Siemens LOGO! PLC Recurring Weekly Stop: Field Diagnostic Guide

Problem Definition: A LOGO!-Controlled Dough Mixer That Halts on a Specific Weekday

Symptom class — recurrent day-of-week stop. The application is a bakery dough-mixer (German: Rührwerk) controlled by a Siemens LOGO! 8 (6ED1052-xxx08-0BA1/BA2) modular logic relay or an equivalent LOGO! 0BA7 / 0BA6 device. The kneading sequence is executed continuously across the working week, but every Thursday evening through Friday the actuator (drive contactor or VFD enable) de-energises intermittently. Because dough is a viscoelastic, fermenting medium, every missed knead cycle allows CO₂ to expand and entrap air pockets, ruining the product. The financial pressure on the integrator is significant and the symptom appears deterministic — same weekday, same shift, every week — which is the strongest diagnostic fingerprint in this failure class.

The fact that the fault is time-correlated is the single most useful piece of information. Three classes of root cause fit this fingerprint:

1. LOGO! real-time clock (RTC) or weekly timer function blocks configured (or left over from a previous program) for Thursday/Friday windowing.
2. Time-of-Day interrupts (TOD) written to the program or triggered externally via a clock input (the I3 wiring pattern mentioned in the original post).
3. Environmental / supply perturbations that are themselves scheduled — a shared mains bus with a large load (cleaning cycle, oven bank, refrigeration defrost) that fires only on a particular day.

This guide walks the field engineer through the diagnostic flow that isolates the cause in roughly twenty minutes, using only the LOGO! on-board HMI and a multimeter.

LOGO! Hardware Reference and Time-Related Function Blocks

Before diagnosing, anchor the model and firmware. The LOGO! 8 (6ED1052-1MD08-0BA2) is the most common current build and supports 400 function blocks, eight digital inputs on the base unit, four digital outputs, and an integrated RTC buffered by a CR2032 coin cell. Time functions include:

The original integrator used what appears to be a 0BA6 / 0BA7 LOGO! with an external clock wired to I3. This is the most common pattern in older bakeries where a central time signal (often 230 V from a 7-day mechanical time switch) is fed into the LOGO! to synchronise start/stop with the daily production schedule. A misconfigured or dirty mechanical time-switch contact is the number one cause of the symptom in the field.

Diagnostic Step 1 — Capture the LOGO! Type, Firmware and Program

1. Press the ESC key on the LOGO! until the main menu appears, then OK > Diagnostics > Software. Record:

LOGO! type : 0BA8 Standard
FS (firmware) : V1.08.04
Program version: V1.0.5
Memory used  : 217 / 400 blocks

2. Use a USB cable (6ED1057-1AA01-0BA0) and LOGO! Soft Comfort V8.2 or later to upload the .lsc program file. Save a read-protected backup before editing, in case the original is encrypted.
3. Open the program in Diagram mode and Function Block Diagram (FBD) view. List every timer block (yellow blocks with clock-face icon) and write down its parameters — trigger edge, time base, on-delay, off-delay, retentivity flag. Open the block dialog box with right-click → Block Properties.
4. Cross-reference each weekly timer against the production calendar. If a weekly timer has a Thursday cam and a Friday cam, you have located the root cause without leaving the office.

Diagnostic Step 2 — Use the LOGO! Onboard Display to Read I/O and RTC

The cheapest diagnostic tool is the LOGO! itself. The original poster was already pointed to this — it is the fastest path.

1. From the main menu press the down arrow repeatedly until Date / Time is displayed. Confirm:

Mo 12.06.2023
14:37:22

2. Press the right arrow to enter the I/O status screen. The first row shows the digital input states (I1–I8 of the base, I9–I16 / I17–I24 of any expansion modules). The second row shows the digital output states (Q1–Q4 of the base, then DM8/DM16 outputs). All values are 0 or 1.
3. Compare the input state to your wiring diagram. If I3 reads 1 during a stop and the program uses I3 as a permission/enable, the contactor is being told to drop out by an upstream signal — most often the mechanical time-switch.
4. Use a multimeter in AC mode across the I3 terminal and 0V. If the mechanical clock is a 230 V type, you should see the line voltage. If you see a slow drop in voltage or chattering, the contactor of the time switch is the failure point.

Diagnostic Step 3 — Isolate the External Time-Switch Hypothesis

This is the most common cause in the author's experience and matches the original poster's wiring description of a clock input on I3.

