Another way to resolve the issue is to put a shroud around the pins so it physically can't go in off by one. The mechanical spec leaves a clearance around the connector explicitly for this. Unfortunately a shrouded version of the pass-through connector is not available. Adding the 60-pin connector to the bottom of the board would also provide mechanical keying.
As with all of the hobby class stuff it is up to the user to ensure everything is plugged in correctly to prevent damage.
How do you protect against off by one when it is an entire row off (up-down, instead of left-right)?
As far as removing one ground, you will get away with it, but then the users will find a new way to blow up the card. This will also create grief when a user plugs a wire into the corner pin expecting it to be ground only to discover it isn't.
Lawrence King email@example.com Engineer, Sr. Staff/Manager Qualcomm Canada Inc. (905)482-5403 desk (x25403) (416)627-7302 cell
-----Original Message----- From: firstname.lastname@example.org [mailto:email@example.com] On Behalf Of Grant Likely Sent: Thursday, December 10, 2015 6:26 AM To: George Grey firstname.lastname@example.org; King, Lawrence email@example.com; Yang Zhang firstname.lastname@example.org; email@example.com; Jorge Ramirez-Ortiz firstname.lastname@example.org Subject: 96B-Grove change request for review
I just made the following change to the Rev C schematic after someone blew up their board by connecting the Sensors board off-by-one. However, I'm not entirely sure if it is a good idea. Full description below, but the jist is I disconnected pin 40 from GND to prevent +12V getting shorted to +5V. There are three other GND pins on the connector, pin 1, 2 & 39. Do you think having one GND pin on the right hand side of the connector provide a sufficient ground connection?
Protect baseboard if Sensors board is installed incorrectly
Disconnect pins 36 (PWR_IN) and 40 (GND) to protect the baseboard if P1 gets installed off-by-one. Keeping PWR_IN pins 36 and 38 tied together will short +12V directly to GPIO_L (34) or GND (40) if the connector was off by one. Having pin 40 attached to GND would cause +12V to be shorted directly to the +5V rail. To protect the baseboard, remove these direct connections. Using only one PWR_IN pin is more than sufficient for this board because it isn't used for anything. There are also three other GND pins on the connector rated at 2A each. Dropping one should not be an issue.
These changes don't guarantee to protect the baseboard. 5V would still get shunted to the 1.8V rail which may still destroy the board, but there is at least a fighting chance of not frying something.
Things would be so much better if we had a keyed connector
Signed-off-by: Grant Likely email@example.com