diff --git a/civ.py b/civ.py
index 984b2da..61aa117 100644
--- a/civ.py
+++ b/civ.py
@@ -24,6 +24,17 @@ SUB_SQL_LEVEL = 0x03
 SUB_SQL_STATUS = 0x01
 SUB_SMETER = 0x02
 
+# --- repeater config (candidate commands; verify on live radio) ---
+CMD_OFFSET = 0x0D       # set duplex offset frequency
+CMD_DUPLEX = 0x0F       # set duplex direction
+DUP_SIMPLEX = 0x10
+DUP_MINUS = 0x11
+DUP_PLUS = 0x12
+CMD_TONE_PARAM = 0x1B   # sub 0x00 = repeater (TX) tone frequency
+SUB_RPT_TONE = 0x00
+CMD_TONE_SWITCH = 0x16  # sub 0x42 = tone encode on/off
+SUB_TONE_ENC = 0x42
+
 Frame = namedtuple("Frame", "to frm cn data")
 
 
@@ -66,6 +77,41 @@ def bcd_to_level(data):
     return value
 
 
+def offset_to_bcd(hz):
+    """Duplex offset Hz -> 3-byte little-endian BCD in 100 Hz units (PROVISIONAL)."""
+    if hz < 0:
+        raise ValueError(f"offset {hz} must be >= 0")
+    units = round(hz / 100)
+    digits = f"{units:06d}"
+    out = bytearray()
+    for i in range(0, 6, 2):
+        out.append((int(digits[4 - i]) << 4) | int(digits[5 - i]))
+    return bytes(out)
+
+
+def bcd_to_offset(data):
+    """3-byte little-endian BCD (100 Hz units) -> Hz (PROVISIONAL)."""
+    units = 0
+    for i, byte in enumerate(data[:3]):
+        units += (byte & 0x0F) * (10 ** (2 * i)) + (byte >> 4) * (10 ** (2 * i + 1))
+    return units * 100
+
+
+def tone_to_bcd(hz):
+    """CTCSS tone Hz -> 2-byte BCD of tone*10 (88.5 -> 08 85)."""
+    digits = f"{round(hz * 10):04d}"
+    return bytes([(int(digits[0]) << 4) | int(digits[1]),
+                  (int(digits[2]) << 4) | int(digits[3])])
+
+
+def bcd_to_tone(data):
+    """2-byte BCD of tone*10 -> CTCSS tone Hz."""
+    t = 0
+    for byte in data[:2]:
+        t = t * 100 + (byte >> 4) * 10 + (byte & 0x0F)
+    return t / 10.0
+
+
 def build_frame(cmd, data=b"", to=RADIO_ADDR, frm=CTRL_ADDR):
     """Assemble FE FE <to> <from> <cmd> [data] FD."""
     return PREAMBLE + bytes([to, frm, cmd]) + bytes(data) + bytes([EOM])
diff --git a/test_civ.py b/test_civ.py
index e07df5d..bf48bc7 100644
--- a/test_civ.py
+++ b/test_civ.py
@@ -73,5 +73,36 @@ class TestParseFrames(unittest.TestCase):
         self.assertEqual(civ.parse_frames(b""), [])
 
 
+class TestOffsetCodec(unittest.TestCase):
+    # PROVISIONAL: 3-byte little-endian BCD in units of 100 Hz.
+    # Round-trip validated until pinned to a real capture (plan Task 6).
+    def test_offset_roundtrip(self):
+        for hz in (600_000, 5_000_000, 1_600_000, 0):
+            self.assertEqual(civ.bcd_to_offset(civ.offset_to_bcd(hz)), hz)
+
+    def test_offset_is_three_bytes(self):
+        self.assertEqual(len(civ.offset_to_bcd(600_000)), 3)
+
+    def test_offset_rejects_negative(self):
+        with self.assertRaises(ValueError):
+            civ.offset_to_bcd(-1)
+
+
+class TestToneCodec(unittest.TestCase):
+    # ICOM convention: CTCSS tone as 2-byte BCD of tone*10. 131.8 -> "13 18".
+    def test_tone_to_bcd_known(self):
+        self.assertEqual(civ.tone_to_bcd(88.5), b"\x08\x85")
+        self.assertEqual(civ.tone_to_bcd(131.8), b"\x13\x18")
+        self.assertEqual(civ.tone_to_bcd(100.0), b"\x10\x00")
+
+    def test_bcd_to_tone_known(self):
+        self.assertAlmostEqual(civ.bcd_to_tone(b"\x13\x18"), 131.8)
+        self.assertAlmostEqual(civ.bcd_to_tone(b"\x08\x85"), 88.5)
+
+    def test_tone_roundtrip(self):
+        for hz in (67.0, 88.5, 131.8, 146.2, 203.5):
+            self.assertAlmostEqual(civ.bcd_to_tone(civ.tone_to_bcd(hz)), hz)
+
+
 if __name__ == "__main__":
     unittest.main()