Resolving Siemens S5-95U Block Status ZYK Cycle Error: Diagnosis, OB31 Workaround, and Verification

The Siemens SIMATIC S5-95U compact PLC is a workhorse controller still running in thousands of machines installed in the 1990s. A classic field symptom that surfaces during commissioning or service work is this: the moment the engineer enables online Block Status (Bausteinanzeige / PB-Status) in STEP 5, the CPU drops to STOP, the red STOP LED illuminates, and the Interrupt Stack (ISTACK) reports ZYK as the active cause. This article documents the precise root cause, the affected CPU variants, the recommended Siemens workarounds (including OB31 call-injection), and a verification procedure you can run on the bench before returning the machine to production.

1. Problem Description

The reported failure mode is reproducible on every transition from offline to online block status:

1. STEP 5 is connected to the S5-95U via the serial programming port (AS511 / TTY / current-loop).
2. The operator opens a Program Block (PB), Function Block (FB), or Data Block (DB) and toggles Block Status (German: Bausteinanzeige).
3. Within milliseconds, the CPU enters the STOP state.
4. The ISTACK, when read with ISTACK in the STEP 5 Test menu, shows the ZYK (cycle time exceeded) flag set.
5. Manual mode restart (MRES / RUN) clears the fault but the cycle repeats on every monitoring session.

The fault occurs even when the user program is "small," which is misleading to first-time S5 service engineers. The threshold for scan-time violation is fixed by the CPU firmware and is unrelated to program size; the program simply needs to be running close to the limit when monitoring is enabled.

2. Affected Siemens SIMATIC S5 Hardware

The default scan-monitoring time on the S5-95U is 100 ms. The CPU polls a hardware timer at the end of every OB1 cycle; if OB1 has not returned within the configured window, the firmware records ZYK in the ISTACK and forces the CPU into STOP. Online status functions inject additional interpreter cycles inside the OB1 scan, raising the effective execution time well above the user's static measurement.

3. Root Cause Analysis

3.1 What the ZYK flag means

ZYK (Zykluszeit) is the bit the S5 firmware sets in the Interrupt Stack when the scan-monitor watchdog expires before OB1 completes. It is not a user-accessible flag; it is visible only through the ISTACK diagnostic on the programming device. The associated error code in the ISTACK stack header is 0x0B / 0x0C family depending on the CPU version, and it indicates Cycle time exceeded.

3.2 Why block status pushes the scan over the limit

Block Status works by having the CPU echo every operand state to the programmer on each cycle. The interpreter walks the active block list at the rate of the scan, adds a status-recording wrapper to each instruction, and ships the resulting trace to the AS511 interface. Empirically, status monitoring adds 15 % to 40 % to the OB1 cycle time, depending on:

• Number of blocks currently being monitored simultaneously.
• Bit-instruction density (each A, O, S, R, =, UN, ON, UW, OW, XOW, etc. is wrapped).
• Program-block nesting depth (PBs calling PBs, FB calls, formal-parameter marshalling).
• Communication load on the AS511 channel (if a PG is also requesting forced-variable writes, the load rises further).

If the unguarded OB1 scan is already at, say, 90 ms, and block status adds 25 %, the effective scan becomes ~112 ms and the 100 ms watchdog fires.

3.3 Why this is the normal S5 behavior, not a defect

The S5-95U does not have any mechanism to suppress scan monitoring during online functions; the watchdog is hard-wired into the firmware. The behavior is by design, and Siemens documents the OB31 workaround explicitly in S5-95U Programming Manual (6ES5 998-8MA22), Section 9.

4. Diagnostic Procedure

Before applying any fix, gather hard numbers. The engineer who is guessing at scan time is the engineer who will apply the wrong workaround.

4.1 Read the ISTACK

1. Connect STEP 5 to the CPU and enter the Test > ISTACK menu (or press F8 in the classic STEP 5 PG software).
2. Confirm the active interrupt cause is ZYK and not QVZ, QVF, PEU, or any other class.
3. Note the Last cycle time and Minimum cycle time the firmware has logged (this is a coarse but useful indicator).

4.2 Measure the actual scan time

Add a temporary diagnostic pair to the top and bottom of OB1 using the system clock word OB 1 scratch flags or, better, the time-of-day clock in DW 0 of a scratch DB:

// Top of OB1
      L   DB 100        // Scratch DB
      L   DW 0          // Last scan time (ms)
      T   FW 200        // Save previous value
      L   KH E000       // System time low word (free-running 10 ms)
      T   DW 1          // Stamp t0

// ... your program ...

// Bottom of OB1
      L   KH E000
      T   DW 2          // Stamp t1
      L   DW 2
      L   DW 1
      -D                // delta in 10 ms units
      SRW 1             // optional /2 for ms approx
      T   DW 0          // publish scan time for monitoring

Read DB100.DW0 in the Status Variable table to capture the live scan time.

4.3 Compute the margin

The S5-95U stores the configured scan-monitor time in system data word SD 5 (default 100 ms). Compare:

If your offline scan is already above 85 ms on a 100 ms monitor, status will always fault without intervention.

5. Solutions

Four field-proven options exist. Pick the one that matches your plant's change-control policy and downtime budget.

