From the Configuration.h file.
"-2 : MAX6675 with Thermocouple, 0°C to +700°C (only for sensors 0-1)"
ie you set
#define TEMP_SENSOR_0 -2
Then just wire it up, its just a SPI device, so you connect it to the SPI pins.
[
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NB these instruction are old and a bit out of date, there is no bug anymore.. and the default CS pin is wrong..
From ramps pin.h
//
// SPI for MAX Thermocouple
//
#ifndef TEMP_0_CS_PIN
#define TEMP_0_CS_PIN 66 // Don't use 53 if using Display/SD card (SDSS) or 49 (SD_DETECT_PIN)
#endif
The default CS is connected to pin D66 on Aux-2 in marlin to avoid conflicts with D53 and D49 if your using a LCD or a SDCARD reader
so change
CS on MAX6675 goes to D66 on RAMPSAUX-2