Hide non-writable channels from Controls picker, gate writes on enabled+writable

Adds ChannelConfig.writable so the Controls editor can only bind to
channels with a real write mapping (VS/MA/ME/MD/ST for CML, do*/DO for
digital I/O, write_cmd-configured SCPI channels, holding-register Modbus
channels) instead of read-only telemetry — this is exactly the class of
mistake hit earlier (binding a motor-speed control to M1_TV instead of
M1_VS). Also carries forward the enabled-channel gating for controls
(hide disabled channels from the picker, block writes to them at
_write() time) done alongside this investigation.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

7 files changed, 21 insertions, 4 deletions

diff --git a/devices/arduino_device.py b/devices/arduino_device.py
index 7c1d21b..f2407e6 100644
--- a/devices/arduino_device.py
+++ b/devices/arduino_device.py
@@ -108,6 +108,7 @@ class ArduinoDevice(BaseDevice):
             channels.append(ChannelConfig(
                 channel_id=f"do{i}", name=f"DO {i}", unit="",
                 min_value=0.0, max_value=1.0,
+                writable=True,
                 color=_DO_COLORS[i % len(_DO_COLORS)],
             ))
 
diff --git a/devices/base_device.py b/devices/base_device.py
index 32fe787..83b8536 100644
--- a/devices/base_device.py
+++ b/devices/base_device.py
@@ -29,6 +29,7 @@ class ChannelConfig:
     max_value:   float          = 100.0
     enabled:     bool           = True
     color:       str            = "#00d4ff"
+    writable:    bool           = False  # True if this channel accepts write_channel() calls
     extra:       Dict[str, Any] = field(default_factory=dict)
 
 
diff --git a/devices/digital_io.py b/devices/digital_io.py
index 53395dc..111224d 100644
--- a/devices/digital_io.py
+++ b/devices/digital_io.py
@@ -60,6 +60,7 @@ class DigitalIODevice(BaseDevice):
                 channel_id=f"do{i}", name=f"DO {i}", unit="",
                 min_value=0.0, max_value=1.0,
                 color=_OUT_COLORS[i % len(_OUT_COLORS)],
+                writable=True,
             ))
 
         info = DeviceInfo(
diff --git a/devices/nidaqmx_device.py b/devices/nidaqmx_device.py
index 88bb932..38cf93d 100644
--- a/devices/nidaqmx_device.py
+++ b/devices/nidaqmx_device.py
@@ -69,6 +69,7 @@ class NidaqmxDevice(BaseDevice):
                 channel_id=f"do{i}", name=f"DO {i}", unit="",
                 min_value=0.0, max_value=1.0,
                 color=_DO_COLORS[i % len(_DO_COLORS)],
+                writable=True,
             ))
 
         info = DeviceInfo(
diff --git a/devices/serial_device.py b/devices/serial_device.py
index 067656a..8062624 100644
--- a/devices/serial_device.py
+++ b/devices/serial_device.py
@@ -264,6 +264,7 @@ class SerialDevice(BaseDevice):
                     unit=sc.get("unit", ""),
                     min_value=-1e9, max_value=1e9,
                     color=_COLORS[i % len(_COLORS)],
+                    writable=bool(sc.get("write_cmd")),
                 )
                 for i, sc in enumerate(self._scpi_channels)
             ]
@@ -275,6 +276,8 @@ class SerialDevice(BaseDevice):
                     unit=mc.get("unit", ""),
                     min_value=-1e9, max_value=1e9,
                     color=_COLORS[i % len(_COLORS)],
+                    # Holding registers (FC03) accept writes (FC06); input registers (FC04) don't.
+                    writable=(mc.get("function_code", 0x03) == 0x03),
                 )
                 for i, mc in enumerate(self._mb_channels)
             ]
@@ -284,8 +287,8 @@ class SerialDevice(BaseDevice):
                 ChannelConfig("force",       "Force",       "N",  -5000.0, 5000.0, color=_COLORS[0]),
                 ChannelConfig("unit_code",   "Unit Code",   "",   0.0, float(len(_MARK10_UNITS) - 1), color=_COLORS[1]),
                 # Write-only action channels (no reading — for control buttons)
-                ChannelConfig("zero",        "Zero Gauge",  "",   0.0, 1.0, color=_COLORS[2]),
-                ChannelConfig("cycle_units", "Cycle Units", "",   0.0, 1.0, color=_COLORS[3]),
+                ChannelConfig("zero",        "Zero Gauge",  "",   0.0, 1.0, color=_COLORS[2], writable=True),
+                ChannelConfig("cycle_units", "Cycle Units", "",   0.0, 1.0, color=_COLORS[3], writable=True),
             ]
         elif fmt == "cml":
             _CMD_UNITS = {"TP": "counts", "TV": "counts/s", "TC": "%×10", "TS": "flags"}
@@ -311,6 +314,7 @@ class SerialDevice(BaseDevice):
                         unit=_WRITE_UNITS.get(cmd, ""),
                         min_value=-1e6, max_value=1e6,
                         color=_COLORS[color_idx % len(_COLORS)],
+                        writable=True,
                     ))
                     color_idx += 1
                 # Write-only action channels — motor must be enabled (ME) before VS/MA take effect
@@ -321,6 +325,7 @@ class SerialDevice(BaseDevice):
                         unit="",
                         min_value=0.0, max_value=1.0,
                         color=_COLORS[color_idx % len(_COLORS)],
+                        writable=True,
                     ))
                     color_idx += 1
             return channels or [ChannelConfig("M1_TP", "M1 TP", "counts", color=_COLORS[0])]
diff --git a/ui/control_editor.py b/ui/control_editor.py
index 48de21d..5140bf3 100644
--- a/ui/control_editor.py
+++ b/ui/control_editor.py
@@ -360,8 +360,12 @@ class ControlEditorDialog(QDialog):
         dev = self.registry.get_instance(dev_id)
         if not dev:
             return
-        # Add actual channels
+        # Add actual channels (skip disabled — can't be driven while switched
+        # off — and skip read-only channels — a control writes, so a channel
+        # with no write mapping should never be offered as a target)
         for ch in dev.info.channels:
+            if not ch.enabled or not ch.writable:
+                continue
             self._ch_cb.addItem(f"{ch.channel_id}  ({ch.name})",
                                 userData=ch.channel_id)
         # For Arduino backends also suggest digital pins for output
diff --git a/ui/control_panel.py b/ui/control_panel.py
index 2b936c4..22229ee 100644
--- a/ui/control_panel.py
+++ b/ui/control_panel.py
@@ -120,7 +120,11 @@ class ControlWidget(QFrame):
         if self.registry and self.device_id and self.channel_id:
             dev = self.registry.get_instance(self.device_id)
             if dev:
-                if dev.status not in (DeviceStatus.CONNECTED, DeviceStatus.SIMULATED):
+                ch = dev.get_channel(self.channel_id)
+                if ch is not None and not ch.enabled:
+                    print(f"[Control] write_channel({self.channel_id}, {value}) skipped on "
+                          f"{self.device_id} — channel is disabled")
+                elif dev.status not in (DeviceStatus.CONNECTED, DeviceStatus.SIMULATED):
                     print(f"[Control] write_channel({self.channel_id}, {value}) skipped on "
                           f"{self.device_id} — device status is {dev.status.value}, not connected")
                 else: