I have a project which attaches a Teensy 3.5 to a vintage computer, supported by some 74LS logic chips. I've produced over 40 boards which all work great, but for the first time it seems like the Teensy is bad. I replaced the board it's plugged into, the logic chips, and all components, and while the Teensy does boot up (I can debug with it), several I/O lines are being pulled low inexplicably, but only while the computer is on. My guess is that a couple pins somewhere are being shorted and pulling these 2 lines low somehow, but I can't figure out how to test it. Is there some kind of test to find out if the Teensy I/O is working improperly, or should I just return this one Teensy?