Simatic S5 PLC Memory Module Programming and LED Diagnostics

Overview

The SIMATIC S5 family — including the compact S5-90U and S5-95U programmable controllers and the modular S5-115U — relies on removable memory submodules to retain user programs across power cycles. Unlike modern S7 controllers with integrated MMC cards, the S5 series separates working RAM from non-volatile backup storage. This division produces three operationally critical behaviors that every commissioning engineer and maintenance technician must internalize:

1. All memory modules and I/O modules must be inserted or removed only in the de-energized state.
2. After backup-battery loss, the CPU does not retain the RAM-resident program; it loads automatically from a plugged memory submodule on the next restart.
3. The same status LED is reused for two distinct meanings — a steady blink indicates an error condition (commonly: no valid program on the submodule), while a rapid flicker indicates an active load or copy operation.

This reference consolidates the operating procedures documented in the S5-90U/S5-95U System Manual (chapter 5) and the S5-115U Programmable Controller manual into a single field-ready procedure covering module selection, hot-swap rules, LED interpretation, and EEPROM copy-button workflows.

SIMATIC S5 Family Background

Siemens released the SIMATIC S5 line beginning in 1979 as a successor to the SIMATIC S3 and S4 ranges. The series spans several form factors intended for different control hierarchies:

Across the entire family the LED signaling scheme on the CPU front panel remains consistent, which is why a single diagnostic table applies regardless of the chassis. The compact S5-95U in particular is documented extensively in the S5-90U/S5-95U System Manual, and the operations vocabulary (AN, A, L, KT, SF, etc.) referenced in the S5-115U manual applies equally to all STEP 5 CPUs.

Prerequisites

Before performing any module handling or program-copy operation, confirm the following:

• The S5 programmer (PG) is loaded with a STEP 5 version compatible with the target CPU firmware. PG 605, PG 615, PG 635, PG 685, PG 710, PG 720, PG 730, PG 740, PG 750, PG 760, and modern Field PG installations with STEP 5 ≥ V6.x are all acceptable for offline programming.
• A backup battery of the correct type is installed and verified. The S5-95U uses a lithium backup battery for RAM retention; without it the program in volatile RAM is lost on power-down.
• An EEPROM programming device (for the S5-90U/S5-95U the integrated copy button procedure, or for older CPUs the external PG with an EPROM/EEPROM driver) is available.
• Anti-static precautions are in place — the S5 submodules are MOS devices sensitive to ESD.
• Documentation for the specific CPU is at hand. The S5-90U/S5-95U System Manual (order number 6ES5 998-8MA22) is the canonical reference for the compact line.

Memory Module Types: EPROM vs EEPROM

S5 memory submodules are not interchangeable. Mis-selecting the technology is the single most common cause of "CPU will not load program" faults after commissioning.

Module Insertion and Removal Rules

The single most important mechanical rule for S5 hardware is:

Modules must be inserted or removed only when the CPU is de-energized.

Hot-plugging any module — memory submodule, digital input, digital output, analog module, communications processor, or interface module — can result in:

• Destruction of the submodule's EPROM/EEPROM cell due to supply rail transients.
• Damage to the backplane connector pins.
• CPU entering an undefined fault state requiring a manual restart.
• In the worst case, blown fuses on the CPU power supply section.

Step-by-Step: Safe Module Handling

1. Place the controller in STOP mode via the mode selector on the CPU front panel.
2. Open the cabinet disconnect and verify zero voltage at the 24 V supply terminals with a calibrated meter.
3. Wait at least one minute to allow DC link capacitors to discharge.
4. Don an ESD wrist strap bonded to the cabinet ground bar.
5. Remove or insert the submodule using the ejector/insert tool where provided.
6. Close the cabinet, restore mains power, and observe the CPU LED for the expected boot pattern.

LED Status Indicators and Their Meaning

The S5-95U and S5-115U CPUs use a small cluster of front-panel LEDs. For memory and program-load diagnostics the relevant indicator is the red status LED adjacent to the mode selector. Its two principal modes are:

Backup Power Loss and Automatic Reload

The S5 CPU architecture splits program storage into two physical locations:

• Working RAM on the CPU module, retained by the lithium backup battery. Holds the executable program plus data blocks, timers, counters, flags, and process-image tables.
• Memory submodule in the front-panel socket. Holds a non-volatile copy of the program.

When the backup battery fails or is removed while mains is also absent, the working-RAM contents are erased. On the next power-up the CPU detects that the working RAM is empty and automatically attempts to copy the program from the plugged submodule into working RAM. The behavior sequence is:

1. Mains power applied. CPU executes POST.
2. CPU checks working RAM. If empty AND a submodule is plugged, CPU begins auto-load.
3. Status LED enters the rapid-flicker state ("load in progress").
4. On successful load the CPU performs a cold restart and enters RUN (or STOP, depending on the mode selector position and the OB21/OB22 configuration in the program).
5. If the submodule is absent, empty, corrupted, or unreadable, the LED enters the slow-blink error state.

