OP7 to OP77B Migration: Resolving MPI Communication Failures

Overview: Why an OP7 to OP77B Field Swap Goes Wrong

Replacing a failed SIMATIC OP7 panel with the newer OP77B is a routine maintenance activity on S7-200/S7-300 lines that were originally commissioned under ProTool. The OP77B is not supported by ProTool/ProTool/ProRT — it must be programmed with WinCC flexible (2005 SP1 or later, ideally 2008 SP5 for cleanest migration). When the panel comes back with no comms to the S7 CPU, the failure pattern is almost always a combination of three issues:

1. Wrong MPI bus parameters on the OP77B (address, baud rate, profile).
2. Lost PLC tag connections because the WinCC flexible migration did not bind all Bit-Meldungen, Wort-Meldungen, and area pointers correctly.
3. Function key / system key reassignment differences between OP7 and OP77B (Chapter 5.2.5 of the WinCC flexible 2008 Migration manual).

This reference walks through diagnosis and repair of all three failure modes, including the exact DIP switch positions for an MPI node, the migration tool behavior, and the verification steps to bring the line back up without a project rewrite.

Hardware and Protocol Background

OP7 vs OP77B Key Differences

The OP77B is a hardware-superset for the OP7 in nearly every category. The migration headache is purely software-tooling, not electrical.

MPI Bus Topology Used at Field

MPI (Multi-Point Interface) is RS-485, isolated, with a Siemens-proprietary token-passing protocol that runs over the same PROFIBUS cable (two-core shielded, 120 Ω termination at both ends). The OP77B sits on the MPI/PROFIBUS port labelled IF1B on the bottom of the device. A typical S7-300 line looks like:

Maximum MPI segment length is 50 m at 1.5 Mbit/s without repeaters. Each node has a unique address 1..32; the OP77B is shipped with default address 1.

Pre-Checks Before Touching the Project

Confirm the four field-bus parameters on the physical device before opening WinCC flexible:

Root Cause 1 — Migration Tool Did Not Re-Connect the Tags

The WinCC flexible 2008 SP5 migration wizard reads the *.pdb/*.hmi file and re-emits an HMI project for OP77B. The wizard reports success on compile even when:

• Bit messages reference markers that no longer exist in the S7 program (because the S7 project is from an archive).
• Area pointer Coordination and Date/Time are not re-bound, so the HMI never refreshes.
• Symbolic tag names (e.g. MOTORE_1_AVVIAMENTO) lose their connection to the DB/MW when the source language was non-English and the symbolic table export was incomplete.

The field symptom is the "cannot read that Marker in the PLC" alarm on every function-key press. The panel has the cable right; it just cannot resolve the tag.

Repair Procedure

1. Open the migrated project in WinCC flexible 2008 SP5 or 2005 SP3 (the migration GUI only ships with 2008).
2. Select Project → Transfer → MPI/DP Settings and verify the connection. Then Project → Compile → Check Consistency. All red rows = broken tag references.
3. Open Project → Connections. The migrated connection will be of type SIMATIC S7-300/400 MPI (correct) or, after a bad import, SIMATIC S7-200 PPI (wrong for OP77B on a 300/400). Re-create with the correct driver if needed.
4. Re-attach each area pointer:

   Pointer	PLC address	Width	Why required
   Coordination	DBn.DBW0 (default)	1 word	HMI life bit, screen refresh
   Date/Time	DBn.DBD2	8 bytes (DT)	PLC → HMI time sync
   PLC ID	DBn.DBW10	1 word	Service ID for diagnostics
   Project ID	DBn.DBW12	1 word	Version number, optional

5. Re-bind symbolic tags. Where the S7 STEP7 archive is not available, switch to absolute addressing (e.g. MW 120 instead of START_PUMP_1). Absolute addresses never break during migration.
6. Recompile (Ctrl+F9) and re-download via MPI or Ethernet (RS232 backplane) into the OP77B.

