Let a tower set its encoder scale, and send no speed after a failed homing

config/hardware.example.ini

@@ -193,6 +193,10 @@ enc_deviation      = 1000
 sw_mode            = 0
 ; Is this axis's encoder aligned to XACTUAL and usable for hold and DIAG?
 has_encoder        = true
+; Microsteps per encoder count (ENC_CONST). Leave it out for the default, 12.8.
+; A drivetrain whose encoder differs must declare it: wrong by 2%, the counter
+; and the encoder drift apart as the axis turns and homing takes that for a stop.
+;enc_const          = 12.5
 
 [Camera.rgb]
 ; The serial of the physical camera fitted to this tower. A USB DEV_ id, or a

docs/configuration.md

@@ -84,6 +84,7 @@ Parsed and validated by `ConfigLoader` ([src/core/Config.cpp](../src/core/Config
 | `Motor.<axis>` | `homing_speed_deg_s` / `center_speed_deg_s` | float | — | Endstop-search and park speeds, also in °/s. **hardware.ini.** Both are needed *before* the first home, so they convert through the declared scale (`steps_per_rev / travel_deg`), not the homed one |
 | `Motor.<axis>` | `steps_per_rev` | int | — | Expected full mechanical travel, **in counts**. **hardware.ini.** The firmware's only guard against a phantom endstop |
 | `Motor.<axis>` | `enc_deviation` | int | — | \|XACTUAL−XENC\| that means "position untrustworthy". **hardware.ini** |
+| `Motor.<axis>` | `enc_const` | float | 12.8 | Microsteps per encoder count (the TMC's `ENC_CONST`), up to 4 decimals. **hardware.ini**, optional: only a drivetrain whose encoder differs declares it. Wrong by a few percent, the step counter and encoder drift apart as the axis turns, and homing takes the drift for a stop (staeffelsberg: 12.5, from its old firmware) |
 | `Motor.<axis>` | `sw_mode` / `has_encoder` | int / bool | — | TMC `SW_MODE` (physical endstop switches on REFL/REFR) and whether this axis's encoder is aligned and usable. **hardware.ini** |
 | `Scan` | `grid_file` | string | — | CSV of `yaw_deg,pitch_deg` waypoints; empty → generate |
 | `Scan` | `yaw_intervals` | int | `56` | Generated yaw positions across `[yaw_min_deg, yaw_max_deg]` |

docs/tower-installation.md

@@ -77,6 +77,10 @@ bash survey-legacy-tower --firmware      # pauses the old service ~20 s; changes
   - its stored north calibration (`y`);
   - its TMC registers (`i`), with the run current worked out for the usual
     sense resistors.
+- the old firmware's source, from the newest build in the Arduino cache:
+  its register table (encoder scale `R3A`, current `R0B`/`R10`, encoder limit
+  `R3D`) and homing speed. The running firmware cannot report these: the
+  registers are write-only, and it answers `i` for them with 0.
 
 The report is saved as `~/legacy-survey-<date>.txt`.
 
@@ -894,6 +898,12 @@ Answer the installer's ambient sensor question with `dht11` and the fan
 question with yes, and choose `loom = legacy`. The firmware is then built with
 the DHT11 and fan (see configuration.md, "The legacy towers").
 
+**The encoder scale.** Take it from the survey's "old firmware's source":
+`R3A` is `ENC_CONST`, the whole part in the high 16 bits and 1/10000ths in the
+low 16, so `0x000C1388` is 12.5. If it is not 12.8, put it in the answers file
+as `yaw.enc_const=12.5` (the installer never asks for it). On staeffelsberg,
+2026-09-28, it was missing: homing took the 2% drift for a stop after 131°.
+
 Check it **on site** after the first flash:
 1. The fan runs at full speed from power-up.
 2. It drops once readings come in, if the box is below 25 °C.

hardware/common/kit.sh

@@ -34,6 +34,9 @@ need_root() { [[ $EUID -eq 0 ]] || die "$1 needs root - re-run with sudo"; }
 # drivetrain that no probe can read.
 KIT_AXIS_KEYS=(r_sense_ohm run_current_ma homing_current_ma travel_deg steps_per_rev
                sw_mode enc_deviation has_encoder)
+# Keys with a firmware default that fits most towers: written only when given,
+# never a TODO. enc_const: a legacy tower's encoder scale (staeffelsberg 12.5).
+KIT_AXIS_OPTIONAL=(enc_const)
 