Workaround: Forcing a Reload from the Submodule

If the working RAM still contains a (possibly corrupted) program but you need to overwrite it from the submodule:

1. Place the mode selector in STOP.
2. Power down the CPU.
3. Wait 30 seconds for capacitor discharge and any RAM self-decay.
4. Remove the backup battery for at least 60 seconds (this guarantees RAM erasure on the next boot).
5. Reinsert the battery, restore mains.
6. Observe the rapid-flicker LED — the CPU is now reloading from the submodule.

Copying a Program to an EEPROM Submodule

The S5-95U (and S5-90U) CPU front panel includes a recessed copy button that initiates an in-CPU transfer from working RAM to the plugged EEPROM submodule. This is the canonical procedure for field backup when no PG is available.

Procedure

1. Verify the plugged submodule is labeled EEPROM, not EPROM. Inspect the device package for the quartz erase window; EEPROMs have an opaque (no window) package, EPROMs have a clear window.
2. Confirm the working RAM contains a valid, fully debugged program: the CPU is in RUN with the process operating as expected.
3. Place the mode selector in STOP. The RAM image becomes the source.
4. Press and hold the recessed copy button on the CPU front panel for at least 3 seconds. Use a non-conductive tool (e.g., a PG stylus or insulated screwdriver) to reach into the recess.
5. Release the button. The status LED transitions to the rapid-flicker state, indicating the write is in progress.
6. Wait until the LED returns to OFF (or to its previous state). Write duration depends on program size and EEPROM technology; allow up to 30 seconds per 8 KB.
7. Verify the copy by powering down the CPU, waiting for capacitor discharge, then powering up and confirming auto-load from the submodule proceeds without an error blink.

Error Conditions During Copy

STEP 5 Operations Reference (S5-115U Excerpt)

The S5-115U programming manual introduces the STEP 5 instruction set with the following illustrative sequence — a timer pulse generator — useful as a verification program after a cold load:

:AN  I 2.5          // AND-NOT input I2.5 (reset)
:A   I 2.6          // AND input I2.6 (start)
:L   KT 5.2         // Load time constant 5.2 s into accumulator
:SF  T 1            // Start timer T1 as off-delay
:A   T 1            // AND timer contact T1
:=   Q 4.0          // Assign to output Q4.0

Operation semantics:

• :AN — AND-NOT (inverse) of a Boolean operand.
• :A — AND of a Boolean operand.
• :L — Load accumulator from constant or operand.
• :KT — Constant of type Time; here 5.2 seconds.
• :SF — Start timer as off-delay (Stored Off-delay).
• := — Assignment to a Boolean operand.

Use this snippet as a known-good test program when verifying that an EEPROM copy operation preserved the program intact: if the timer pulses output Q4.0 with the expected 5.2-second period, both RAM and submodule images are confirmed valid.

Troubleshooting Matrix

Field Commissioning Sequence

For a green-field or replacement S5-95U installation, follow this condensed procedure:

1. Visual inspection: Verify all submodules, battery, and I/O modules are seated fully. Confirm no bent pins on the backplane connectors.
2. Power-on smoke test: Apply 24 V supply with no program loaded. Expect slow-blink LED if no submodule is present.
3. Submodule insert: Power down, insert the production EEPROM submodule carrying the validated program.
4. Auto-load verification: Power up. Confirm rapid-flicker LED for the load duration, then transition to RUN (or STOP if OB21 not configured).
5. Backup creation: With CPU in STOP and the program confirmed in RAM, press the copy button ≥ 3 s to write the program to a fresh EEPROM submodule. Verify by cold-restarting the CPU with the new submodule.
6. I/O loop check: Force each input and read back the corresponding process image; force each output and verify field-side voltage with a meter.
7. Documentation: Record program version, submodule serial number, battery install date, and ISTACK baseline on the cabinet door card.

Verification Checklist

After any module-handling or copy operation, confirm:

• Mode selector returns to the operating position (RUN or RUN-P).
• Status LED behavior matches the table above (no slow-blink error).
• BAT LED is OFF (battery healthy).
• Cycle-time LED (where fitted) shows a normal scan period.
• ISTACK cleared of any error entries (read via PG).
• A known-good test program — for example the timer snippet from the S5-115U manual — executes as expected after a power-cycle.

Notes on Long-Term Storage of S5 Spares

S5 systems remain in service globally decades after their official end-of-production. When warehousing spare submodules, observe:

• Store EPROM/EEPROM submodules in anti-static foam at 15–30 °C and < 60 % relative humidity.
• For EPROMs, opaque tape must remain over the erase window to prevent UV-induced data loss.
• Rotate spares into service every 24 months and re-verify contents against the master copy, as long-term data retention on aged EEPROM cells degrades.
• Keep at least two independent backup copies of each production program: one EEPROM stored at the machine, one EPROM stored off-site. This dual-media strategy insulates the install from a single point of failure on either technology.

Migration Considerations

When an S5 installation is replaced with an S7-1200, S7-1500, or ET 200 substitute, the S5-95U copy-button procedure and submodule handling rules described above do not carry over. Modern S7 CPUs use the SIMATIC Memory Card (MMC) or System Memory Card (SMC) with file-system-based storage, hot-pluggable under defined conditions, and load via a different mechanism (boot loader rather than ISTACK-driven cold restart). Consult the specific S7 manual rather than assuming procedural continuity.