Siemens S5 CPU 928/928B Program Backup with RAM and EPROM Memory Modules

The Siemens SIMATIC S5-135U/155U CPU 928 and CPU 928B controllers remain in service across process plants, packaging lines, and machine tools decades after their original commissioning. The single most common field-service question on these CPUs is how to make a non-volatile program backup that can be reloaded without dragging a PG (Programmiergerät) or laptop to the site. This reference covers the memory architecture of the CPU 928 family, the difference between RAM and EPROM sub-modules, the role of the backup battery, the STEP5 upload/download workflow, the location of the memory module slot, and a verified field-replacement procedure.

1. CPU 928 Family Overview

The CPU 928 was introduced as a mid-range controller for the S5-135U and S5-155U rack systems. Variants found in service include:

The programming and operations manuals for these variants are still available from Siemens Industry Online Support:

• S5-135U/155U CPU 928B Programming Guide (covers 3UB11 / 3UB12)
• CPU 928B-3UB21 Programming Guide
• S5-135U/155U CPU 928/928B/948 List of Operations

2. Memory Architecture of the CPU 928/928B

The CPU 928 family separates work memory (RAM, volatile) from load memory (sub-module slot). The load memory sub-module can be populated with one of three media types:

When the CPU is delivered with only onboard RAM and a lithium backup battery, the battery alone preserves the program during a power loss. The sub-module slot may be present but unpopulated; on earlier units it is covered with a removable blanking plate approximately 1 cm wide by 5 cm high.

2.1 Battery Backup vs. EPROM

The internal lithium battery (typically 3.6 V, 1.7 Ah, type LTC-3PN or equivalent A2 cells in the battery compartment) maintains both the onboard RAM and any RAM sub-module for several years. A failing or removed battery erases the work memory on the next power cycle. EPROM, in contrast, retains data without any power source and is immune to battery failure, which is the principal reason a service technician would convert a CPU 928B to EPROM boot.

3. The Memory Sub-Module Slot

The sub-module slot is located on the front edge of the CPU module, accessible without removing the module from the rack. The slot accepts the standard Siemens memory card footprint. Procedure to access:

1. Place the CPU in STOP using the mode selector.
2. Open the front access cover.
3. Identify the card (label side up; notch orientation toward the front).
4. Release the ejector tab and pull the card straight out.
5. When inserting, ensure the card seats fully; an improperly seated EPROM will cause the CPU to enter STOP with a SUB-MODULE FAULT.

The slot is keyed to prevent reverse insertion. On a 928B-3UB21 the slot is labelled EP/EEP on the silk-screen.

4. Determining Whether the Sub-Module Is Required

Before designing a backup strategy, you must know whether the user program fits entirely in the onboard work memory. The standard procedure is:

1. Go online with STEP5 via the TTY/serial interface (PG cable 6ES5734-2BD20 or USB-to-TTY converter such as a SIEMENS PC-TTY adapter, 6ES5722-.../USB variant).
2. Select File → Compress Memory (German: Speicher verdichten) so the entire work memory is contiguous.
3. Switch to the block list view and verify the program size against the CPU's work-memory capacity (see Section 8).
4. Power down.
5. Remove the RAM sub-module.
6. Power up.
7. If the CPU enters RUN, the program fits in onboard memory and the sub-module can be replaced with EPROM. If the CPU enters STOP with a memory-fault indicator, reinsert the RAM module and reload from PG.

5. STEP5 Backup Procedure

STEP5 is the dedicated Siemens programming environment for the S5 family. It is no longer sold by Siemens but is widely retained in service organizations. The backup dialog (German UI: Sicherung, English UI: Backup) requires the operator to select which block classes to include.

5.1 Connecting a PC to the CPU 928B

1. Set the CPU mode selector to STOP.
2. Connect the PG (or PC) cable to the 25-pin PG port on the CPU front panel. The pinout is 20 mA current loop (TTY), not RS-232. A passive 6ES5734-2BD20 cable works on a real PG; on a modern PC use a 6ES5722-... USB-to-TTY converter or a legacy SIEMENS 6ES5444-3UA11 PC-TTY card.
3. Start STEP5. Set the interface to COM1 (or the relevant USB COM port) and baud to 9600 (default) or 19200 for newer CPU 928B firmware.
4. Issue Online → Backup → B (or A in English UI).
5. Save the file as S5_<project>_<date>.S5D on a PC folder or directly onto a 3.5" disk if using a PG.

