6ES5928-3UB12 SIMATIC S5 928

SKU: 6ES5928-3UB12 Category: PLC

Description
Additional information

The Siemens SIMATIC S5 928 (part number 6ES5928-3UB12) is a central processing unit (CPU) module belonging to the classic SIMATIC S5 programmable logic controller family. This CPU is designed for use in the S5-90U/S5-95U compact PLC series and serves as the core processing element responsible for executing the user program, managing I/O communication, and coordinating system functions. It represents Siemens' established S5 generation of industrial controllers, which were widely deployed across manufacturing and process automation environments.

The 6ES5928-3UB12 CPU features an integrated program memory and supports STEP 5 programming language, which includes Statement List (STL), Ladder Diagram (LAD), and Function Block Diagram (FBD) representations. The module communicates via the S5 bus backplane and supports connection to expansion modules for digital and analog I/O. It includes a battery-backed RAM to retain program data and process values during power loss, and provides diagnostic LEDs for run, stop, and fault status indication. The CPU operates on a 24 V DC supply and is housed in a compact, rail-mountable format suited for panel installation.

The SIMATIC S5 928 CPU is used extensively in legacy automation systems across industries such as automotive manufacturing, chemical processing, food and beverage production, and material handling. Its robust design and compatibility with the broad S5 peripheral module ecosystem make it a long-standing choice for maintaining and extending existing S5-based installations. Many facilities continue to operate S5 systems, making refurbished and tested units of this CPU highly sought after for spare parts and system continuity.

Technical Specifications

Series/Product Family	SIMATIC S5
Compatible PLC Series	S5-90U / S5-95U
Programming Language	STEP 5 (STL, LAD, FBD)
Input/Operating Voltage	24 V DC
Memory Type	Battery-backed RAM
Bus System Compatibility	S5 backplane bus
Mounting	DIN rail
Operating Temperature	0 °C to +60 °C
Storage Temperature	-40 °C to +70 °C
Relative Humidity	15 % to 95 % (non-condensing)
Certifications	CE
Status Indicators	RUN, STOP, FAULT LEDs

Common System Event Codes

STOP (Red LED): CPU has entered STOP mode due to a programming error, hardware fault, or manual switch activation.
Check the diagnostic buffer via STEP 5 programming device. Identify the cause of STOP, correct the program or hardware fault, and perform a restart.
SF (System Fault): A system fault has been detected, which may relate to memory errors, module failure, or configuration mismatch.
Connect a STEP 5 PG/PC and read the diagnostic buffer. Replace faulty modules and verify hardware configuration matches the programmed configuration.
Battery Fault: The backup battery is low or missing, risking loss of retentive data and program memory upon power loss.
Replace the backup battery with the correct type as specified in the S5 manual. Reload the user program if memory content has been lost.
I/O Access Error: The CPU cannot access a configured I/O module, indicating a missing, faulty, or incorrectly addressed peripheral module.
Verify that all configured modules are correctly seated in the rack. Check module addressing and replace any faulty I/O modules.
Program Memory Error: An error has been detected in the user program memory, possibly due to data corruption or a failed RAM module.
Reload the user program from a programming device or EPROM memory card. If the error persists, the CPU module may require repair or replacement.
Watchdog Timeout: The CPU watchdog timer has expired, indicating the scan cycle exceeded the maximum permissible cycle time.
Review the user program for loops or time-intensive routines. Increase the watchdog time setting if appropriate, or optimize the program to reduce cycle time.
Configuration Error: The actual hardware configuration does not match the configuration stored in the CPU program.
Use a STEP 5 programming device to compare the configured and actual hardware. Correct the hardware configuration or update the program to match the installed modules.

Frequently Asked Questions

What PLC series is the 6ES5928-3UB12 compatible with?

The 6ES5928-3UB12 CPU module is designed for use with the Siemens SIMATIC S5-90U and S5-95U compact PLC series.

What programming software is required for this CPU?

This CPU is programmed using Siemens STEP 5 software, which runs on a compatible programming device (PG) or a PC with the appropriate interface adapter.

Does the CPU retain its program during a power outage?

Yes, the CPU uses a battery-backed RAM to retain the user program and retentive data during power loss. The backup battery must be in good condition to ensure data retention.

Can this CPU be used as a direct replacement in an existing S5-95U system?

Yes, the 6ES5928-3UB12 is a direct CPU replacement for compatible S5-90U/S5-95U systems, provided the user program is reloaded and the hardware configuration matches.

Is the SIMATIC S5 series still supported by Siemens?

The SIMATIC S5 series has been discontinued by Siemens and is no longer in active production. Spare parts and refurbished units are available through specialist suppliers, and migration to the S7 or TIA Portal platform is recommended for new projects.

Troubleshooting

CPU remains in STOP mode after power-up

Check the mode selector switch position on the CPU front panel. Connect a STEP 5 programming device and read the diagnostic buffer to identify any recorded faults.

Set the mode selector to RUN, clear any faults identified in the diagnostic buffer, and reload the user program if memory has been lost due to a flat battery.

No communication between programming device and CPU

Verify the programming cable connection and check that the correct COM port and transfer rate are configured in the STEP 5 software. Inspect the CPU's programming interface port for physical damage.

Replace the programming cable if faulty, correct the interface settings in STEP 5, and ensure the CPU is in STOP or RUN mode and not in a fault state that prevents communication.

Program memory lost after power cycle

Check the backup battery voltage on the CPU module. A flat or missing battery will cause loss of RAM-stored program data when power is removed.

Replace the backup battery with the correct type specified in the S5-95U hardware manual. Reload the user program from a programming device or EPROM memory card after battery replacement.

I/O modules not responding or showing errors

Inspect the physical seating of all I/O modules in the rack. Verify that module addresses are correctly set using the address switches and match the configuration in the STEP 5 program.

Reseat all modules firmly in the backplane connectors. Correct any address switch settings and update the hardware configuration in the STEP 5 program if necessary. Replace any confirmed faulty modules.

CPU overheating or thermal shutdown

Check the ambient temperature around the control cabinet and verify that ventilation slots on the CPU and rack are not blocked. Inspect any cooling fans fitted to the cabinet.

Ensure the operating environment does not exceed 60 °C. Clear any obstructions from ventilation slots, repair or replace cabinet cooling fans, and consider adding additional ventilation if the thermal load is high.

Condition	Repair, Used, Refurbished, New in box (sealed)