diff --git a/pico_tnc/cmd.c b/pico_tnc/cmd.c
index b08a9c6..2530a84 100644
--- a/pico_tnc/cmd.c
+++ b/pico_tnc/cmd.c
@@ -1,5 +1,5 @@
 /*
-Copyright (c) 2021, Kazuhisa Yokota, JN1DFF
+Copyright (c) 2021, JN1DFF
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
@@ -99,6 +99,9 @@ static const uint8_t *gps_str[] = {
 
 // indicate converse mode
 bool converse_mode = false;
+// indicate calibrate mode
+bool calibrate_mode = false;
+uint8_t calibrate_idx = 0;
 
 static uint8_t *read_call(uint8_t *buf, callsign_t *c)
 {
@@ -547,7 +550,33 @@ static bool cmd_txdelay(tty_t *ttyp, uint8_t *buf, int len)
 
 static bool cmd_calibrate(tty_t *ttyp, uint8_t *buf, int len)
 {
-    tty_write_str(ttyp, "CALIBRATE\r\n");
+    //tty_write_str(ttyp, "CALIBRATE\r\n");
+    tnc_t *tp = &tnc[0];
+    if (tp->send_state != SP_IDLE) {
+        tty_write_str(ttyp, "Transmitter busy\r\n");
+        return false;
+    }
+
+    tp->send_state = SP_CALIBRATE;
+    tp->do_nrzi = false;
+    calibrate_mode = true;
+    calibrate_idx = 0;
+    tp->cal_data = 0x00;
+    tp->ttyp = ttyp;
+    tp->cal_time = tnc_time();
+    tty_write_str(ttyp, "Calibrate Mode. SP to toggle; ctl C to Exit\r\n");
+    return true;
+}
+
+void calibrate(void)
+{
+    tnc_t *tp = &tnc[0];
+    if (tp->send_state != SP_CALIBRATE_OFF) return;
+
+    tp->send_state = SP_IDLE;
+    tp->do_nrzi = true;
+    calibrate_mode = false;
+    tty_write_str(tp->ttyp, "Exit Calibrate Mode\r\ncmd: ");
 }
 
 static bool cmd_converse(tty_t *ttyp, uint8_t *buf, int len)
@@ -719,7 +748,7 @@ void cmd(tty_t *ttyp, uint8_t *buf, int len)
     if (matched == 1) {
 
         if (mp->func(ttyp, param, param_len)) {
-            if (!converse_mode) tty_write_str(ttyp, "\r\nOK\r\n");
+            if (!(converse_mode | calibrate_mode)) tty_write_str(ttyp, "\r\nOK\r\n");
             return;
         }
     }
diff --git a/pico_tnc/cmd.h b/pico_tnc/cmd.h
index f243b5a..c24350e 100644
--- a/pico_tnc/cmd.h
+++ b/pico_tnc/cmd.h
@@ -1,5 +1,5 @@
 /*
-Copyright (c) 2021, Kazuhisa Yokota, JN1DFF
+Copyright (c) 2021, JN1DFF
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
@@ -32,5 +32,8 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 
 extern bool converse_mode;
+extern bool calibrate_mode;
+extern uint8_t calibrate_idx;
 
 void cmd(tty_t *ttyp, uint8_t *buf, int len);
+void calibrate(void);
diff --git a/pico_tnc/main.c b/pico_tnc/main.c
index 89c250b..d0cc8b4 100644
--- a/pico_tnc/main.c
+++ b/pico_tnc/main.c
@@ -1,5 +1,5 @@
 /*
-Copyright (c) 2021, Kazuhisa Yokota, JN1DFF
+Copyright (c) 2021, JN1DFF
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
@@ -148,6 +148,9 @@ int main()
         // send beacon
         beacon();
 
+        // calibrate off
+        calibrate();
+
 #ifdef BUSY_PIN
 //        gpio_put(BUSY_PIN, 0);
 #endif
diff --git a/pico_tnc/send.c b/pico_tnc/send.c
index a914913..e97aa80 100644
--- a/pico_tnc/send.c
+++ b/pico_tnc/send.c
@@ -63,6 +63,8 @@ static const int ptt_pins[] = {
 
 #define ISR_PIN 15
 
+#define CAL_TIMEOUT (60 * 100)  // 60 sec
+
 static void __isr dma_handler(void)
 {
     int int_status = dma_hw->ints0;
@@ -152,7 +154,11 @@ int send_byte(tnc_t *tp, uint8_t data, bool bit_stuff)
         int bit = byte & 1;
         while (byte > 1) { // check sentinel
 
-            if (!bit) tp->level ^= 1; // NRZI, invert if original bit == 0
+            if (tp->do_nrzi) {
+                if (!bit) tp->level ^= 1; // NRZI, invert if original bit == 0
+            } else {
+                tp->level = bit;
+            }
 
             // make Bell202 CPAFSK audio samples
             tp->dma_blocks[tp->next][idx++] = phase_tab[tp->level][tp->phase]; // 1: mark, 0: space
@@ -309,17 +315,6 @@ void send_init(void)
     gpio_put(SMPS_PIN, 1);
 }
 
