SIMATIC S5 115U CPU 944 LED Diagnostics and Freeware Test

Problem Definition: SIMATIC S5 115U CPU 944 with QV, BA, and ST LED Anomaly

The SIMATIC S5-115U with CPU 944 is a legacy Siemens programmable logic controller widely found in service on packaging lines, water-treatment skids, and small process cells commissioned in the 1990s. A field-typical diagnostic question is: "When I power up the rack, QV and BA are glowing red, ST is blinking red, and no other LEDs are lit — is the CPU damaged?" Without software diagnostics this question has to be answered by interpreting front-panel LEDs, then by attempting a serial online connection to confirm.

This article consolidates the Siemens S5-115U front-panel LED definitions from the original programming manual, maps each LED state to a probable cause, and walks the engineer through a freeware-based procedure to verify CPU 944 operability over an RS232-TTY programming cable. The procedure requires only a PC with a real or virtual serial port, a Siemens PG-2000 / STEP5 cable (or 6ES5 734-1... adapter with TTY-to-RS232 converter), and either Siemens STEP5, the open-source STEP5/STEP7 PG-2000 clone, or KEPServerEx in demo mode.

CPU 944 Hardware Identification

Before interpreting LEDs, confirm the exact module. The S5-115U family includes CPU 941, CPU 942, CPU 943, CPU 944, and CPU 945. The CPU 944 is the most common "mid-range" 115U processor with the following relevant front-panel markings:

The CPU 944 is single-width and inserts in any central rack slot configured for a CPU. Its front face carries four status LEDs arranged vertically, in this order (top to bottom):

1. QVZ — Quittierverzögerung (acknowledgement timeout)
2. BA — Batterie (battery / RAM backup status)
3. BASP — Befehlsausgabe gesperrt (command output disabled)
4. ST — Stop (run/stop/fault indicator)

Many CPU 944 faceplates also show additional mode LEDs (RUN, STOP) and a green "PWR" indicator driven by the rack power supply. Refer to the S5-115U Programmable Controller System Manual, chapter 5, for the exact LED layout of your hardware revision.

LED Definitions and Normal States on Power-Up

Section 5.1.3 of the S5-115U Programmable Controller system manual enumerates the LED error-signalling matrix. The relevant entries for a CPU 944 with no project loaded and no online connection are:

Power-On Self-Test Sequence

When the rack is energised, every S5 module performs a self-test. The CPU 944 executes a hardware check of its RAM, EPROM, watchdog, and the bus interface. The expected LED sequence is:

1. All front-panel LEDs illuminate briefly (≈ 100-300 ms) — this is a lamp test, not a fault.
2. QVZ and BASP turn on, BA may also turn on if the battery is low or absent.
3. ST turns on steady. The CPU waits for a cold-restart OB (OB20 for manual cold restart, OB21 for automatic) or remains in STOP if no user program is present.

The crucial diagnostic test is: does the ST LED flicker during step 1? If the ST LED begins a high-frequency alternating on/off pattern (commonly interpreted as "flickering" — frequency not specified in the Siemens manual, but observationally 5-10 Hz) and remains in that pattern, the manual states: the CPU module is defective and must be replaced. There is no software recovery path for this hardware fault.

If, after the lamp test, the ST LED transitions to a slower flash (0.5-1 Hz) or steady-on, the CPU is operable but the system is reporting a logic fault — most commonly a missing or corrupt user program.

Interpreting the Reported Symptom: QV + BA Red, ST Blinking Red

Mapping the field report against the LED table:

• QV (red, steady): Expected in STOP because the cyclic OB1 is not being executed. Indicates "no I/O cycle in progress." Not a fault indicator in this state.
• BA (red, steady): Backup battery is missing, discharged, or the battery jumper on the CPU is not fitted. The RAM is volatile. This means the program is not retained across power cycles — but it does not mean the CPU is broken.
• ST (red, blinking): Two interpretations:
  1. If the blink rate is the high-frequency "flicker" (5-10 Hz) seen during self-test, the CPU is defective per Siemens manual section 5.1.3.
  2. If the blink rate is ≈ 1 Hz and the QVZ LED is pulsing in a complementary pattern, the CPU is signalling "STOP requested, no user program loaded". This is recoverable.

Diagnostic Procedure Without Dedicated Software

Step 1: Battery Replacement

1. Power down the rack. Wait 30 seconds for capacitor discharge on the CPU.
2. Open the CPU front cover; locate the 3.6 V lithium backup cell (typically a 1/2 AA or AA form factor).
3. Replace the cell. Observe polarity: positive terminal toward the front of the CPU.
4. Re-apply power. Note whether the BA LED extinguishes. If BA remains lit after 30 s, the on-board charging circuit may be damaged — proceed with serial diagnostic.

Step 2: Overall Reset (Cold-Restart Memory Clear)

If a programmer is available:

1. Connect the PG-2000 cable (or compatible) between the CPU's TTY port and the PC's COM port.
2. Open STEP5 or the STEP5/STEP7 PG-2000 clone.
3. Select PLC > Memory Reset > Overall Reset (German: Gesamtlöschen).
4. After overall reset, the CPU performs a cold restart automatically. With no user program it will return to STOP with QVZ, BA (if battery still absent), and BASP illuminated.