6. Backing Up to an EPROM Sub-Module

Once you have a verified STEP5 backup file, the same file can be written to an EPROM using a Siemens PROMMER (e.g. PROMMER 6ES5 868-2MA11 or the later Field PG PROMMER) or a third-party EPROM programmer capable of 27C256/27C512 pin-compatible Siemens cards.

1. Convert the STEP5 .S5D file to an EPROM image using the PROMMER utility. The PROMMER expects the file in raw EPROM format (no header).
2. Place the EPROM sub-module in the PROMMER socket. The orientation is the same as in the CPU.
3. Program and verify. A checksum error at this stage means the EPROM is suspect — discard it.
4. Label the EPROM with project name, date, and CRC.
5. Insert the EPROM into the CPU 928B sub-module slot.
6. Perform an overall reset (cold restart) so the CPU re-stores its operating system and copies the EPROM contents into work memory.

After the EPROM has been accepted, the CPU 928B will boot from EPROM into RUN with no PG, no battery, and no laptop in the loop — the original goal of the field-service request.

7. Cold Restart vs. Warm Restart

The CPU 928 front-panel mode selector has two mechanical positions and a small toggle for restart behavior:

Neither the RUN/STOP switch nor the reset toggle is required to reload a program from PG. The reload is performed with the CPU in STOP via the STEP5 online dialog. The reset/overall-reset toggle is required only when an EPROM has just been inserted and the CPU must copy it into work memory.

8. CPU 928 vs. CPU 928B Work-Memory Capacity

The capacity determines whether an EPROM-only solution is feasible without using the sub-module as data extension.

Reference: CPU 928B Programming Guide (Section 2, Memory Model).

9. Field Service Workflow for a Lost Program

The most common service event is a CPU 928B that has lost its program after a battery failure. The condensed field workflow is:

1. Arrive on site with a verified EPROM (or RAM sub-module) containing the project.
2. Power down the rack.
3. Insert the EPROM sub-module into the CPU's slot.
4. Power up.
5. Move the mode selector to Overall Reset for 3–5 seconds, then release to RUN.
6. Verify the CPU enters RUN, the BASP LED clears, and the outputs follow process conditions.
7. Document the CPU firmware version (visible in the IM 308 or online in STEP5: Module Information → CPU) and the EPROM CRC.

Total elapsed time once on site: typically 5–10 minutes, with no PG required.

10. Troubleshooting Matrix

11. Practical Recommendations

• Maintain at least two verified EPROM copies of every deployed project, stored in ESD bags with project name, date, and CRC on the label.
• Replace the lithium backup battery every 5 years, irrespective of the BAT LED state. Use a battery tested under load; a multimeter open-circuit reading alone is not sufficient.
• When replacing a battery on a CPU 928 without an EPROM, the new battery must be installed before power is removed. A capacitor inside the CPU holds the RAM for typically 30–60 minutes — schedule the swap accordingly.
• Archive the STEP5 source on a modern medium. STEP5 .S5D files can be read by STEP5 on a virtualized Windows XP or DOS PC; convert them to PDF listing and source printouts as a long-term insurance.
• On a CPU 928B-3UB21, the EPROM slot accepts both UV-erasable EPROM and electrically erasable EEPROM cards. Prefer EEPROM for in-the-field updates, as it does not require a UV eraser.

12. Specifications Summary

13. Field Commissioning Checklist

1. Verify the MLFB on the CPU module matches the project documentation.
2. Confirm firmware version (printed label or STEP5 Module Information).
3. Inspect the lithium battery, replace if older than 5 years.
4. Insert EPROM with the project image.
5. Apply power.
6. Perform overall reset.
7. Verify RUN status; check process I/O via the I/Q status display.
8. Back up the new state to STEP5 Backup → A for archive.
9. Sign off with date, technician, project ID, and EPROM CRC.

FAQ