 # What to measure, per key - the text of each `; TODO`.
 kit_axis_todo() {
@@ -46,6 +49,7 @@ kit_axis_todo() {
         sw_mode)           echo "TMC SW_MODE. 0 = no physical switches (StallGuard homing); set the bits if this axis has REFL/REFR wired" ;;
         enc_deviation)     echo "|XACTUAL - XENC| that means 'position untrustworthy'. Measure the free-running gap on THIS axis and set ~4-5x it (tower 121: gap ~200, set 1000). Re-measure whenever the homing current changes" ;;
         has_encoder)       echo "true if an encoder is fitted to this axis and aligned to XACTUAL (usable for hold and DIAG), else false" ;;
+        enc_const)         echo "microsteps per encoder count (the TMC's ENC_CONST); leave out for the default 12.8. A legacy tower's own firmware names it (staeffelsberg: 12.5)" ;;
     esac
 }
 
@@ -421,6 +425,9 @@ EOF
             kv_or_todo "$key" "${MOTOR[$ax.$key]:-}" "$(kit_axis_todo "$key")"
             [[ $key == steps_per_rev ]] && center_note "${MOTOR[$ax.$key]:-}" "$ax"
         done
+        for key in "${KIT_AXIS_OPTIONAL[@]}"; do
+            [[ -n ${MOTOR[$ax.$key]:-} ]] && printf '%-18s = %s\n' "$key" "${MOTOR[$ax.$key]}"
+        done
         cat <<'EOF'
 ; Speeds are DEGREES PER SECOND, not VMAX: a register value means different
 ; things on different gear ratios, so it is not portable between towers. These
@@ -494,7 +501,7 @@ Wiring ([Pins], which the firmware is built from):
                             only the lines for what is declared
     --pin KEY=N             one pin at a time instead (yaw_cs=1, spi_sck=2, ...)
 The drivetrain, per axis - anything not given is written as a ; TODO:
-    --motor AXIS.KEY=VALUE  KEY one of: ${KIT_AXIS_KEYS[*]}
+    --motor AXIS.KEY=VALUE  KEY one of: ${KIT_AXIS_KEYS[*]} ${KIT_AXIS_OPTIONAL[*]}
     --r-sense, --run-current, --homing-current, --travel-deg, --steps-per-rev,
     --sw-mode               the same, for the FIRST axis in --axes
 Other:
@@ -567,8 +574,8 @@ kit_main() {
     for key in "${!MOTOR[@]}"; do
         local ax="${key%%.*}" k="${key#*.}" ok=0 known
         [[ ",${AXES// /}," == *",$ax,"* ]] || die "--motor $key: $ax is not in --axes $AXES"
-        for known in "${KIT_AXIS_KEYS[@]}"; do [[ $k == "$known" ]] && ok=1; done
+        for known in "${KIT_AXIS_KEYS[@]}" "${KIT_AXIS_OPTIONAL[@]}"; do [[ $k == "$known" ]] && ok=1; done
-        [[ $ok -eq 1 ]] || die "--motor $key: unknown key (one of: ${KIT_AXIS_KEYS[*]})"
+        [[ $ok -eq 1 ]] || die "--motor $key: unknown key (one of: ${KIT_AXIS_KEYS[*]} ${KIT_AXIS_OPTIONAL[*]})"
     done
 
     local board; board="$(probe_board)"

include/fgc/Config.h

@@ -115,6 +115,10 @@ struct AxisDrivetrainConfig {
     // The firmware holds none of these once it asks for them.
     long   steps_per_rev = 0;  // expected full mechanical travel, in counts
     long   enc_deviation = 0;  // |XACTUAL-XENC| that means "position untrustworthy"
+    // Microsteps per encoder count (the TMC's ENC_CONST). 0 = undeclared: the
+    // firmware keeps its default, 12.8. Only a drivetrain whose encoder differs
+    // declares it - staeffelsberg's legacy firmware used 12.5.
+    double enc_const     = 0.0;
 
     // Wiring: TMC SW_MODE (physical endstop switches on REFL/REFR), and whether
     // this axis's encoder is aligned to XACTUAL and usable for hold and DIAG.