1. Identify the upstream device feeding I3. Common makes: Theben TR 610 series, Grasslin Talento 111, Hager EG 103, Finder 12.01, or a Siemens 7LF4 511 mechanical timer.
2. Pull the LOGO! program and search for I3. If I3 goes into a NOT gate or feeds an OR that can force the run coil low, the program is designed to honour the external clock.
3. Open the mechanical timer's dial. Look at the Friday segment. In many legacy 7-day dials, the Friday segment was set for "no production — mixer off" because the bakery originally ran a Mon–Fri schedule. When production expanded to seven days, the dial was never updated. The same is true for cleaning-shift windows on Thursday evenings.
4. Add a bypass jumper across the I3 contact (or simply disconnect the wire at I3 and tie it to 24 V for test). Run the line for 24 hours. If the mixer does not stop, the external clock is conclusively the cause.

Diagnostic Step 4 — Internal Weekly Timer and TOD Interrupt Hunt

If I3 is not the trigger, scan the program for hidden time functions. The most likely candidates in LOGO! Soft Comfort V8 are:

• Block type Weekly timer (clock with seven day ticks)
• Block type Time-of-day interrupt (single clock icon)
• Block type Astronomical timer (sun icon — usually dismissed in indoor industrial plants)

For each block, expand the parameter dialog and check the cam table. The dialog shows one row per day with an on-time and off-time. Disable the block by setting both on-time and off-time to --:--, then upload and run. If the stop ceases, you have found it. Do not delete the block until production management signs off, because it may have been intended to silence the machine for a sanitation cycle that has since moved to a different day.

A common field pattern is:

// I3 → AND1.in1
// AND1.in2 = WeeklyTimer_01.Q
// AND1.out → RunCoil (Q1)
//
// WeeklyTimer_01:
//   Thursday  17:00 ON
//   Friday    06:00 OFF

The integrator was told "run 24/7" but copied a starter program from a 5-day week. The on-time for Thursday is 17:00 — exactly when the bakery switches from morning production to evening cleaning, and the operator assumes the machine is "tired" and reboots the LOGO! by pulling the supply. The internal RTC never loses the time, so the block fires again next Thursday.

Diagnostic Step 5 — Counter / Timer Saturation in Long-Running Programs

Check the diagnostics screen OK > Diagnostics > Output > OpHours for each output. If a value is at 9999 and you can still command the output manually, the counter has not wrapped — it is only a display limit. The real wrap occurs in the LOGO! Retentive Counter block (Counter / Up-Down Counter), which uses signed 32-bit. Wrap to 0 is mathematically possible after 2³¹ increments — over 2 billion events — and is not realistic in a kneading application.

The realistic saturation case is a non-retentive timer whose time base overflows. The longest single-shot the LOGO! supports is 99 h 59 m on a 0BA6, but with cascaded On-Delay blocks (one triggers the next) integrators have built timers that exceed 31 days. Power-glitch immunity on the LOGO! is good, but on a bakery where the supply is shared with high-inductive loads (ovens, walk-in coolers), a brown-out of 80 ms is enough to reset the cascaded timer and shift its phase by one cycle. Over a 7-day period, the cumulative phase shift can place a periodic block in the production window. The fix is to make all long-duration timers retentive (tick the retentivity box in the block properties) so they survive a power dip.

Diagnostic Step 6 — Mains, Noise and the Thursday-Oven Hypothesis

Several respondents suggested external electrical interference. The "is there something special on that day" hint is well-founded: most industrial bakeries run a deep-cleaning cycle on Thursday evening, energising a bank of high-power three-phase ovens, walk-in refrigeration defrost heaters, and a high-current water heater. The inrush of a refrigeration compressor or a 60 kW oven bank can:

1. Cause a voltage dip below the LOGO! hold-up range (LOGO! 24 CE tolerates 20.4–28.8 V; transient immunity is good but not infinite).
2. Inject common-mode noise on the I3 cable if it is run in the same tray as the oven feeders.
3. Cause a 24 VDC supply on a Siemens LOGO! Power 24 V (6EP1331-1SH03) to drop out if the power supply is undersized.

To confirm, fit a plug-in data logger (Hioki PW3198, Fluke 1730 or even a cheap Voltcraft Energy Logger 4000) on the LOGO! 24 V supply rail for one full week. The trigger threshold is any drop below 21 V lasting > 20 ms, or any peak above 32 V. A second clamp meter around the L1 conductor of the oven MCC will correlate inrush with the LOGO! reset. If the LOGO! resets, the output is held low during the brown-out, the dough-mixer stops, the operator intervenes — and the integrator blames the LOGO!. The cure is a 24 V buffer module such as the 6EP1931-2DC21 SITOP DC-UPS or simply a 24 V supply with at least 50 % headroom (a 10 A supply for a 4 A load).

