Which board generation is which, and how their pins differ.

Generations

	Board files	Relays	Thermocouples	Firmware profile
Gen1	archive/hw_archive/osf_2022-05-19, osf_2022-06-30, osf_2023-02-01, osf_2023-12-06	1	2× MAX6675	gen1_nano, gen1_nanoevery
Gen2	archive/hw_archive/finn_recent	2	2× MAX6675	gen2_nano, gen2_nanoevery
—	hardware/ (current KiCad)	2	2× MAX31855K	None (#22)

hardware/ is not gen2. It uses MAX31855K converters, which the firmware’s MAX6675 driver cannot read, and routes the thermocouple pins differently.

Pin maps

Each net below is the Nano A1 pad’s net, read out of the PCB.

Arduino pin	Gen1	Gen2	hardware/
D2	/E_STOP	/E_STOP	/E_STOP
D5	—	/LED	/LED
D6	/PWM (relay K1)	/Relay1	/Relay1
D7	—	/Relay2	/Relay2
D8, D9	—	—	—
D10	/SS1	/SS1	/TC_CLK
D11	/SS2	/SS2	/SS2
D12	/TC_DATA	/TC_DATA	/TC_DATA
D13	/TC_CLK	/TC_CLK	/SS1

— is unconnected.

Gen1 has one relay

Gen1 has one relay, K1, driven from D6. D7 is not connected. Both thermocouple channels are populated.

The gen1 firmware profile sets RLHT_RELAY_COUNT to 1 and does not define RELAY2, so D7 is never driven. Channel-2 commands are still accepted, so the wire format is the same on both generations, but they drive nothing: GET_STATE never reports RLHT_FLAG_RELAY2_ON or a channel-2 on-time. A channel can still read either thermocouple through SET_TC_SELECT.

GET_CAPS is the same on both generations, so a host cannot tell them apart over the bus.

Gen1 has no status LED either: D5 is unconnected, and the gen1 profile compiles the LED out (RLHT_HAS_STATUS_LED).