diff --git a/arena_studio.html b/arena_studio.html
index 126d66e1..902fa506 100644
--- a/arena_studio.html
+++ b/arena_studio.html
@@ -2779,10 +2779,9 @@ <h2 id="modalTitle">Import error</h2>
           </div>
           <div class="cmenu-sep" title="Rig-declared controller I/O roles + power-on defaults (rig YAML io: block, #135). Applied automatically at connect; the role selectors are SESSION-LOCAL overrides — they never edit the rig YAML. Greyed roles need firmware that doesn't exist yet. Names match the board's BNC silkscreen.">Rig I/O — session roles</div>
           <div class="cmenu-row">
-            <label title="The BNC silkscreened 'Digital IO 1 (5V)' on the controller (J3; rig io: port 1 = 0xAA channel 1)">Digital IO 1</label>
-            <select id="cIoRoleDio1" title="'Digital IO 1 (5V)' role — session-local override of the rig's io: declaration">
+            <label title="The BNC silkscreened 'Digital IO 1 (5V)' on the controller (J3; rig io: port 1 = 0xAA channel 1). It can't be the trigger input: only Digital IO 2 (5V) is wired to the panels' trigger (EINT).">Digital IO 1</label>
+            <select id="cIoRoleDio1" title="'Digital IO 1 (5V)' role — session-local override of the rig's io: declaration. No in_trigger here: only Digital IO 2 reaches the panels' trigger input (EINT).">
               <option value="off">off</option>
-              <option value="in_trigger" disabled title="Needs io_ext firmware (0xC2 capability bit 5) — enables automatically when the connected controller advertises it.">in_trigger</option>
               <option value="out_programmable">out_programmable</option>
               <option value="out_debug_framescan" disabled title="Needs io_ext firmware (0xC2 capability bit 5) — enables automatically when the connected controller advertises it.">out_debug_framescan</option>
             </select>
@@ -6938,7 +6937,8 @@ <h2 id="modalTitle">Import error</h2>
         opt.title = on ? title
             : 'Needs io_ext firmware (0xC2 capability bit 5) — connect a controller running feat/dio-roles-ao-modes or later.';
     };
-    gate('cIoRoleDio1', 'in_trigger', 'Input role — on Digital IO 2 this is the EINT trigger route (J30); pulse-semantics bench validation pending.');
+    // Digital IO 1 has no in_trigger option at all: that BNC has no route to the panels' EINT net
+    // (#168). Only Digital IO 2 can be the trigger input.
     gate('cIoRoleDio2', 'in_trigger', 'Input role — the EINT trigger route to the panels (J30 shunt); pulse-semantics bench validation pending.');
     gate('cIoRoleDio1', 'out_debug_framescan', 'Scope envelope: HIGH exactly while each frame\'s SPI transmission runs (bench-verified).');
     gate('cIoRoleDio2', 'out_debug_framescan', 'Scope envelope: HIGH exactly while each frame\'s SPI transmission runs. NOTE: with the J30 shunt installed this also pulses the panels\' EINT net.');
@@ -6993,6 +6993,11 @@ <h2 id="modalTitle">Import error</h2>
         const r = roleOf(d.port === 1 ? 'cIoRoleDio1' : 'cIoRoleDio2', d.role);
         const ch = d.port; // io: port == wire channel == board "Digital IO n (5V)" label
         if (r === 'off') continue; // off = DON'T TOUCH (keeps fw boot roles, e.g. port 2 in_trigger)
+        if (ch === 1 && r === 'in_trigger') {
+            // Digital IO 1 has no EINT route: the trigger would be silently dead (#168).
+            clog('rig io: Digital IO 1 cannot be the trigger input (only Digital IO 2 reaches the panels\' EINT) — not applied', 'warn');
+            continue;
+        }
         if (ext) {
             // Explicit role first (SET_DIO_ROLE 0xAC), then the level for outputs.
             try {
diff --git a/js/plugin-registry.js b/js/plugin-registry.js
index 4001884e..50f75c81 100644
--- a/js/plugin-registry.js
+++ b/js/plugin-registry.js
@@ -1253,6 +1253,14 @@ function parseRigIo(rigData) {
         }
         var slot = result.dio[port - 1];
         slot.role = _rigIoRole(entry.role, RIG_IO_ROLES.dio, 'dio port ' + port, result.warnings);
+        // Only "Digital IO 2 (5V)" is wired to the panels' EINT trigger net (U3 + J30 shunt).
