diff --git a/rack/README.md b/rack/README.md
index 5617781..eab1924 100644
--- a/rack/README.md
+++ b/rack/README.md
@@ -77,7 +77,8 @@ bash rack/deploy/deploy.sh          # 构建 → 传 → 服务器上 Python 3.8
 | 门户 | 应用 id 16，key `rack`、端口 7764（`phonerail` 已被需求文档站占用）；授权 admin + staff，**不给 viewer**（没有只读模式，能断电重启） |
 | 保留期 | 原始 1 s 遥测 2 天；分钟聚合 90 天；事件 / 审计 / 恢复记录永久 |
 
-加一块新板：USB 配网（`firmware/tools/prlink.py` 发 `wifi` / `mqtt`）+ 服务器上给它开 MQTT 账号（见 memory 里的 broker 说明），上线后自动出现在总览。
+加一块新板：配网（固件 ≥0.1.19：手机连板子自己开的热点 `PhoneRail-xxxxxx`，在 192.168.4.1 网页里填 WiFi 与 MQTT，
+热点口令 `88888888`；或 USB：`firmware/tools/prlink.py` 发 `wifi` / `mqtt`）+ 服务器上给它开 MQTT 账号（见 memory 里的 broker 说明），上线后自动出现在总览。
 
 ## 踩过的坑
 
diff --git a/rack/backend/prrack/protocol.py b/rack/backend/prrack/protocol.py
index 9087e67..d369159 100644
--- a/rack/backend/prrack/protocol.py
+++ b/rack/backend/prrack/protocol.py
@@ -172,7 +172,15 @@ def build_command(req: dict) -> dict:
         duty = req.get("duty")
         if not isinstance(duty, int) or isinstance(duty, bool) or not 0 <= duty <= 1000:
             raise CmdError("风扇下限必须是 0~1000‰（0 = 自动）")
-        return {"cmd": cmd, "duty": duty}
+        out = {"cmd": cmd, "duty": duty}
+        # 固件 ≥0.1.21：`sec` = 这个下限保持多少秒，到期自动回落到自动（没给 = 一直保持）。
+        sec = req.get("sec")
+        if sec is not None:
+            if not isinstance(sec, int) or isinstance(sec, bool) or not 0 <= sec <= 86400:
+                raise CmdError("风扇下限的保持时间必须是 0~86400 秒（0 / 不给 = 一直保持）")
+            if sec:
+                out["sec"] = sec
+        return out
     raise CmdError("不支持的命令：%r" % (cmd,))
 
 
diff --git a/rack/backend/prrack/simboard.py b/rack/backend/prrack/simboard.py
index 247afcd..b18ae19 100644
--- a/rack/backend/prrack/simboard.py
+++ b/rack/backend/prrack/simboard.py
@@ -185,7 +185,10 @@ class SimBoard:
             return {"ok": True, "oldest": 0, "next": len(self.events), "ev": ev}
         if cmd == "fan":
             self.fan_floor = req["duty"]
-            return {"ok": True, "floor": self.fan_floor}
+            rsp = {"ok": True, "floor": self.fan_floor}
+            if "sec" in req:
+                rsp["sec"] = req["sec"]  # 模拟板不倒计时：只把形状回对
+            return rsp
         if req.get("seq") is None:
             return {"ok": False, "err": "no_seq"}
         if cmd == "hub":  # 0.1.14：固件 hubctl.h 的命令形状；不模拟倒计时
diff --git a/rack/backend/tests/test_core.py b/rack/backend/tests/test_core.py
index 28ff92a..61b3c8b 100644
--- a/rack/backend/tests/test_core.py
+++ b/rack/backend/tests/test_core.py
@@ -59,12 +59,24 @@ def test_build_command_whitelist_and_ranges():
         {"cmd": "power", "rail": True, "on": True},         # bool 不是 int
         {"cmd": "vbus", "rail": 1, "on": 1},
         {"cmd": "fan", "duty": 1001},
+        {"cmd": "fan", "duty": 500, "sec": 86401},
+        {"cmd": "fan", "duty": 500, "sec": -1},
+        {"cmd": "fan", "duty": 500, "sec": "60"},
+        {"cmd": "fan", "duty": 500, "sec": True},
     ]
     for b in bad:
         with pytest.raises(protocol.CmdError):
             protocol.build_command(b)
 
 
+def test_fan_floor_may_carry_a_hold_time():
+    """固件 0.1.21：`sec` 原样带下去；0 / 不给 = 一直保持，命令里就不出现这个键（老固件不认它也没关系）。"""
+    assert protocol.build_command({"cmd": "fan", "duty": 600}) == {"cmd": "fan", "duty": 600}
+    assert protocol.build_command({"cmd": "fan", "duty": 600, "sec": 0}) == {"cmd": "fan", "duty": 600}
+    assert protocol.build_command({"cmd": "fan", "duty": 600, "sec": 90}) == {"cmd": "fan", "duty": 600, "sec": 90}
+    assert protocol.build_command({"cmd": "fan", "duty": 0, "sec": 86400}) == {"cmd": "fan", "duty": 0, "sec": 86400}
+
+
 def test_seq_is_strictly_increasing_and_in_range():
     t = [1700000000.0]
     g = protocol.SeqGen(clock=lambda: t[0])
diff --git a/rack/frontend/src/pages/Board.vue b/rack/frontend/src/pages/Board.vue
index b62d73a..cb605ae 100644
--- a/rack/frontend/src/pages/Board.vue
+++ b/rack/frontend/src/pages/Board.vue
@@ -46,9 +46,10 @@
         <div class="g"><div class="gh"><span class="gk">风扇</span>
             <el-select v-model="fanFloor" size="small" class="floor" aria-label="风扇占空比下限" @change="setFan">
               <el-option :value="0" label="自动" />
-              <el-option v-for="d in [500, 700, 850, 1000]" :key="d" :value="d" :label="`不低于 ${d / 10}%`" />
+              <el-option v-for="d in [300, 500, 700, 850, 1000]" :key="d" :value="d" :label="`不低于 ${d / 10}%`" />
             </el-select></div>
-          <div class="gvs"><span class="gv readout">{{ (b.status.fan?.duty ?? 0) / 10 }}<small>%</small></span>
+          <div class="gvs"><span v-if="b.status.fan?.en === false" class="gv readout" title="板温低于 40 °C：风扇断电（固件 ≥0.1.21）">停</span>
+            <span v-else class="gv readout">{{ (b.status.fan?.duty ?? 0) / 10 }}<small>%</small></span>
             <span v-for="(r, i) in b.status.fan?.rpm || []" :key="i" class="gv sm readout" :class="{ bad: b.status.fan?.stall?.[i] }">{{ r ?? '—' }}<small>rpm</small></span></div></div>
         <div class="g"><div class="gh"><span class="gk">巡检</span></div>
           <div class="gvs"><span class="gv readout">{{ b.status.safety?.cycles }}<small>圈</small></span>