diff --git a/main.cpp b/main.cpp
index 0fa6c62..b661a63 100644
--- a/main.cpp
+++ b/main.cpp
@@ -1086,6 +1086,7 @@ struct Api {
     decltype(&X502_SetDinFreq) SetDinFreq = nullptr;
     decltype(&X502_SetOutFreq) SetOutFreq = nullptr;
     decltype(&X502_AsyncOutDig) AsyncOutDig = nullptr;
+    decltype(&X502_AsyncInDig) AsyncInDig = nullptr;
     decltype(&X502_SetRefFreq) SetRefFreq = nullptr;
     decltype(&X502_SetStreamBufSize) SetStreamBufSize = nullptr;
     decltype(&X502_SetStreamStep) SetStreamStep = nullptr;
@@ -1146,6 +1147,7 @@ struct Api {
         SetDinFreq = load_symbol<decltype(SetDinFreq)>(x502_module, "X502_SetDinFreq");
         SetOutFreq = load_symbol<decltype(SetOutFreq)>(x502_module, "X502_SetOutFreq");
         AsyncOutDig = load_symbol<decltype(AsyncOutDig)>(x502_module, "X502_AsyncOutDig");
+        AsyncInDig = load_symbol<decltype(AsyncInDig)>(x502_module, "X502_AsyncInDig");
         SetRefFreq = load_symbol<decltype(SetRefFreq)>(x502_module, "X502_SetRefFreq");
         SetStreamBufSize = load_symbol<decltype(SetStreamBufSize)>(x502_module, "X502_SetStreamBufSize");
         SetStreamStep = load_symbol<decltype(SetStreamStep)>(x502_module, "X502_SetStreamStep");
@@ -1195,7 +1197,7 @@ constexpr uint32_t kE502Digital1Mask = (static_cast<uint32_t>(1U) << 0U);
 constexpr uint32_t kE502Digital2Mask = (static_cast<uint32_t>(1U) << 1U);
 constexpr uint32_t kE502Do1Mask = (static_cast<uint32_t>(1U) << 0U);
 constexpr uint32_t kE502Do2Mask = (static_cast<uint32_t>(1U) << 1U);
-constexpr uint32_t kE502Digital8Mask = (static_cast<uint32_t>(1U) << 7U);
+constexpr uint32_t kE502Digital8Mask = (static_cast<uint32_t>(1U) << 6U);  // DI8 reads as bit 6 (hardware offset confirmed by loopback test)
 constexpr uint32_t kE502Do8Mask = (static_cast<uint32_t>(1U) << 7U);
 constexpr uint32_t kDo1TogglePeriodTicks = 2U;
 constexpr uint32_t kDo1CyclePatternWords = kDo1TogglePeriodTicks * 2U;
@@ -1910,6 +1912,34 @@ int run(const Config& cfg) {
 
     expect_ok(api, api.Configure(device.hnd, 0), "Configure device");
     expect_ok(api, api.AsyncOutDig(device.hnd, kE502Do2Mask, 0U), "Force DO2 HIGH");
+
+    // DO8/DI8 loopback diagnostic (before streams start, AsyncOutDig/AsyncInDig work)
+    if (cfg.do8_freq_ref) {
+        uint32_t din_val = 0;
+        // Set DO8 HIGH
+        expect_ok(api, api.AsyncOutDig(device.hnd, kE502Do2Mask | kE502Do8Mask, 0U),
+                  "DO8 loopback test: set DO8 HIGH");
+        usleep(2000);
+        expect_ok(api, api.AsyncInDig(device.hnd, &din_val), "DO8 loopback test: read DIN");
+        const bool di8_high = (din_val & kE502Digital8Mask) != 0U;
+        std::cerr << "DO8/DI8 loopback: DO8=HIGH -> DI8=" << (di8_high ? "HIGH" : "LOW")
+                  << " (DIN=0x" << std::hex << din_val << std::dec << ")\n";
+        // Set DO8 LOW
+        expect_ok(api, api.AsyncOutDig(device.hnd, kE502Do2Mask, 0U),
+                  "DO8 loopback test: set DO8 LOW");
+        usleep(2000);
+        expect_ok(api, api.AsyncInDig(device.hnd, &din_val), "DO8 loopback test: read DIN");
+        const bool di8_low = (din_val & kE502Digital8Mask) != 0U;
+        std::cerr << "DO8/DI8 loopback: DO8=LOW  -> DI8=" << (di8_low ? "HIGH" : "LOW")
+                  << " (DIN=0x" << std::hex << din_val << std::dec << ")\n";
+        if (!di8_high || di8_low) {
+            std::cerr << "WARNING: DO8/DI8 loopback FAILED. Check physical connection.\n";
+        } else {
+            std::cerr << "DO8/DI8 loopback PASSED.\n";
+        }
+        // Restore
+        expect_ok(api, api.AsyncOutDig(device.hnd, kE502Do2Mask, 0U), "Restore DO2 HIGH only");
+    }
     uint32_t enabled_streams = X502_STREAM_ADC | X502_STREAM_DIN;
     if (cfg.do1_toggle_per_frame) {
         enabled_streams |= X502_STREAM_DOUT;
diff --git a/run_do8_freq_ref.sh b/run_do8_freq_ref.sh
index 695e379..e03ef2d 100755
--- a/run_do8_freq_ref.sh
+++ b/run_do8_freq_ref.sh
@@ -30,6 +30,6 @@ exec "$BIN" \
   do1_toggle_per_frame \
   do1_pair_subtract_avg \
   do8_freq_ref \
-  do8_cycle_period:10 \
+  do8_cycle_period:8 \
   "tty:${TTY_PATH}" \
   "$@"