-7 LEDs Pin13 status LED

Pins 2-7 + Ground go to an 8-pin connector flush-mounted to my project box. I am able to power on Arduino when the signal cable (15') is not patched to that 8-pin connector, but if the connection is made then Arduino will not power on.

Does anyone know why I can't boot Arduino if the signal cable is patched?

TIA.

(Of course I hope I've posted this in the right place. Please inform if not.)

How do you determine whether it's booting?

Have you monitored 3V3 and 5V lines to make sure the supply is holding up? Have you got anything connected to the reset pin?

If you can try disconnecting individual wires rather than the entire connector, perhaps you'll be able to narrow down which pin is causing the problem.

Most easily, I can tell because Pin13 is a status LED. But I am also able to run the sketch once I am booted.

I have tried going directly from the Digital I/Os to LEDs without using the 8-pin connector or its extension and everything is fine. And while using the 8-pin connector hookup, once I have booted I can connect the extension then run the sketch and everything seems fine. The only hitch is that if the extension is in place before I throw the power switch then there is seemingly no power getting to the Uno.

Thanks again.

sonvivant: seemingly no power getting to the Uno.

What does that mean? (Are you actually measuring voltages anywhere on board? Or do you just mean that the things you expect your sketch to do aren't happening?)

This means that I had not actually measured voltages, no, but since there was no light in on-board the status LEDs, I assumed that there was no power getting to the board.

I have now measured and I thank you for reminding me the price of assumption. Power does get to the board - I read 4.98V and 3.29V on the +5V and +3.3V pins no matter whether I see status LEDs or not.

If power gets to the board but the on-board LED on pin13 doesn't flash then go solid, does this means that my Arduino Uno isn't booting up?

I will try to disconnect pins one by one to try to isolate the problem pin. In the meantime I continue to invite any other helpful suggestions...

sonvivant:
If power gets to the board but the on-board LED on pin13 doesn’t flash then go solid, does this means that my Arduino Uno isn’t booting up?

If you have a sketch loaded that turns pin13 on then it indicates that the sketch isn’t running. Does your sketch do that?

I wonder if your circuit is stopping the board from coming out of reset. I haven’t experimented, but I guess you could do it either by holding the reset pin or by doing something that kept DTR low on the serial port. Hopefully once you’ve narrowed down which pin is causing the problem it will become clearer how it is causing the problem.

Not that I particularly wanted to revive a thread that is nearly a year old, but a few months went by before I finally had a chance to push forward on the problem described above, and thought I would offer my findings in case it might help someone else later on.

It turns out that there was a short in the connector at the other end of my signal cable; so yes, PeterH, my circuit was causing the problem. Replacing the connector at that end allowed me to run the sketch. Thank you for your help.