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Siemens
6ES5928-3UB21
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SKU: 2100000018741
$1,100.00
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2-Year Warranty
Free Diagnostics
5–30 Day Turnaround
Nationwide Service
Repair option: component-level repair is available for this model — free diagnostics, a 2-year warranty, and a 5–30 business day turnaround. Repair typically costs 40–70% of a new unit, saving you 30–60%. Request a repair quote.
6ES5928-3UB21 repair service is available for this discontinued Siemens SIMATIC S5 CPU module — the Siemens 6ES5928-3UB21 repair process begins with free diagnostics, and 6ES5928-3UB21 refurbished units are also available for facilities that need an immediate swap-out while their original is being serviced. The 6ES5928-3UB21 is a CPU 928B central processing unit engineered specifically for the S5-135U and S5-155U programmable controller racks, operating on a 24 VDC supply and featuring a single-width form factor with an interface receptacle for a second communication interface module. Though discontinued by Siemens since October 1, 2006, this CPU remains active in legacy process control systems across petrochemical, automotive assembly, and heavy manufacturing facilities that have not yet migrated to the S7 platform. Engineers performing 6ES5928-3UB21 troubleshooting often find that units fail due to capacitor degradation on the internal power regulation board or corrupted EPROM firmware after battery-backed RAM loss. For facilities asking about 6ES5928-3UB21 repair near me, Flexa Systems ships nationwide from Lewisville, TX and can service units sourced from anywhere in the country. Industrial VFD repair and legacy CPU module restoration require component-level expertise that most OEM distributors no longer support for S5-series hardware, making third-party repair the most cost-effective path. The unit's second interface receptacle was commonly used to connect PROFIBUS-DP or programmable interface modules, making it a critical communication hub within the S5 rack architecture.
Technical Specifications
Part Number
6ES5928-3UB21
Manufacturer
Siemens
Voltage
24 VDC
Package / Type
CPU 928B CPU FOR S5-135U/155U
Compatibility
The 6ES5928-3UB21 is compatible with the Siemens S5-135U and S5-155U central controller racks, where it occupies a single slot in the central rack backplane and communicates with I/O and function modules over the S5 bus. Siemens 6ES5928-3UB21 integration with interface submodules such as the 6ES5318-8MA12 (PROFIBUS-DP interface) and 6ES5524-3UA15 (serial communications processor) allows the CPU to serve as a network master in multi-drop industrial networks. The 6ES5928-3UB21 compatible with memory submodules including the 6ES5374-1FH21 EPROM cartridge and 6ES5375-1LA15 RAM module, which plug directly into the CPU faceplate to extend program and data storage. For facilities evaluating a 6ES5928-3UB21 replacement for aging CPU 928A units such as the 6ES5928-3UA11, the 928B variant offers expanded memory addressing and the additional second interface slot, though both share the same rack and power supply footprint within the S5-135U/155U system.
Service Options
Repair Service
Ship your faulty 6ES5928-3UB21 to our facility. Our certified technicians perform IGBT testing, capacitor inspection, gate driver diagnostics, and full load testing. Typical turnaround: 5–7 business days. 2-year warranty included.
Refurbished
In-stock Siemens 6ES5928-3UB21 variable frequency drive, reconditioned to OEM specifications and verified under load. Ships immediately. 2-year warranty included.
Brand New
New 6ES5928-3UB21 in original Siemens packaging. Full manufacturer warranty applies. Contact us to confirm availability and lead time.
Common Failure Modes
Loss of user program after battery discharge — the 6ES5928-3UB21 relies on a 3.6V lithium backup cell (Varta CR1/2AA or equivalent) to retain RAM contents; when this cell drops below threshold, the CPU enters STOP mode with the SF LED lit and all program memory is cleared on power cycle.
PROFIBUS interface submodule communication failure causing Bus Fault LED activation — 6ES5928-3UB21 troubleshooting in multi-node S5 networks frequently reveals that the second interface receptacle's edge connector pins develop oxidation-related intermittent contact resistance, resulting in cyclic DP master timeout errors and slave station dropouts.
