fix: add SequenceNumber to GetMultiVariables/SetMultiVariables payloads

[gijzelaerr]

From #735 — Bug 3

All GetMultiVariables / SetMultiVariables payloads require a VLQ-encoded SequenceNumber field immediately after the ObjectQualifier. Without it the PLC rejects the request with error 11862009.

Fix

In all 6 payload-builder functions, insert encode_uint32_vlq(1) between encode_object_qualifier() and the trailing struct.pack(">I", 0):

payload += encode_object_qualifier()
payload += encode_uint32_vlq(1)   # SequenceNumber
payload += struct.pack(">I", 0)

Affected functions:

• _build_read_payload
• _build_write_payload
• _build_area_read_payload
• _build_area_write_payload
• _build_symbolic_read_payload
• _build_symbolic_write_payload

Hardware tested on

• S7-1200 CPU 1214C DC/DC/DC, FW V4.5

Related

• S7-1200 Q/I/M symbolic reads: findings from real hardware testing (FW V4.5) #735 (parent issue)

[gijzelaerr]

Fix is up in PR #742.

Inserted encode_uint32_vlq(1) (SequenceNumber) after encode_object_qualifier() in all 6 payload builder functions.

Could you verify against your S7-1200 (FW V4.5)?

pip install git+https://github.com/gijzelaerr/python-snap7.git@fix-payload-sequence-number