+        // Digital IO 1 has no route, so in_trigger there would be a silently dead trigger (#168).
+        if (port === 1 && slot.role === 'in_trigger') {
+            result.warnings.push(
+                'dio port 1 cannot be in_trigger: only "Digital IO 2 (5V)" reaches the panels\' trigger input (EINT); treated as off'
+            );
+            slot.role = 'off';
+        }
         // default is outputs-only; clamp anything truthy-numeric/boolean to 0|1.
         if (entry.default != null)
             slot.default = entry.default === true || Number(entry.default) === 1 ? 1 : 0;
diff --git a/tests/test-protocol-roundtrip-v3.js b/tests/test-protocol-roundtrip-v3.js
index 702cecab..5bc8f657 100644
--- a/tests/test-protocol-roundtrip-v3.js
+++ b/tests/test-protocol-roundtrip-v3.js
@@ -3846,6 +3846,53 @@ console.log('\n--- Suite N12io: rig io: block (#135) ---');
         bad.warnings.join(' | ')
     );
 
+    // (c2) #168: Digital IO 1 has no route to the panels' EINT trigger net — in_trigger on
+    // port 1 degrades to off with a warning naming Digital IO 2; port 2 keeps it.
+    const trig = parseRigIo({
+        io: {
+            dio: [
+                { port: 1, role: 'in_trigger' },
+                { port: 2, role: 'in_trigger' }
+            ]
+        }
+    });
+    check('N12io-c2: port 1 in_trigger → off (no EINT route)', trig.dio[0].role, 'off');
+    check('N12io-c2: port 2 in_trigger kept (the EINT route)', trig.dio[1].role, 'in_trigger');
+    checkTrue(
+        'N12io-c2: one warning, naming Digital IO 2',
+        trig.warnings.length === 1 &&
+            /port 1 cannot be in_trigger/.test(trig.warnings[0]) &&
+            /Digital IO 2/.test(trig.warnings[0]),
+        trig.warnings.join(' | ')
+    );
+    // The Studio side: Digital IO 1 offers no in_trigger option at all (the reason is in its
+    // tooltip), capability gating never adds it back, and the apply path refuses to send it.
+    const studioHtml = fs.readFileSync(path.join(__dirname, '..', 'arena_studio.html'), 'utf8');
+    const dio1Select = (studioHtml.match(/<select id="cIoRoleDio1"[\s\S]*?<\/select>/) || [''])[0];
+    checkTrue('N12io-c2: Studio DIO 1 select found', dio1Select.length > 0, 'select missing');
+    checkTrue(
+        'N12io-c2: Studio DIO 1 has no in_trigger option',
+        !/value="in_trigger"/.test(dio1Select),
+        dio1Select
+    );
+    checkTrue(
+        'N12io-c2: Studio DIO 1 tooltip says why (only Digital IO 2 reaches EINT)',
+        /id="cIoRoleDio1" title="[^"]*only Digital IO 2 reaches the panels' trigger input \(EINT\)/.test(
+            studioHtml
+        ),
+        'tooltip changed'
+    );
+    checkTrue(
+        'N12io-c2: capability gating never enables DIO 1 in_trigger',
+        !/gate\('cIoRoleDio1',\s*'in_trigger'/.test(studioHtml),
+        'found a gate() call for cIoRoleDio1 in_trigger'
+    );
+    checkTrue(
+        'N12io-c2: apply path skips in_trigger on channel 1',
+        /if \(ch === 1 && r === 'in_trigger'\)/.test(studioHtml),
+        'apply guard missing'
+    );
+
     // (d) value clamping.
     const clamp = parseRigIo({
         io: {