include/fgc/DumpParser.h

@@ -36,6 +36,7 @@ struct DumpAxis {
     // config handshake, which is a version skew, not a value of zero.
     long        steps_per_rev = -1;
     long        enc_deviation = -1;
+    long        enc_const     = -1;   // ENC_CONST as the driver holds it
     long        homing_speed  = -1;
     long        center_speed  = -1;
     long        sw_mode       = -1;

include/fgc/FirmwareConfig.h

@@ -68,6 +68,12 @@ struct AxisConfigSource {
     bool     have_wiring  = false;
     unsigned sw_mode      = 0;
     bool     has_encoder  = true;
+
+    // Optional: the encoder scale as the ENC_CONST register value (see
+    // encConstRegister). Never in a NEED line - the firmware has a default - so
+    // it is sent first, unasked, when declared.
+    bool     have_enc_const = false;
+    long     enc_const      = 0;
 };
 
 struct FirmwareConfigPlan {
@@ -90,4 +96,10 @@ struct FirmwareConfigPlan {
 // existing CENTER and MARGIN commands use and hand-typed sessions rely on them.
 FirmwareConfigPlan planFirmwareConfig(const NeedRequest& need, const AxisConfigSource& src);
 
+// Microsteps per encoder count as the TMC5160 holds it in decimal mode
+// (ENCMODE bit 10): the whole part in the high 16 bits, the fraction in
+// 1/10000ths in the low 16. 12.5 -> 0x000C1388. nullopt for anything that
+// format cannot hold exactly: zero or less, 32768 or more, or a fifth decimal.
+std::optional<long> encConstRegister(double microsteps_per_count);
+
 }  // namespace fgc

include/fgc/mock/MockMotorController.h

@@ -145,6 +145,7 @@ public:
                             " soft_margin=" + val("margin", "5000") +
                             " steps_per_rev=" + val("steps_per_rev", "0") +
                             " enc_dev=" + val("enc_deviation", "0") +
+                            " enc_const=" + val("enc_const", "794432") +
                             " homing_speed=" + val("homing_speed", "0") +
                             " center_speed=" + val("center_speed", "0") +
                             " sw_mode=" + val("sw_mode", "0") +

scripts/install-tower

@@ -609,6 +609,14 @@ if [[ ! -r $HW ]]; then
             ask_value "$ax.$key" "  $ax $key" "$pat"
             [[ -n $REPLY_VAL ]] && KIT_ARGS+=(--motor "$ax.$key=$REPLY_VAL")
         done
+        # Optional keys are taken from the answers file only (yaw.enc_const=12.5):
+        # asking every install about a default nearly every tower keeps is noise.
+        for key in "${KIT_AXIS_OPTIONAL[@]}"; do
+            [[ -n ${ANS[$ax.$key]:-} ]] || continue
+            [[ ${ANS[$ax.$key]} =~ ^[0-9]+(\.[0-9]{1,4})?$ ]] || die "bad value for '$ax.$key': ${ANS[$ax.$key]}"
+            say "$ax $key = ${ANS[$ax.$key]} (from the answers)"
+            KIT_ARGS+=(--motor "$ax.$key=${ANS[$ax.$key]}")
+        done
     done
     "$KIT/install.sh" "${KIT_ARGS[@]}" > "$WORK/hardware.ini" 2> "$WORK/kit.err" || { cat "$WORK/kit.err" >&2; die "the kit could not generate hardware.ini"; }
     HW_NEW="$WORK/hardware.ini"

scripts/survey-legacy-tower

@@ -93,6 +93,24 @@ for c in /opt/fire_gimbal_control/config.ini "$HOME"/*/config.ini "$HOME"/*/*/co
     grep -vE '^[[:space:]]*(;|#|$)' "$c" | sed -E 's/^(mqtt_pw[[:space:]]*=).*/\1 *****/' | sed 's/^/  /'
 done
 
