diff --git a/frontend/src/App.tsx b/frontend/src/App.tsx
index ccbe142..0730314 100644
--- a/frontend/src/App.tsx
+++ b/frontend/src/App.tsx
@@ -855,6 +855,7 @@ function App() {
               onFormatChange={setSendFormat}
               onSend={handleSendData}
               isConnected={connectionStatus.is_connected}
+              config={config}
               checksumConfig={checksumConfig}
               onChecksumConfigChange={setChecksumConfig}
               quickCommandLists={quickCommandLists}
diff --git a/frontend/src/components/SendPanel.tsx b/frontend/src/components/SendPanel.tsx
index daee370..841b65d 100644
--- a/frontend/src/components/SendPanel.tsx
+++ b/frontend/src/components/SendPanel.tsx
@@ -1,6 +1,6 @@
 import React, { useRef, useEffect, useState, useCallback, useMemo } from 'react';
-import { Send, Shield, ChevronDown, ChevronUp, List, FileText } from 'lucide-react';
-import { DataFormat, ChecksumType, ChecksumConfig, QuickCommandList, QuickCommand, LineEnding } from '../types';
+import { Send, Shield, ChevronDown, ChevronUp, List, FileText, AlertTriangle } from 'lucide-react';
+import { DataFormat, ChecksumType, ChecksumConfig, QuickCommandList, QuickCommand, LineEnding, SerialConfig } from '../types';
 import QuickCommandPanel from './QuickCommandPanel';
 import { useTheme } from '../contexts/ThemeContext';
 import { useTranslation } from '../i18n';
@@ -51,6 +51,7 @@ interface SendPanelProps {
   onFormatChange: (format: DataFormat) => void;
   onSend: () => void;
   isConnected: boolean;
+  config: SerialConfig;
   checksumConfig: ChecksumConfig;
   onChecksumConfigChange: (config: ChecksumConfig) => void;
   // Quick Command props
@@ -71,6 +72,7 @@ const SendPanel: React.FC<SendPanelProps> = ({
   onFormatChange,
   onSend,
   isConnected,
+  config,
   checksumConfig,
   onChecksumConfigChange,
   quickCommandLists,
@@ -92,6 +94,29 @@ const SendPanel: React.FC<SendPanelProps> = ({
   const [isChecksumExpanded, setIsChecksumExpanded] = useState(false);
   const [isConverting, setIsConverting] = useState(false);
 
+  // Line capacity in bytes/s: 1 start bit + data bits + optional parity bit
+  // + stop bits per byte on the wire.
+  const lineCapacityBps = useMemo(() => {
+    const dataBits = { Five: 5, Six: 6, Seven: 7, Eight: 8 }[config.data_bits];
+    const parityBit = config.parity === 'None' ? 0 : 1;
+    const stopBits = { One: 1, OnePointFive: 1.5, Two: 2 }[config.stop_bits];
+    return config.baud_rate / (1 + dataBits + parityBit + stopBits);
+  }, [config]);
+
+  // Payload per scheduled send in bytes (UTF-8 approximation in Text mode;
+  // this is a guard rail, not an exact meter).
+  const payloadBytes = useMemo(() => {
+    const base = format === 'Hex'
+      ? value.replace(/\s/g, '').length / 2
+      : new TextEncoder().encode(value).length;
+    return base + getChecksumLength(checksumConfig.type);
+  }, [value, format, checksumConfig.type]);
+
+  const requiredRateBps = (payloadBytes * 1000) / scheduledInterval;
+  const showRateWarning = isScheduledEnabled && payloadBytes > 0 && requiredRateBps > lineCapacityBps * 0.9;
+  const formatRate = (bps: number) =>
+    bps >= 1024 ? `${(bps / 1024).toFixed(1)} KB/s` : `${Math.round(bps)} B/s`;
+
   // Calculate disabled state for normal mode (depends on both connection AND content)
   const isNormalSendDisabled = !isConnected || !value.trim() || isScheduledEnabled || isConverting;
   // Calculate disabled state for quick mode (depends ONLY on connection)
@@ -577,6 +602,18 @@ const SendPanel: React.FC<SendPanelProps> = ({
               </div>
             </div>
 
