fix: match real engineering-tool SZL read and continuation request layouts

CHANGES.md

@@ -36,6 +36,9 @@ Unreleased
 * Compress now uses the PI service `_GARB` and copy RAM to ROM uses file system `EP` with `_MODU`, matching real
   engineering tools and native Snap7, instead of the invented `_MSZL`. The bundled server recognises the new
   requests (#941).
+* Read-SZL requests now carry the `0xFF`/octet-string data header and continuation requests the 12-byte parameter
+  block (method `0x12`) that real engineering tools send; both previously used the header of a request without data
+  and an 8-byte parameter block (#942).
 
 3.2.1
 -----

snap7/s7protocol.py

@@ -1059,8 +1059,8 @@ def build_read_szl_request(self, szl_id: int, szl_index: int) -> bytes:
         # Data section: SZL ID and Index
         data_section = struct.pack(
             ">BBHHH",
-            0x0A,  # Return value (request)
-            0x00,  # Transport size
+            0xFF,  # Return value (0xFF, as engineering tools and native Snap7 send for requests carrying data)
+            0x09,  # Transport size (octet string)
             0x0004,  # Length (4 bytes for ID + Index)
             szl_id,  # SZL ID
             szl_index,  # SZL Index
@@ -1091,18 +1091,23 @@ def build_userdata_followup_request(self, group: int, subfunction: int, sequence
         Returns:
             Complete S7 PDU for follow-up request
         """
-        # Parameter section: same as initial but with DataRef = sequence_number
+        # Parameter section as sent by engineering tools for continuation requests: 12 bytes, method 0x12,
+        # DataRef = sequence_number, followed by last-data-unit and error code fields set to zero
         type_group = 0x40 | (group & 0x0F)  # Type 4 (request) | group
         param_data = struct.pack(
-            ">BBBBBBBB",
+            ">BBBBBBBBBBBB",
             0x00,  # Reserved
             0x01,  # Parameter count
             0x12,  # Type/length header
-            0x04,  # Length of following data
-            0x11,  # Method (0x11 = request)
+            0x08,  # Length of following data
+            0x12,  # Method (0x12)
             type_group,  # Type | Group
             subfunction,  # Subfunction
             sequence_number,  # DataRef from previous response
+            0x00,  # Last data unit
+            0x00,  # Reserved
+            0x00,  # Error code high
+            0x00,  # Error code low
         )
 
         # Minimal data section for follow-up

tests/test_multipacket.py

@@ -136,19 +136,20 @@ def test_szl_followup(self) -> None:
         # Extract param_len and data_len from header
         param_len = struct.unpack(">H", pdu[6:8])[0]
         data_len = struct.unpack(">H", pdu[8:10])[0]
-        assert param_len == 8
+        assert param_len == 12
         assert data_len == 4
 
         # Parameter section starts at offset 10
         params = pdu[10 : 10 + param_len]
         assert params[0] == 0x00  # Reserved
         assert params[1] == 0x01  # Param count
         assert params[2] == 0x12  # Type header
-        assert params[3] == 0x04  # Length
-        assert params[4] == 0x11  # Method (request)
+        assert params[3] == 0x08  # Length
+        assert params[4] == 0x12  # Method, as engineering tools send for continuation requests
         assert params[5] == 0x44  # Type(4) | Group(4=SZL)
         assert params[6] == 0x01  # Subfunction
         assert params[7] == 0x02  # DataRef = sequence_number
+        assert params[8:12] == bytes(4)
 
         # Data section
         data = pdu[10 + param_len :]

tests/test_s7protocol.py

@@ -284,6 +284,12 @@ def test_copy_ram_to_rom_request_matches_real_job(self) -> None:
         request = self.proto.build_copy_ram_to_rom_request()
         assert request[10:] == bytes.fromhex("28000000000000fd00024550055f4d4f4455")
 
+    def test_read_szl_request_matches_real_tool(self) -> None:
+        # Captured from a real S7-300 session: SZL id 0x0132 index 4. Bytes 4-5 are the PDU reference.
+        real = bytes.fromhex("320700000300000800080001120411440100ff09000401320004")
+        request = self.proto.build_read_szl_request(0x0132, 4)
+        assert request[:4] + request[6:] == real[:4] + real[6:]
+
     def test_request_db_download(self) -> None:
         assert self.proto.build_download_request(0x41, 1, bytes(64)).hex() == (
             "320100000001002000001a00010000000000095f30413030303031500d31303030303634303030303238"