+hr "the old firmware's source"
+# What the running firmware cannot report: ENC_CONST, GLOBALSCALER and IHOLD_IRUN
+# are write-only on the TMC5160, and the old sketch answers `i` for them from its
+# own copy, which reads 0. Its register table is in its source - the newest build
+# in the Arduino IDE's cache is most likely the one flashed.
+sketch="$(ls -t "$HOME"/.cache/arduino/sketches/*/sketch/*.ino.cpp 2>/dev/null |
+          xargs -r grep -l chipCS_enc 2>/dev/null | head -1)"
+if [[ -z $sketch ]]; then
+    echo "no build of the old sketch in ~/.cache/arduino; look for FWT_firmware*.ino by hand"
+else
+    echo "newest build: $sketch ($(date -r "$sketch" '+%Y-%m-%d %H:%M'))"
+    echo "  source: $(sed -n 's/^#line 1 "\(.*\)"/\1/p' "$sketch" | head -1)"
+    grep -hE '^#define (V_ENC_INIT|ONE_ROUND_STEPS)|gear_red_var *=|one_round_in_steps *=' "$sketch" | sed 's/^/  /'
+    # ENC_CONST is decimal: 0x000C1388 is 12 + 0x1388/10000 = 12.5 microsteps per count.
+    grep -hE '^#define R(0B|10|3A|3D|26|27|28) ' "$(dirname "$sketch")"/TMC5160.h 2>/dev/null | sed 's/^/  /'
+    echo "  (R3A = ENC_CONST: hardware.ini [Motor.yaw] enc_const = whole + low 16 bits/10000)"
+fi
+
 [[ $FIRMWARE -eq 1 ]] || { hr "firmware"; echo "not asked (run with --firmware)"; echo; echo "saved: $OUT"; exit 0; }
 
 hr "the old firmware"

src/core/Application.cpp

@@ -1227,7 +1227,16 @@ struct Application::Impl {
         // Production move speeds for subsequent MOVE commands, converted from the
         // declared deg/s. Sent here rather than earlier because deriveCountsPerDeg()
         // above has just refreshed the scale this conversion depends on.
-        sendProductionSpeeds();
+        //
+        // Only after a successful home. A failed homing leaves the driver in
+        // velocity mode, where firmware before 2026-09-28 wrote SPEED straight to
+        // VMAX: on staeffelsberg that turned the axis on its own until it jammed on
+        // its stop and ground there. The firmware now refuses that too; this keeps
+        // towers still running the old build safe.
+        if (homed)
+            sendProductionSpeeds();
+        else
+            LOG_WARN << "production speed not sent: the gimbal is not homed";
         LOG_INFO << "Gimbal init finished";
     }
 
@@ -1286,6 +1295,10 @@ struct Application::Impl {
         s.steps_per_rev  = d.steps_per_rev;
         s.have_deviation = d.enc_deviation > 0;
         s.enc_deviation  = d.enc_deviation;
+        if (const auto r = encConstRegister(d.enc_const)) {
+            s.have_enc_const = true;
+            s.enc_const      = *r;
+        }
 
         s.have_wiring = d.wiring_declared;
         s.sw_mode     = d.sw_mode;

src/core/Config.cpp

@@ -1,4 +1,5 @@
 #include "fgc/Config.h"
+#include "fgc/FirmwareConfig.h"
 #include "fgc/Logger.h"
 #include "fgc/Paths.h"
 
@@ -512,6 +513,7 @@ AppConfig ConfigLoader::fromMap(const std::map<std::string, std::string>& kv) {
         d.center_speed_deg_s = getDouble(kv, pre + "center_speed_deg_s", d.center_speed_deg_s);
         d.steps_per_rev      = getInt(kv, pre + "steps_per_rev", static_cast<int>(d.steps_per_rev));
         d.enc_deviation      = getInt(kv, pre + "enc_deviation", static_cast<int>(d.enc_deviation));
+        d.enc_const          = getDouble(kv, pre + "enc_const", d.enc_const);
         d.wiring_declared    = kv.count(pre + "sw_mode") || kv.count(pre + "has_encoder");
         d.sw_mode     = static_cast<unsigned>(getInt(kv, pre + "sw_mode", static_cast<int>(d.sw_mode)));
         d.has_encoder = getBool(kv, pre + "has_encoder", d.has_encoder);
@@ -567,6 +569,10 @@ AppConfig ConfigLoader::fromMap(const std::map<std::string, std::string>& kv) {
             throw std::runtime_error(sec + ".travel_deg must be within (0, 720]; the "
                                            "direction of travel is carried by the sign of "
                                            "counts_per_deg, not by this angle");
+        if (d.enc_const != 0.0 && !encConstRegister(d.enc_const))
+            throw std::runtime_error(sec + ".enc_const must be between 0 and 32768 with at "
+                                           "most 4 decimals (microsteps per encoder count; "
+                                           "leave it out for the default 12.8)");
     };
     checkDrivetrain("yaw", cfg.drivetrain.yaw);
     checkDrivetrain("pitch", cfg.drivetrain.pitch);