This confirms the CPU's ability to execute its bootstrap. If the CPU enters STOP and stays there with no error code, the processor core is functional.

Step 3: Online Connection Test

The simplest confirmation that the CPU is alive is a successful online connection. Tools and their availability:

For a quick check using KEPServerEx:

1. Install KEPServerEx. Launch the Configuration tool.
2. Add a Channel of type Serial. Set COM port parameters: 9600 baud, 8 data bits, even parity, 1 stop bit.
3. Add a Device with the Siemens S5 Serial driver. Enter the PLC address (default 1).
4. Click Quick Client. Try to read DB1 or Merker (M) bits. If the server returns values without error, the CPU is responsive.

RS232-to-TTY Cable Wiring

The CPU 944 communicates over a 20 mA current-loop TTY interface, not RS-232. A direct serial cable will not work. Use one of:

Standard TTY pinout (15-pin D-sub on the CPU side):

Modern PCs lack a 9-pin RS-232 port. Either a USB-to-RS232 adapter (FTDI FT232R-based devices work reliably with the Siemens TTY converter) or a direct USB-to-TTY cable is acceptable. Verify the cable supports 9600 baud, which the CPU 944 uses as the default.

Test Program for the CPU 944

If the online connection succeeds, write a minimal OB1 to confirm that the CPU can execute user code:

; OB1 - Test program for CPU 944
; Single rung, no inputs, no outputs
;
  BE

Transfer this to the CPU using PLC > Download. Issue a cold restart. Expected behaviour:

• CPU transitions to RUN (green RUN LED on, if fitted).
• QVZ extinguishes because the cyclic OB is now executing.
• BASP extinguishes because outputs are now enabled.
• ST LED extinguishes (CPU is in RUN).
• BA remains lit if the backup battery is still missing or discharged.

If the CPU enters RUN with this single BE instruction in OB1, the processor, RAM, firmware, and I/O bus are all functional. The CPU is operational.

Fault Code Reference for CPU 944

The S5-115U CPU 944 indicates software faults through the combination of ST and QVZ LEDs and the binary fault number visible only to STEP5. The most common codes that an engineer will see on a CPU that has stopped are:

Step-by-Step Verification Procedure

1. Replace the backup battery if BA is lit.
2. Apply power. Observe the ST LED for the first 1-2 seconds.
3. If the ST LED flickers at 5-10 Hz, the CPU is defective — replace it.
4. If the ST LED transitions to a slow flash or steady on, continue to step 5.
5. Connect the PG-2000 cable to the PC. Configure COM port to 9600, 8, E, 1.
6. Attempt an online connection with STEP5, the STEP5/STEP7 PG-2000 clone, or KEPServerEx in demo mode.
7. If the connection succeeds, the CPU is responsive. Read or write a test word and verify the response.
8. Program an empty OB1 with only a BE instruction and download it.
9. Perform a cold restart. The CPU should enter RUN. ST, QVZ, and BASP LEDs should all extinguish.
10. If the CPU enters RUN, the module is operational. The BA LED will remain lit if the battery was not replaced — this is non-fatal but the program will not survive a power cycle.

Troubleshooting Matrix

Recommended Learning Path for the SIMATIC S5 Platform

For an engineer newly assigned to an S5-115U fleet, the following reference materials are essential:

1. S5-115U Programmable Controller System Manual (Siemens, 6ES5 998-0UBx5) — covers hardware, addressing, and LED diagnostics.
2. STEP5 Programming Manual — describes the line-oriented, structured text, and ladder representations used by S5.
3. PG-2000 Programming Device Manual — covers the programming cable and the COM port settings.
4. CPU 944 Reference Manual — describes instruction set, scan times, and memory organisation.

Siemens continues to host legacy documentation on its product support portal. Searching the MLFB (e.g. 6ES5 944-7UB11) on the Siemens Support site returns the full PDF library for that module. For an S5-115U overview and the complete family tree, refer to the Siemens Legacy PLC Knowledge Base.

Safety Considerations Before Working on a Live S5-115U

When working with a 24 V DC powered S5-115U, the rack may remain live even after the CPU is removed if the power supply is separate. Verify zero energy on the backplane before any contact.

Field-Commissioning Notes

• Always replace the backup battery with the rack powered up, to avoid RAM loss and program reload work. The CPU 944 charges the lithium cell only when the rack is energised.
• Document the cold-restart OB selection in your project documentation. A CPU 944 that has OB20 but not OB21 will not auto-restart after power recovery.
• If the S5-115U is part of a process plant with HazOp requirements, treat any CPU replacement as a SIL-relevant event. Test all safety functions before returning the line to production.
• Keep at least one spare CPU 944 and one spare PG-2000 cable in the maintenance locker. Lead time on new-old-stock units is unpredictable.