5.1 Solution A — Call OB31 mid-program (recommended, no firmware change)

OB31 is the Scan Time Trigger organization block on the S5-95U. Calling OB31 from anywhere inside OB1 re-arms the watchdog, which is exactly what status monitoring needs. Siemens describes this technique in the S5-95U programming manual (6ES5 998-8MA22), pages 9-5 to 9-7.

1. Open OB1 in STEP 5.
2. Place a single unconditional call at the geometric midpoint of the program:

      SPA FB 31        // or "JU OB 31" if FB31 alias is unavailable

If your OB31 block does not yet exist in the program file, copy it from the standard library disk supplied with STEP 5 (S5DOS.S5D → OB31.S5D). It contains nothing more than a BE in its default form, but its presence is what the firmware needs to reset the timer.

3. Download the modified OB1 to the CPU.
4. Restart the CPU in RUN.
5. Re-enable block status. The CPU should remain in RUN with ZYK no longer asserted.

5.2 Solution B — Raise the scan-monitor time in SD 5

The S5-95U allows the scan-monitor time to be parameterized in system data word SD 5 via the STEP 5 System Data editor. The supported range is typically 10 ms to 2550 ms in 10 ms steps.

1. Open the System Data editor (F8 in some builds; otherwise Edit > System Data).
2. Locate SD 5 (Scan Time Monitoring Period).
3. Set the new value, e.g., KH 01F4 (500 ms decimal) for headroom.
4. Transfer system data to the CPU and cold-restart.

5.3 Solution C — Refactor the program to lower OB1 peak time

If the OB1 scan time is genuinely close to 100 ms because the program is doing too much per cycle, the correct long-term fix is to:

• Move slow or rarely-changed logic into OB32 (time-of-day interrupt) or OB21/OB22 (warm/cold restart).
• Split a long PB into two PBs and call the second from a slower task.
• Replace time-wasting constructs (long DO/DI loops, table scans in bit logic) with PB calls gated by a 100 ms timer flag.

This is the only solution that survives a controller swap or a future engineer who is unaware of the workaround.

5.4 Solution D — Use a static variable watch instead of full block status

For most debugging tasks, Status Variable (Variable beobachten) plus a single Force window consumes far less CPU time than full block status. Use this technique when you only need to see a handful of flags or a data word.

6. Step-by-Step Implementation of Solution A (OB31 Re-trigger)

6.1 Prerequisites

• STEP 5 (any version from PG 635 through PG 720/740, including the Windows-based STEP 5 V7.x).
• Current offline backup of the S5 program on a working PG.
• AS511 or MPI-to-RS232 converter cable appropriate to the CPU port.
• Knowledge of the machine's safety state and the operator's permission to cycle the CPU.

6.2 Procedure

1. Switch the controlled machine to its safe-state mode per local lock-out/tag-out.
2. Place the CPU in STOP (key switch or remote STOP).
3. Open the offline project in STEP 5 and load OB1.
4. Identify the line of OB1 closest to the temporal midpoint of the program. Empirically this is the line where the cumulative instruction count is roughly half of OB1's total length, weighted by instruction type (load/transfer is cheap; arithmetic and word logic are expensive).
5. Insert the unconditional OB31 call:

      SPA FB 31

6. If FB 31 is not in the program file, copy it from the STEP 5 library. The default OB31 body is just BE. Some sites prefer to replace the call with the literal JU OB 31 to remove the FB/DB parameterization overhead; both forms work.
7. Document the change in the program header comment of OB1.
8. Transfer OB1 to the CPU with PG > CPU File > Blocks.
9. Cold-restart the CPU and bring the machine back to a controlled RUN state.
10. Open PB status. Confirm RUN LED stays lit and ISTACK shows no ZYK.

6.3 Verification

1. Enable Block Status on at least three PBs and one FB simultaneously.
2. Watch the STOP LED: it must remain dark.
3. Read the ISTACK again. ZYK must not be set.
4. Read DB100.DW0 (your scan-time stamp) and confirm the live scan time stays below the SD 5 value with at least 10 % headroom.
5. Force a worst-case load (cycle every digital input, run any high-speed counters, etc.) and repeat steps 1–4.
6. Sign off the change in the machine logbook with date, firmware version of STEP 5, and SD 5 value.

7. ISTACK Error Code Reference

If you see anything other than ZYK, the symptom is not what this article addresses. Resolve that primary cause first.

8. Troubleshooting Matrix

9. Safety and Lifecycle Considerations

The S5-95U has been formally Discontinued by Siemens for many years, but spare-part service continues in selected regions. Any modification you make to OB1 today must survive the next engineer who services the machine without access to your notes. Apply these rules:

• Always write the rationale as a comment in OB1's header section.
• Keep a paper copy of the original OB1 alongside the modified one in the machine documentation binder.
• Never raise SD 5 above the value the safety analysis (EN ISO 13849-1) tolerates.
• When replacing the CPU with a modern S7-1200 or ET 200S controller (typical migration path), remove the OB31 call before generating the migration project — the workaround has no equivalent and is not required on any current Siemens platform.

10. Related Siemens Documentation

• S5-95U Programming Manual (6ES5 998-8MA22) — Section 9, "Cycle Monitoring" with the OB31 re-trigger technique.
• Siemens Industry Online Support — entry point for all SIMATIC S5 manuals and FAQs.
• STEP 5 V7.23 Help — keyword scan monitoring time.

FAQ