src/core/DumpParser.cpp

@@ -182,6 +182,7 @@ DumpData parseDump(const std::string& block) {
             ax.homing_irun     = asInt(get(m, "irun_home", "-1"));
             ax.steps_per_rev   = asInt(get(m, "steps_per_rev", "-1"));
             ax.enc_deviation   = asInt(get(m, "enc_dev", "-1"));
+            ax.enc_const       = asInt(get(m, "enc_const", "-1"));
             ax.homing_speed    = asInt(get(m, "homing_speed", "-1"));
             ax.center_speed    = asInt(get(m, "center_speed", "-1"));
             ax.sw_mode         = asInt(get(m, "sw_mode", "-1"));

src/core/FirmwareConfig.cpp

@@ -1,5 +1,7 @@
 #include "fgc/FirmwareConfig.h"
 
+#include <algorithm>
+#include <cmath>
 #include <sstream>
 
 namespace fgc {
@@ -34,6 +36,7 @@ Answer answerFor(const std::string& key, const AxisConfigSource& s) {
 
     if (key == "sw_mode")     return {s.have_wiring, static_cast<long long>(s.sw_mode)};
     if (key == "has_encoder") return {s.have_wiring, s.has_encoder ? 1 : 0};
+    if (key == "enc_const")   return {s.have_enc_const, s.enc_const};
 
     return {};  // not in the vocabulary
 }
@@ -43,6 +46,7 @@ bool known(const std::string& key) {
         "global_scaler", "run_irun",     "homing_irun", "center_count", "center",
         "soft_margin",   "margin",       "steps_per_rev", "enc_deviation",
         "normal_speed",  "homing_speed", "center_speed",  "sw_mode",    "has_encoder",
+        "enc_const",
     };
     for (const char* k : kKeys)
         if (key == k) return true;
@@ -72,7 +76,14 @@ FirmwareConfigPlan planFirmwareConfig(const NeedRequest& need, const AxisConfigS
     FirmwareConfigPlan plan;
     const std::string axis(1, need.axis);
 
-    for (const auto& key : need.keys) {
+    // The encoder scale goes first, asked for or not: it is optional, so no NEED
+    // names it, and the firmware writes it to the driver with the rest of the
+    // set only if it has arrived by the time the last required key does.
+    std::vector<std::string> keys = need.keys;
+    if (src.have_enc_const && std::find(keys.begin(), keys.end(), "enc_const") == keys.end())
+        keys.insert(keys.begin(), "enc_const");
+
+    for (const auto& key : keys) {
         if (!known(key)) {
             // The firmware wants something this host has never heard of, which
             // means it is newer. Naming it is the whole diagnostic.
@@ -89,4 +100,14 @@ FirmwareConfigPlan planFirmwareConfig(const NeedRequest& need, const AxisConfigS
     return plan;
 }
 
+std::optional<long> encConstRegister(double v) {
+    if (!(v > 0.0) || v >= 32768.0) return std::nullopt;
+    const double scaled = v * 10000.0;
+    const double rounded = std::round(scaled);
+    if (std::fabs(scaled - rounded) > 1e-6) return std::nullopt;
+    const long whole = static_cast<long>(rounded) / 10000;
+    const long frac  = static_cast<long>(rounded) % 10000;
+    return (whole << 16) | frac;
+}
+
 }  // namespace fgc

src/core/TestRunner.cpp

@@ -1,4 +1,5 @@
 #include "fgc/TestRunner.h"
+#include "fgc/FirmwareConfig.h"
 
 #include "fgc/sensors/McuEnvSensor.h"
 
@@ -834,6 +835,7 @@ long axisField(const DumpData& d, char code, long DumpAxis::*field) {
 }
 long echoSteps(const DumpData& d, char c)        { return axisField(d, c, &DumpAxis::steps_per_rev); }
 long echoDev(const DumpData& d, char c)          { return axisField(d, c, &DumpAxis::enc_deviation); }
+long echoEncConst(const DumpData& d, char c)     { return axisField(d, c, &DumpAxis::enc_const); }
 long echoSwMode(const DumpData& d, char c)       { return axisField(d, c, &DumpAxis::sw_mode); }
 long echoHomingSpeed(const DumpData& d, char c)  { return axisField(d, c, &DumpAxis::homing_speed); }
 long echoCenterSpeed(const DumpData& d, char c)  { return axisField(d, c, &DumpAxis::center_speed); }
@@ -970,6 +972,8 @@ void TestRunner::testHwMotor(TestReport& r) {
         const DeclaredValue vals[] = {
             {"steps_per_rev", cfg.steps_per_rev > 0, cfg.steps_per_rev, echoSteps(d, code)},
             {"enc_deviation", cfg.enc_deviation > 0, cfg.enc_deviation, echoDev(d, code)},
+            {"enc_const",     encConstRegister(cfg.enc_const).has_value(),
+                              encConstRegister(cfg.enc_const).value_or(0), echoEncConst(d, code)},
             {"sw_mode",       cfg.wiring_declared,   (long)cfg.sw_mode, echoSwMode(d, code)},
             {"homing_speed",  cfg.homing_speed_deg_s > 0,
                               speedVmax(cfg.homing_speed_deg_s, cfg, map), echoHomingSpeed(d, code)},