+            {/* Rate guard: scheduled sending faster than the line can carry */}
+            {showRateWarning && (
+              <div className="mt-2 flex items-center space-x-1.5 flex-shrink-0">
+                <AlertTriangle size={13} style={{ color: '#f59e0b' }} className="flex-shrink-0" />
+                <span className="text-xs" style={{ color: '#f59e0b' }}>
+                  {t('sendPanel.rateExceeded')
+                    .replace('{rate}', formatRate(requiredRateBps))
+                    .replace('{capacity}', formatRate(lineCapacityBps))}
+                </span>
+              </div>
+            )}
+
             {/* Quick Insert Row (Hex mode only) */}
             {format === 'Hex' && (
               <div
diff --git a/frontend/src/i18n/translations/en.json b/frontend/src/i18n/translations/en.json
index 4f83bcc..d52a16f 100644
--- a/frontend/src/i18n/translations/en.json
+++ b/frontend/src/i18n/translations/en.json
@@ -89,6 +89,7 @@
     "stopScheduled": "Stop scheduled sending",
     "startScheduled": "Start scheduled sending",
     "intervalTitle": "Send interval in milliseconds",
+    "rateExceeded": "Send rate ~{rate} exceeds line capacity {capacity} — sends will keep timing out",
     "characters": "Characters",
     "bytes": "Bytes",
     "quickInsert": "Quick Insert",
diff --git a/frontend/src/i18n/translations/zh-CN.json b/frontend/src/i18n/translations/zh-CN.json
index 4b3d579..b6d8775 100644
--- a/frontend/src/i18n/translations/zh-CN.json
+++ b/frontend/src/i18n/translations/zh-CN.json
@@ -89,6 +89,7 @@
     "stopScheduled": "停止定时发送",
     "startScheduled": "开始定时发送",
     "intervalTitle": "发送间隔（毫秒）",
+    "rateExceeded": "发送速率约 {rate}，超过线路容量 {capacity}，将持续超时",
     "characters": "字符数",
     "bytes": "字节数",
     "quickInsert": "快速插入",
diff --git a/src-tauri/src/serial_manager.rs b/src-tauri/src/serial_manager.rs
index 8d9f97e..f2ce1af 100644
--- a/src-tauri/src/serial_manager.rs
+++ b/src-tauri/src/serial_manager.rs
@@ -84,6 +84,12 @@ impl PortOpener for SystemPortOpener {
     }
 }
 
+/// Write-side timeout (POSIX only, see `connect`). Much longer than the
+/// 50 ms read timeout so large or bursty payloads tolerate a full kernel TX
+/// buffer draining at line rate instead of failing spuriously.
+#[cfg(unix)]
+const WRITE_TIMEOUT_MS: u64 = 1000;
+
 impl SerialManager {
     pub fn new() -> Self {
         // Default log directory - will be overridden by frontend settings
@@ -170,7 +176,10 @@ impl SerialManager {
             self.disconnect()?;
         }
 
-        let port = self.port_opener.open(port_name, &config)?;
+        // `mut` is only exercised by the POSIX write-timeout tweak below;
+        // Windows shares timeouts across cloned handles and leaves it alone.
+        #[allow(unused_mut)]
+        let mut port = self.port_opener.open(port_name, &config)?;
         info!("Successfully opened serial port: {}", port_name);
 
         // Reset and start reading thread
@@ -187,6 +196,15 @@ impl SerialManager {
         let shutdown_flag = Arc::clone(&self.shutdown_flag);
         let mut read_port = port.try_clone()?;
 
+        // Give the write side a longer timeout than the read-friendly 50 ms.
+        // On POSIX the timeout lives on each handle, so this does not slow
+        // the read loop; on Windows cloned handles share COMMTIMEOUTS, so we
+        // leave the write side at the builder's value there.
+        #[cfg(unix)]
+        if let Err(e) = port.set_timeout(Duration::from_millis(WRITE_TIMEOUT_MS)) {
+            warn!("Failed to set write timeout on {}: {}", port_name, e);
+        }
+
         thread::spawn(move || {
             let mut read_buffer = [0u8; 1024];
             let initial_config = frame_segmentation_config.lock()