-enum SEND_STATE {
-    SP_IDLE = 0,
-    SP_WAIT_CLR_CH,
-    SP_P_PERSISTENCE,
-    SP_WAIT_SLOTTIME,
-    SP_PTT_ON,
-    SP_SEND_FLAGS,
-    SP_DATA_START,
-    SP_DATA,
-    SP_ERROR,
-};
 
 int send_queue_free(tnc_t *tp)
 {
@@ -434,6 +429,15 @@ void send(void)
                 while (queue_try_remove(&tp->send_queue, &data)) {
                 }
                 tp->send_state = SP_IDLE;
+                break;
+
+            case SP_CALIBRATE:
+                if (tnc_time() - tp->cal_time >= CAL_TIMEOUT) {
+                    tp->send_state = SP_CALIBRATE_OFF;
+                } else {
+                    send_byte(tp, tp->cal_data, false);
+                }
+                break;
         }
 
         tp++;
diff --git a/pico_tnc/serial.c b/pico_tnc/serial.c
index 2b99170..bfb0587 100644
--- a/pico_tnc/serial.c
+++ b/pico_tnc/serial.c
@@ -1,5 +1,5 @@
 /*
-Copyright (c) 2021, Kazuhisa Yokota, JN1DFF
+Copyright (c) 2021, JN1DFF
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
diff --git a/pico_tnc/tnc.c b/pico_tnc/tnc.c
index 0a5cc7a..db5d461 100644
--- a/pico_tnc/tnc.c
+++ b/pico_tnc/tnc.c
@@ -1,5 +1,5 @@
 /*
-Copyright (c) 2021, Kazuhisa Yokota, JN1DFF
+Copyright (c) 2021, JN1DFF
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
@@ -95,12 +95,16 @@ void tnc_init(void)
 
         // send queue
         queue_init(&tp->send_queue, sizeof(uint8_t), SEND_QUEUE_LEN);
+        tp->send_state = SP_IDLE;
 
         tp->cdt = 0;
         tp->kiss_txdelay = 50;
         tp->kiss_p = 63;
         tp->kiss_slottime = 10;
         tp->kiss_fullduplex = 0;
+
+        // calibrate
+        tp->do_nrzi = true;
     }
 
     //printf("%d ports support\n", PORT_N);
diff --git a/pico_tnc/tnc.h b/pico_tnc/tnc.h
index 916198d..07dc40b 100644
--- a/pico_tnc/tnc.h
+++ b/pico_tnc/tnc.h
@@ -1,5 +1,5 @@
 /*
-Copyright (c) 2021, Kazuhisa Yokota, JN1DFF
+Copyright (c) 2021, JN1DFF
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
@@ -85,6 +85,8 @@ typedef struct {
   int high_q;
 } values_t;
 
+typedef struct TTY tty_t;
+
 typedef struct TNC {
     uint8_t port;
 
@@ -177,6 +179,12 @@ typedef struct TNC {
     uint16_t packet_len;
     int wait_time;
 
+    // calibrate
+    uint8_t cal_data;
+    bool do_nrzi;
+    uint32_t cal_time;
+    tty_t *ttyp;
+
 } tnc_t;
 
 extern tnc_t tnc[];
@@ -256,3 +264,18 @@ typedef struct TTY {
 } tty_t;
 
 extern tty_t tty[];
+
+// send process state
+enum SEND_STATE {
+    SP_IDLE = 0,
+    SP_WAIT_CLR_CH,
+    SP_P_PERSISTENCE,
+    SP_WAIT_SLOTTIME,
+    SP_PTT_ON,
+    SP_SEND_FLAGS,
+    SP_DATA_START,
+    SP_DATA,
+    SP_ERROR,
+    SP_CALIBRATE,
+    SP_CALIBRATE_OFF,
+};
diff --git a/pico_tnc/tty.c b/pico_tnc/tty.c
index e97e704..1ae9e93 100644
--- a/pico_tnc/tty.c
+++ b/pico_tnc/tty.c
@@ -1,5 +1,5 @@
 /*
-Copyright (c) 2021, Kazuhisa Yokota, JN1DFF
+Copyright (c) 2021, JN1DFF
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without
@@ -112,8 +112,21 @@ void tty_write_str(tty_t *ttyp, uint8_t const *str)
 #define BELL '\a'
 #define CTRL_C '\x03'
 #define FEND 0xc0
+#define SP ' '
 
 #define KISS_TIMEOUT (1 * 100) // 1 sec
+ 
+#define CAL_DATA_MAX 3
+
+static const uint8_t calibrate_data[CAL_DATA_MAX] = {
+    0x00, 0xff, 0x55,
+};
+
+static const char *calibrate_str[CAL_DATA_MAX] = {
+    "send space (2200Hz)\r\n",
+    "send mark  (1200Hz)\r\n",
+    "send 0x55  (1200/2200Hz)\r\n",
+};
 
 void tty_input(tty_t *ttyp, int ch)
 {
@@ -128,6 +141,28 @@ void tty_input(tty_t *ttyp, int ch)
         ttyp->kiss_state = KISS_OUTSIDE;
     }
 
+    // calibrate mode
+    if (calibrate_mode) {
+        tnc_t *tp = &tnc[0];
+
+        switch (ch) {
+            case SP: // toggle mark/space
+                if (++calibrate_idx >= CAL_DATA_MAX) calibrate_idx = 0;
+                tp->cal_data = calibrate_data[calibrate_idx];
+                tty_write_str(ttyp, calibrate_str[calibrate_idx]);
+                tp->cal_time = tnc_time();
+                break;
+
+            case CTRL_C:
+                tp->send_state = SP_CALIBRATE_OFF;
+                break;
+
+            default:
+                tty_write_char(ttyp, BELL);
+        }
+        return;
+    }
+
     switch (ch) {
         case FEND: // KISS frame end
             kiss_input(ttyp, ch);
@@ -153,7 +188,7 @@ void tty_input(tty_t *ttyp, int ch)
                     cmd(ttyp, ttyp->cmd_buf, ttyp->cmd_idx);
                 }
             }
-            if (!converse_mode) tty_write_str(ttyp, "cmd: ");
+            if (!(converse_mode | calibrate_mode)) tty_write_str(ttyp, "cmd: ");
             ttyp->cmd_idx = 0;
             break;