tests/test_dumpparser.cpp

@@ -172,8 +172,8 @@ TEST_CASE("the host-owned drivetrain values are read back from DUMP") {
         "DUMP BEGIN build=abc uptime=1 mcusr=0x01 free_ram=4096\n"
         "DUMP Y state=2 hsub=0 enabled=1 lim_neg=0 lim_pos=1000000 hold_target=0 speed=39738 "
         "center=500000 soft_margin=5000 gs=80 irun=31 irun_home=7 steps_per_rev=1210778 "
-        "enc_dev=1500 homing_speed=9958 center_speed=29873 sw_mode=0 cfg_rx=0xFFE "
+        "enc_dev=1500 enc_const=791432 homing_speed=9958 center_speed=29873 sw_mode=0 "
-        "hold_corr=0 hold_peak=0 hold_cumul=0 has_encoder=1\n"
+        "cfg_rx=0xFFE hold_corr=0 hold_peak=0 hold_cumul=0 has_encoder=1\n"
         "DUMP END\n";
     auto d = fgc::parseDump(text);
     REQUIRE(d.valid);
@@ -181,6 +181,7 @@ TEST_CASE("the host-owned drivetrain values are read back from DUMP") {
     const auto& a = d.axes[0];
     CHECK(a.steps_per_rev == 1210778);
     CHECK(a.enc_deviation == 1500);
+    CHECK(a.enc_const == 791432);
     CHECK(a.homing_speed == 9958);
     CHECK(a.center_speed == 29873);
     CHECK(a.sw_mode == 0);

tests/test_firmwareconfig.cpp

@@ -303,3 +303,36 @@ TEST_CASE("the mock leaves NEEDCFG per axis, as the firmware does") {
     CHECK(d.find("DUMP Y state=1") != std::string::npos);
     CHECK(d.find("DUMP P state=5") != std::string::npos);
 }
+
+// ---- the encoder scale (ENC_CONST), optional per drivetrain ----
+
+TEST_CASE("the encoder scale converts to the driver's decimal format, exactly or not at all") {
+    CHECK(encConstRegister(12.8) == 0x000C1F40L);   // the firmware default
+    CHECK(encConstRegister(12.5) == 0x000C1388L);   // staeffelsberg's legacy firmware
+    CHECK(encConstRegister(1.0) == 0x00010000L);
+    CHECK(encConstRegister(0.0001) == 0x00000001L);
+    CHECK_FALSE(encConstRegister(0.0));
+    CHECK_FALSE(encConstRegister(-12.5));           // a reversed encoder is not modelled
+    CHECK_FALSE(encConstRegister(32768.0));
+    CHECK_FALSE(encConstRegister(12.34565));        // a fifth decimal the driver cannot hold
+}
+
+TEST_CASE("a declared encoder scale is sent first, asked for or not") {
+    auto src = iotaYaw();
+    src.have_enc_const = true;
+    src.enc_const      = 0x000C1388L;
+    const auto need = parseNeedLine("NEED Y steps_per_rev global_scaler");
+    REQUIRE(need);
+    const auto p = planFirmwareConfig(*need, src);
+    REQUIRE(p.sets.size() == 3);
+    CHECK(p.sets[0] == "SET Y enc_const 791432");
+    CHECK(p.complete());
+}
+
+TEST_CASE("an undeclared encoder scale is never sent, so the firmware keeps its default") {
+    const auto need = parseNeedLine("NEED Y steps_per_rev global_scaler");
+    REQUIRE(need);
+    const auto p = planFirmwareConfig(*need, iotaYaw());
+    CHECK(p.sets.size() == 2);
+    for (const auto& s : p.sets) CHECK(s.find("enc_const") == std::string::npos);
+}