From 109eacecd58b0a4b54cf20f64763ded33a4dffaa Mon Sep 17 00:00:00 2001
From: Cameron Brooks <95648474+CameronBrooks11@users.noreply.github.com>
Date: Tue, 3 Nov 2026 20:48:15 -0500
Subject: [PATCH] docs: record gen1's ungrounded thermocouple T- and its rework

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).
---
 docs/hardware-revisions.md | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/docs/hardware-revisions.md b/docs/hardware-revisions.md
index d846919..fd665f3 100644
--- a/docs/hardware-revisions.md
+++ b/docs/hardware-revisions.md
@@ -50,3 +50,27 @@ over the bus.
 
 Gen1 has no status LED either: D5 is unconnected, and the gen1 profile compiles
 the LED out (`RLHT_HAS_STATUS_LED`).
+
+## Gen1 thermocouple T-
+
+On gen1 the MAX6675's T- pin is not connected to ground. In all four gen1 board
+files, U3 pin 2 (`/T-1`) and U4 pin 2 (`/T-2`) reach only the filter capacitor,
+the ferrite bead and the thermocouple terminal (C3, FB2, J7; C6, FB4, J8). The
+MAX6675 datasheet's pin description for T- reads "Alumel Lead of Type-K
+Thermocouple. Should be connected to ground externally.", and under Open
+Thermocouple:
+
+> In order to allow the operation of the open thermocouple detector, T- must
+> be grounded. Make the ground connection as close to the GND pin as possible.
+
+The gen1 boards in use carry a hand rework for this: a solder bridge from pin 1
+(GND) to pin 2 (T-) on each MAX6675. A gen1 board without the bridge has a
+floating thermocouple input.
+
+Gen2 (`finn_recent`) grounds T- in the layout: U3 and U4 pin 2 are on GND. The
+current `hardware/` design uses MAX31855K converters, whose datasheet says the
+opposite for T- ("Thermocouple Input. See Table 1. Do not connect to GND."), and
+leaves it off GND.
+
+Gen1 thermocouples also read high as the board warms, by about 2 °C after
+hours powered (#86).