Root Cause 2 — DIP Switch / PROFIBUS-DP Override Active

The OP77B inherits the OP7 hardware foot-print, including the 8-way DIP bank behind the rear cover. Switches 1..3 encode a PROFIBUS DP slave address (binary, switch 1 = LSB, switch 3 = MSB). If the original OP7 had the switches in any non-up position left from a commissioning mistake, the OP77B interprets that as a DP address and runs the bus as DP slave — the S7 CPU never sees it as an MPI peer.

Forcing MPI Mode

1. Power off the OP77B.
2. Open the rear cover (4 captive screws, OP77B Service Manual Chapter 4.1).
3. Set all 8 DIP switches to UP (= off / logical 0).
4. Re-power and verify: Control Panel → Info/System shows MPI in the Interface field, not DP.

Root Cause 3 — Function Key and System Key Reassignment

Chapter 5.2.5 of the WinCC flexible 2008 Migration manual states explicitly that OP7 K-softkeys (K1..K8) and OP7 system keys (S1..S16) cannot be mapped 1:1 to OP77B. The OP77B has only 8 F-keys + 4 K-keys with fixed assignment:

If the original Italian-language project used K-keys to acknowledge alarms and to drive bit M 200.0, the migrated project will compile but the keypress is dead. Open every screen in WinCC flexible, examine the Events tab of every key, and verify the function list (e.g. SetBit, ResetBit) points to a tag the panel can read.

Step-by-Step Recovery Procedure

1. Verify the bus: with the OP77B powered off, connect a PG/PC with STEP7 to the same MPI segment, run PLC → Accessible Nodes. The CPU must show up. If it does not, the bus is broken before the HMI is the problem.
2. Verify the panel: power only the OP77B, open Control Panel → Transfer, set MPI addr 1, baud 187.5 kbit/s, HSA 31, all DIP up. The panel should boot to the project without error code 0x1F in the system line.
3. Re-migrate: in WinCC flexible 2008 SP5, open the source *.hmi, choose target OP77B, tick Create complete migration. Save the new project under a new name so the source is never lost.
4. Rebind pointers: re-create the Coordination and Date/Time pointers (Root Cause 1, step 4).
5. Convert symbolic tags to absolute (optional but recommended when STEP7 archive is unavailable).
6. Re-author K-keys and S-keys (Root Cause 3).
7. Compile / Download: Project → Compiler → All; Transfer → to HMI; choose MPI, addr 1.
8. Cycle test: from the OP77B, press each F-key, observe the corresponding marker with STEP7 Monitor/Modify. The bit should toggle within 200 ms.

Verification Matrix

Cable and Pinout Reference

For an OP7→OP77B swap on an existing S7-300/200 segment, use Siemens 6XV1 830-0EH10 (purple PROFIBUS cable, 2-core shielded) or 6XV1 830-3BH10 (fast-connect). The OP77B port is a standard PROFIBUS 9-pin D-sub (IF1B):

Fault-Code / Status Quick Reference

Advanced: When the STEP7 Archive Is Missing

Often the S7 archive is lost; only the running CPU is available. To recover tag bindings without a STEP7 source:

1. Use STEP7 Accessible Nodes to upload the S7 program to a temporary PG project. This brings the symbol table only if it is online-readable (S7-300 ≥ V3.0 with adapted symbol online).
2. Use the OP77B Tag Simulation feature: Control Panel → Tags → Simulation. Tag values update locally without the PLC, letting you see which tag addresses are dead.
3. Watch the OP77B log buffer: Control Panel → Logs → System. A persistent Tag error, address 0x0000C8 = the marker is not in the PLC, or the wrong DB is referenced.

Commissioning Checklist (Print and Tick)

1. ☐ DIP bank on OP77B = all UP
2. ☐ MPI address = 1 (or per project), unique on segment
3. ☐ Baud = 187.5 kbit/s, profile MPI
4. ☐ HSA = 31
5. ☐ Accessible Nodes sees CPU and OP77B
6. ☐ Coordination pointer bound to a writable DB
7. ☐ Date/Time pointer bound
8. ☐ Every F-key Events tab has a valid function + tag
9. ☐ No K-key or S-key > 8 references remain in project
10. ☐ Power-cycle and re-test 3 times

Safety Note

FAQ