6ES5928-3UA12 SIMATIC S5 928
The Siemens SIMATIC S5 928 (part number 6ES5928-3UA12) is a central processing unit (CPU) module belonging to the SIMATIC S5 programmable logic controller family. This CPU is designed for use in the S5-90U/S5-95U/S5-100U compact PLC series and serves as the core processing element responsible for executing the user program, managing I/O, and coordinating communication within the controller system. It represents Siemens' established S5 generation of industrial controllers, which were widely deployed across manufacturing and process industries.
The 6ES5928-3UA12 CPU features an integrated programming interface for connection to a programming device (PG) via the standard S5 PG port, enabling program upload, download, and online monitoring. It supports STEP 5 programming software and executes programs written in Statement List (STL), Ladder Diagram (LAD), and Function Block Diagram (FBD). The module includes onboard RAM for user program and data storage, with battery-backed memory retention to preserve the program during power loss. Cycle time performance is suited to small-to-medium automation tasks with deterministic scan cycle execution.
The SIMATIC S5 928 CPU is commonly used in legacy manufacturing automation systems, machine tool control, conveyor systems, and process control applications across industries such as automotive, food and beverage, chemical processing, and general factory automation. Its compact form factor makes it suitable for panel-mounted installations where space is limited. Many installations continue to operate this CPU in maintenance and retrofit scenarios, where compatibility with existing S5 hardware and STEP 5 programs is a primary requirement.
Technical Specifications
| Series/Product Family | SIMATIC S5 |
|---|---|
| Compatible PLC Systems | S5-90U, S5-95U, S5-100U |
| Programming Software | STEP 5 |
| Programming Languages | STL, LAD, FBD |
| Programming Interface | S5 PG port (TTY/RS-232) |
| Memory Type | Battery-backed RAM |
| Input/Operating Voltage | 24 V DC |
| Mounting | DIN rail or panel mounting |
| Operating Temperature | 0 °C to +60 °C |
| Storage Temperature | -40 °C to +70 °C |
| Relative Humidity | 15 % to 95 % (non-condensing) |
| Certifications | CE, UL |
Common System Event Codes
- BASP (Bus Error)
- Bus access error detected on the S5 backplane bus, indicating a communication fault between the CPU and connected I/O modules.
Check seating of all modules on the backplane. Inspect the bus connector for damage or contamination. Re-seat the CPU and I/O modules and restart the PLC. - STOP (Red LED)
- CPU has entered STOP mode due to a programming error, hardware fault, or watchdog timeout.
Connect a programming device (PG) running STEP 5, read out the diagnostic buffer, identify the cause of the STOP, correct the fault, and perform a memory reset (MRES) before restarting. - SF (System Fault)
- A system fault has been detected, which may relate to a module failure, configuration mismatch, or internal CPU error.
Check the diagnostic buffer via STEP 5 for detailed fault information. Verify module configuration matches the hardware configuration. Replace faulty modules as indicated. - Battery Fault
- The backup battery for RAM memory retention is low or missing, risking loss of user program and data on power cycle.
Replace the backup battery with the correct Siemens replacement type. Perform a program reload from the programming device after battery replacement if program loss has occurred. - I/O Access Error
- The CPU cannot access one or more configured I/O modules, typically due to a missing or faulty module.
Verify all configured I/O modules are physically present and correctly seated. Check module order numbers against the STEP 5 configuration. Replace any defective I/O modules. - Watchdog Timeout
- The CPU cycle time has exceeded the configured watchdog time limit, causing the CPU to transition to STOP mode.
Review the user program for loops or lengthy processing blocks that may extend cycle time. Increase the watchdog time setting in STEP 5 if appropriate, or optimize the program structure. - Memory Reset Required
- The CPU requires a complete memory reset (MRES) before it can accept a new program download or recover from a corrupted program state.
Perform a memory reset using the mode selector switch on the CPU (turn to MRES position and hold as per the S5 manual procedure), then reload the user program from the programming device.
Frequently Asked Questions
What programming software is required for the 6ES5928-3UA12?
This CPU requires Siemens STEP 5 programming software for program creation, upload, download, and online diagnostics. STEP 5 runs on older Windows or DOS-based programming devices (PG) or compatible PCs.
Is the user program retained during a power loss?
Yes, the 6ES5928-3UA12 uses battery-backed RAM to retain the user program and data during power loss. The backup battery must be in good condition to ensure program retention; a depleted battery will result in program loss on power cycle.
Can the 6ES5928-3UA12 communicate with SIMATIC S7 controllers?
Direct native communication between S5 and S7 systems is not supported without additional interface modules or gateways. Siemens offers S5-compatible communication processors and third-party gateways to bridge S5 and S7 networks where integration is required.
What is the compatible PLC hardware for this CPU?
The 6ES5928-3UA12 is designed for use in the SIMATIC S5-90U, S5-95U, and S5-100U compact PLC systems. It is not interchangeable with S5-115U, S5-135U, or S5-155U rack-based systems.
Is this CPU still supported by Siemens?
The SIMATIC S5 product line has been discontinued by Siemens and is no longer in active production or standard support. Spare parts and repair services are available through specialist suppliers and refurbishers, and migration to the SIMATIC S7 or S7-1500 platform is recommended for new installations.
Troubleshooting
CPU remains in STOP mode and will not transition to RUN after power on
Connect a STEP 5 programming device and read the diagnostic buffer to identify the specific fault causing the STOP condition. Check the mode selector switch position and verify the user program is present in memory.
Correct the identified fault (e.g., missing I/O module, program error, or memory corruption). If the program is corrupted or missing, perform a memory reset (MRES) and reload the user program from a backup. Ensure the mode selector is set to RUN.
User program is lost after power cycle
Check the backup battery voltage and condition. A depleted or missing battery will cause RAM contents to be lost when the 24 V DC supply is removed.
Replace the backup battery with the correct Siemens replacement type. After battery replacement, reload the user program from the programming device. Verify battery replacement intervals are followed according to the S5 maintenance schedule.
Programming device cannot establish communication with the CPU
Check the PG interface cable connection between the programming device and the CPU's PG port. Verify the correct COM port and interface settings are configured in STEP 5. Inspect the PG port connector pins for damage or contamination.
Replace the PG interface cable if damaged. Ensure the STEP 5 interface settings match the physical connection (TTY or RS-232 as applicable). Clean the PG port connector carefully. Try a known-good programming cable or alternative programming device to isolate the fault.
SF (System Fault) LED is illuminated during operation
Use a STEP 5 programming device to access the diagnostic buffer and identify the specific system fault. Common causes include a missing or faulty I/O module, configuration mismatch, or internal CPU hardware fault.
Address the specific fault identified in the diagnostic buffer. Re-seat or replace faulty I/O modules. If the CPU itself is identified as faulty, replace the 6ES5928-3UA12 CPU module with a tested replacement unit.
Outputs do not respond correctly despite CPU being in RUN mode
Verify the output modules are correctly addressed in the STEP 5 program. Check the output module status LEDs to determine if the module is receiving commands from the CPU. Measure output terminal voltages to distinguish between a program logic issue and a hardware fault.
Correct any addressing errors in the STEP 5 program. If the output module is not responding to CPU commands, re-seat the module and check the backplane bus connection. Replace the output module if it is confirmed faulty after substitution testing.
| Condition | Repair, Used, Refurbished, New in box (sealed) |
|---|
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