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docs: record gen1's ungrounded thermocouple T- and its rework - #87
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In all four gen1 board files, each MAX6675's T- (pin 2) reaches only its filter capacitor, ferrite bead and terminal, never GND, though the datasheet says T- "should be connected to ground externally" and must be grounded for open-thermocouple detection. The gen1 boards in use carry a hand-soldered bridge from pin 1 (GND) to pin 2 on U3 and U4. docs/hardware-revisions.md now records both, notes that gen2 grounds T- in the layout and that the MAX31855K in hardware/ must not have T- on GND, and points to the gen1 cold-junction error (#86).
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In all four gen1 board files, each MAX6675's T- (pin 2) reaches only
its filter capacitor, ferrite bead and terminal, never GND, though the
datasheet says T- "should be connected to ground externally" and must
be grounded for open-thermocouple detection. The gen1 boards in use
carry a hand-soldered bridge from pin 1 (GND) to pin 2 on U3 and U4.
docs/hardware-revisions.md now records both, notes that gen2 grounds T-
in the layout and that the MAX31855K in hardware/ must not have T- on
GND, and points to the gen1 cold-junction error (#86).
Pins confirmed on the board in use: the bridge joins pin 1 and pin 2 on both U3 and U4. Net checks are from the four osf_* board files and finn_recent on main; the quotes are from the MAX6675 and MAX31855 datasheets.
Refs #86.