fix: add SequenceNumber to Get/SetMultiVariables payloads (#738)

S7-1200 FW V4.5 requires a VLQ-encoded SequenceNumber field after the
ObjectQualifier in all Get/SetMultiVariables requests. Without it the
PLC rejects with error 11862009.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: gijzelaerr

s7/_s7commplus_client.py

@@ -484,6 +484,7 @@ def _build_read_payload(items: list[tuple[int, int, int]]) -> bytes:
     for addr in addresses:
         payload += addr
     payload += encode_object_qualifier()
+    payload += encode_uint32_vlq(1)
     payload += struct.pack(">I", 0)
 
     return bytes(payload)
@@ -570,6 +571,7 @@ def _build_write_payload(items: list[tuple[int, int, bytes]]) -> bytes:
         payload += encode_pvalue_blob(data)
     payload += bytes([0x00])
     payload += encode_object_qualifier()
+    payload += encode_uint32_vlq(1)
     payload += struct.pack(">I", 0)
 
     return bytes(payload)
@@ -622,6 +624,7 @@ def _build_area_read_payload(area_rid: int, start: int, size: int) -> bytes:
     payload += encode_uint32_vlq(field_count)
     payload += addr_bytes
     payload += encode_object_qualifier()
+    payload += encode_uint32_vlq(1)
     payload += struct.pack(">I", 0)
     return bytes(payload)
 
@@ -643,6 +646,7 @@ def _build_area_write_payload(area_rid: int, start: int, data: bytes) -> bytes:
     payload += encode_pvalue_blob(data)
     payload += bytes([0x00])
     payload += encode_object_qualifier()
+    payload += encode_uint32_vlq(1)
     payload += struct.pack(">I", 0)
     return bytes(payload)
 
@@ -675,6 +679,7 @@ def _build_symbolic_read_payload(access_area: int, lids: list[int], symbol_crc:
     payload += encode_uint32_vlq(field_count)
     payload += addr_bytes
     payload += encode_object_qualifier()
+    payload += encode_uint32_vlq(1)
     payload += struct.pack(">I", 0)
     return bytes(payload)
 
@@ -702,6 +707,7 @@ def _build_symbolic_write_payload(access_area: int, lids: list[int], data: bytes
     payload += encode_pvalue_blob(data)
     payload += bytes([0x00])
     payload += encode_object_qualifier()
+    payload += encode_uint32_vlq(1)
     payload += struct.pack(">I", 0)
     return bytes(payload)
 

tests/test_s7_unit.py

@@ -9,8 +9,12 @@
     _parse_read_response,
     _build_write_payload,
     _parse_write_response,
+    _build_area_read_payload,
+    _build_area_write_payload,
+    _build_symbolic_read_payload,
+    _build_symbolic_write_payload,
 )
-from s7.codec import encode_pvalue_blob
+from s7.codec import encode_object_qualifier, encode_pvalue_blob
 from s7.connection import S7CommPlusConnection, _element_size
 from s7.protocol import DataType, ElementID, ObjectId
 from s7.vlq import (
@@ -175,6 +179,42 @@ def test_write_read_consistency(self) -> None:
         assert isinstance(write_payload, bytes)
 
 
+class TestSequenceNumber:
+    """Verify all payload builders include a SequenceNumber after ObjectQualifier."""
+
+    @staticmethod
+    def _has_sequence_number(payload: bytes) -> bool:
+        oq = encode_object_qualifier()
+        idx = bytes(payload).find(oq)
+        assert idx >= 0, "ObjectQualifier not found in payload"
+        seq_offset = idx + len(oq)
+        return payload[seq_offset : seq_offset + 1] == encode_uint32_vlq(1)
+
+    def test_read_payload_has_sequence_number(self) -> None:
+        payload = _build_read_payload([(1, 0, 4)])
+        assert self._has_sequence_number(payload)
+
+    def test_write_payload_has_sequence_number(self) -> None:
+        payload = _build_write_payload([(1, 0, bytes([1, 2, 3, 4]))])
+        assert self._has_sequence_number(payload)
+
+    def test_area_read_payload_has_sequence_number(self) -> None:
+        payload = _build_area_read_payload(82, 0, 4)
+        assert self._has_sequence_number(payload)
+
+    def test_area_write_payload_has_sequence_number(self) -> None:
+        payload = _build_area_write_payload(82, 0, b"\x00\x00\x00\x00")
+        assert self._has_sequence_number(payload)
+
+    def test_symbolic_read_payload_has_sequence_number(self) -> None:
+        payload = _build_symbolic_read_payload(0x8A0E0001, [1, 4])
+        assert self._has_sequence_number(payload)
+
+    def test_symbolic_write_payload_has_sequence_number(self) -> None:
+        payload = _build_symbolic_write_payload(0x8A0E0001, [1, 4], b"\x01")
+        assert self._has_sequence_number(payload)
+
+
 # -- Connection unit tests --