Diagnostic Step 7 — Verifying Output Wiring and Contactor Drop

A surprisingly common root cause is the contactor, not the LOGO!. A 230 V AC coil on the motor contactor that has been running for > 100 000 operations will chatter and intermittently fail to pull in. Bake-house humidity accelerates this. The visual symptom is identical: the mixer stops and the LOGO! shows the coil is on. The author has seen this mis-diagnosed three times in twelve years.

Test pattern:

1. Force the output Q1 to ON from LOGO! Soft Comfort online monitor or via the ESC > OK > Start > Output menu on the display.
2. Measure the voltage at the contactor coil (A1–A2). If > 195 V AC, the contactor should be energised. If the contactor is silent, the coil is open-circuit or the auxiliary contact is sticking. Replace the contactor (typical: Siemens SIRIUS 3RT2015 series, or equivalent).
3. If a 24 VDC contactor is used, the LOGO! output is rated 0.3 A resistive only — a low-power contactor pull-in of 0.5 A will pull the output transistor into its current-limit fold-back and the output will oscillate at roughly 1 Hz. This oscillation is fast enough to defeat the mechanical contactor but slow enough to release the motor starter. The cure is a 24 V interposing relay (Phoenix PLC-OSC or Siemens 3RQ1).

Diagnostic Step 8 — Time-Set the LOGO! Internal RTC to Verify the Time-Correlation Hypothesis

One of the most elegant diagnostic moves in the source thread is the suggestion to set the internal date to the wrong date. The reasoning is: if the time-correlation persists, the cause is not the LOGO!'s clock.

1. From the LOGO! front panel, navigate ESC > OK > Setup > Date/Time.
2. Set the day-of-week to Monday when the actual day is Thursday, and set the time forward by 12 hours.
3. Run the line for 24 hours.
4. If the stop occurs on the original calendar day at the original wall-clock time, the LOGO! is innocent — go look upstream at the supply, the contactor, the I3 wiring.
5. If the stop now occurs on the "new Thursday" 12 hours from now, the LOGO!'s internal clock is the culprit and you have located the weekly-timer block.

Diagnostic Step 9 — Field-Proven Verification Checklist

Repair Procedure Once the Root Cause Is Identified

1. Document the program in a Word or PDF file with screenshots of every block, the parameter dialog, and the program header (version, author, date).
2. If the weekly timer is unwanted, set its on/off times to --:-- instead of deleting it — easier to roll back if the bakery changes shift pattern.
3. If the external time-switch is the cause, replace it with a digital weekly timer (e.g. Siemens 7LF5 301-0) and label the I3 cable at both ends.
4. Add a 24 V SITOP buffer (6EP1931-2DC21, 2.5 kWs) to ride through oven-bank inrush.
5. Replace the contactor with a SIRIUS 3RT2015-1BB41 or 3RT2026 if the old one has > 1 000 000 operations. Bake-house humidity halves contact life.
6. Add a system flag in the LOGO! program that latches when the run coil drops. This is a one-line RS flip-flop wired to the run coil and reset by the operator. The flag can be viewed from the LOGO! display, which lets the next shift know if a stop was PLC-induced.

Long-Term Hardening for 24/7 Bakery Duty

Bakeries are particularly hostile to PLCs: high humidity, flour dust, frequent wash-downs, and high ambient temperature at the dough-mixer location. Recommendations for the next retrofit:

• Specify the LOGO! 8 pure variant (6ED1052-1CC08-0BA2) only if the application is digital-only. The LOGO! 8.3 (6ED1052-xxx08-0BA3) introduces MQTT and HTTPS and is the correct choice for new builds.
• Mount the LOGO! in an Spelsberg Abox-i or Rittal AE IP65 enclosure with a Gore PMF100 breather drain.
• Provide a 24 VDC UPS as above.
• Replace any I3 wired to a mechanical clock with the LOGO! internal RTC and a digital calendar block. The internal RTC is accurate to ±2 s/day, drift is negligible, and there is no mechanical wear.
• Use a second LOGO! in parallel as a watchdog: a simple program that watches the run coil and raises a fault flag if it drops for more than 30 s without operator intervention. This turns a mystery stop into an obvious fault the next morning.

Troubleshooting Matrix — Symptoms vs. Root Cause

FAQ