diff --git a/docs/tower-installation.md b/docs/tower-installation.md
index 6f8afae..0ada254 100644
--- a/docs/tower-installation.md
+++ b/docs/tower-installation.md
@@ -738,6 +738,128 @@ gimbal heading setup     # ties yaw to true north by sighting landmarks; no IMU
 gimbal pitch setup       # needs an IMU; skip on a rig without one
 ```
 
+## 10b. Bringing up the config handshake on an existing tower
+
+This is the one upgrade that changes what an **unconfigured** tower does: the new
+firmware boots into `NEEDCFG` and refuses `HOME` / `MOVE` / `ENABLE` until the
+host has answered its `NEED` lines. Flashing it onto a tower whose `hardware.ini`
+predates the drivetrain keys leaves the gimbal inert — working, talking, and not
+turning.
+
+So the order matters. **Host first, firmware second.** Each step below either
+changes nothing or is reversible, and the step that could strand the tower comes
+after the one that prevents it.
+
+### 0. Record what you are rolling back to
+
+```bash
+gimbal dump                 # the running firmware's build id and register state
+cat /etc/fire_gimbal_control/hardware.ini
+git -C ~/projects/fwt_2a/firmware rev-parse --short HEAD
+```
+
+Keep the dump. It is the only record of what the old firmware was holding, and
+the comparison that tells you whether the new one adopted the same values.
+
+### 1. Update `hardware.ini` — nothing else, and no restart yet
+
+Add the `[Motor.<axis>]` block (section 5). The five keys the new firmware will
+not run without are `steps_per_rev`, `enc_deviation`, `sw_mode`, plus the two
+speeds; `install.sh --stdout` generates the block if you would rather not type it.
+
+Doing this **before** flashing is what keeps the window at zero: the host is ready
+to answer the moment the firmware starts asking.
+
+### 2. Deploy the new host against the OLD firmware
+
+A real test, and it risks nothing. The old firmware never sends `NEED`, so the
+host must behave exactly as it did before — which is also the guarantee the whole
+Delta fleet depends on.
+
+```bash
+scripts/fgc start --start
+scripts/fgc peek
+test hardware motor
+```
+
+Expect: no handshake lines at all, and the motor leaf reporting a **version skew**
+rather than a mismatch —
+
+```
+; yaw_current: yaw: firmware echoes only part of the current (2 of 3 fields
+  absent), so it cannot be confirmed -- it predates the full CURRENT echo. Re-flash it
+```
+
+If instead you see wrong *values*, stop: something in `hardware.ini` disagrees
+with what the tower is really running, and flashing will not fix it.
+
+### 3. Flash
+
+```bash
+cd firmware && ./deploy.sh          # uploads; --compile-only if you want a dry run first
+```
+
+Watch the banner. A `NEED` line per axis should follow it:
+
+```
+BOOT build=<rev>
+NEED Y steps_per_rev global_scaler run_irun homing_irun center_count soft_margin ...
+```
+
+**No `NEED` line means the axis is not in `NEEDCFG`** — either the flash did not
+take, or the image is older than you think. Check the build id before going on.
+
+### 4. Confirm the handshake completed
+
+The host answers within a control-loop tick. Expect:
+
+```
+firmware config: answered 11 keys for axis Y
+```
+
+Then prove it landed rather than trusting the log:
+
+```
+gimbal dump                 # state should be 1 (RESET), NOT 5 (NEEDCFG)
+                            # cfg_rx should be the full required mask
+test hardware motor         # both metrics PASS
+```
+
+`test hardware motor` is the real gate: it compares every declared value against
+what the firmware echoes, so a `SET` that was lost on the wire shows up here and
+nowhere else.
+
+### 5. Prove the refusal actually refuses
+
+Worth doing once per firmware version, because a guard that silently stopped
+working looks exactly like a guard that is working.
+
+On a **bench** board, not a mast: power-cycle it without the host running, then
+send `HOME` over a serial monitor. Expect `ERR need config`. Send `RESET` and try
+again — still `ERR need config`, because a RESET is deliberately not a way out.
+`USE DEFAULTS` is, and should then let `HOME` through.
+
+### 6. Only then, move the axis
+
+```
+gimbal home                 # at the reduced homing current
+```
+
+Check against the recorded figures: travel ~1,210,778 counts, repeatable to
+~0.3 %, and the derived `counts_per_deg` near 3552.87. A travel that disagrees by
+more than a few percent means the axis homed against something that was not the
+endstop.
+
+### If it goes wrong
+
+The board is recoverable over the network as long as USB CDC comes up:
+`BOOTSEL` over the serial port, then `./deploy.sh` with the previous firmware
+checked out. Only an image that wedges *before* `Serial.begin()` needs a physical
+button press — which is why `Serial.begin()` is the first thing `setup()` does.
+
+`USE DEFAULTS` is the in-place escape: it puts every axis on the compile-time
+table and lets the tower move while you work out what `hardware.ini` is missing.
+
 ## 11. Verification checklist
 
 | Check | How | Tower 121 |
diff --git a/include/fgc/mock/MockMotorController.h b/include/fgc/mock/MockMotorController.h
index 63264b9..3f42756 100644
--- a/include/fgc/mock/MockMotorController.h
+++ b/include/fgc/mock/MockMotorController.h
@@ -87,6 +87,8 @@ public:
             deviation_ = true;
         } else if (verb == "RESET") {
             deviation_ = false;
+            // A RESET is not a way out of NEEDCFG (firmware motor.cpp says why).
+            if (needcfg_ && !configured_) homed_ = false;
         } else if (verb == "MOVE") {
             std::string a;
             in >> a;
diff --git a/tests/test_firmwareconfig.cpp b/tests/test_firmwareconfig.cpp
index 3ae4972..af6e890 100644
--- a/tests/test_firmwareconfig.cpp
+++ b/tests/test_firmwareconfig.cpp
@@ -247,3 +247,19 @@ TEST_CASE("a partial answer leaves the axis in NEEDCFG") {
     m.sendCommand("HOME");
     CHECK_FALSE(m.telemetry().yaw.ready());
 }
+
+TEST_CASE("RESET is not a way out of NEEDCFG") {
+    // Found by reading the firmware before flashing it: Motor::reset() set
+    // ST_RESET unconditionally, so an axis that was never told its drivetrain
+    // would come back accepting HOME and MOVE on the compile-time table -
+    // reached by a command typed for unrelated reasons.
+    MockMotorController m(/*needcfg=*/true);
+    m.start();
+    m.takeNeedLines();
+
+    m.sendCommand("RESET");
+    CHECK_FALSE(m.configured());
+    m.sendCommand("HOME");
+    CHECK_FALSE(m.telemetry().yaw.ready());
+    CHECK(m.telemetry().yaw.state == AxisState::NeedConfig);
+}