Internal 5VDC logic rail collapse due to failed electrolytic capacitors on the CPU board — a 6ES5928-3UB21 fault of this type presents as a unit that powers on briefly, illuminates the BASP output-disable LED, then shuts down within seconds; measurable ripple on the 5V rail exceeds 200mV under load.
EPROM submodule read errors causing F-CPU halt at startup — units with 6ES5374-1FH21 EPROM cartridges stored in high-humidity environments develop bit-level data retention failures that trigger an immediate CPU STOP condition before the operating system completes its self-test, requiring EPROM reprogramming or submodule replacement.
Frequently Asked Questions
Is the 6ES5928-3UB21 compatible with other S5 CPU modules, and can it replace a CPU 928A?
The 6ES5928-3UB21 is a direct physical replacement for the earlier CPU 928A (6ES5928-3UA11) within the S5-135U and S5-155U racks, sharing the same single-width slot footprint and 24 VDC power requirement. The primary functional difference is that the 928B variant adds the second interface receptacle, so existing programs compiled for the 928A will run on the 928B without modification. For facilities migrating away from S5 entirely, the Siemens S7-400 CPU 414-2 or CPU 416-2 are the recommended forward migration targets, though these require full project conversion in STEP 7 V5.x and are not pin-compatible. The 6ES5928-3UB21 is not interchangeable with S5-115U CPUs such as the 6ES5944-3UA11, as those units use a narrower rack bus architecture.
How do I know if my 6ES5928-3UB21 is worth repairing or should be replaced outright?
A Siemens 6ES5928-3UB21 is generally worth pursuing 6ES5928-3UB21 repair when the failure is isolated to a subcomponent — such as the backup battery circuit, a failed filter capacitor, a corroded interface connector, or a corrupted EPROM submodule — rather than widespread motherboard-level damage from a surge event or physical impact. Units exhibiting burn marks on the PCB, cracked or delaminated board substrate near the backplane connector, or multiple simultaneous power rail failures are candidates for replacement rather than repair, as component procurement and rework labor costs can exceed the value of a tested refurbished unit. The most repairable 6ES5928-3UB21 fault conditions include RAM loss, intermittent communication errors, and DC input undervoltage trips, all of which respond well to board-level component replacement. Sending the unit in for free diagnostics is the fastest way to get a definitive repair-vs-replace recommendation without committing to any cost upfront.
Where can I get 6ES5928-3UB21 repair service with a warranty?
Flexa Systems, based in Lewisville, TX, provides depot-level 6ES5928-3UB21 repair service with a 2-year warranty on all completed repairs and a standard 5–7 business day turnaround from receipt of the unit. Every unit goes through a full functional test under simulated S5 rack conditions before being returned, and free diagnostics are performed on any unit that is ultimately deemed non-repairable at no charge to the customer. Nationwide shipping is supported with prepaid inbound labels available on request, so facilities outside Texas can access the same service without a local Siemens authorized repair center nearby. If you need to buy 6ES5928-3UB21 USA stock rather than repair your existing unit, Flexa Systems also carries 6ES5928-3UB21 for sale as tested refurbished inventory ready for immediate shipment.
Where can I get 6ES5928-3UB21 repair service with a warranty?
Flexa Systems provides professional 6ES5928-3UB21 repair service from our facility in Lewisville, TX, serving customers nationwide. Standard turnaround is 5–7 business days from receipt and all repaired units ship with a 2-year warranty. Rush service (2–3 business days) is also available.
Flexa Systems specializes in Siemens 6ES5928-3UB21 variable frequency drive repair, supplying refurbished and new units backed by a 2-year warranty. All drives are load-tested before shipping. Serving industrial facilities nationwide from Lewisville, TX.
Technical Specifications
Part Number:
6ES5928-3UB21
Manufacturer:
Siemens
Voltage:
24 VDC
Type:
CPU 928B CPU FOR S5-135U/155U
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