Siemens 6ES5947-3UR21 Replacement Upgrade for SIMATIC S5

Product Overview

Siemens 6ES5947-3UR21 Replacement Upgrade for SIMATIC S5: Smooth Transition & Legacy System Compatibility

The Siemens 6ES5947-3UR21 is the CPU 947 central processing unit designed for the SIMATIC S5-115U, S5-115H, and S5-115F programmable logic controller families. As Siemens has officially discontinued the SIMATIC S5 product line, facilities running aging S5-115U control systems face increasing pressure to source reliable replacement units or plan a structured migration to a supported platform. TOPNLMS maintains verified stock of the 6ES5947-3UR21 to support both immediate spare-part replacement and phased system modernization projects.

Whether your plant is replacing a failed CPU in a running production line or executing a planned retrofit from an end-of-life S5 architecture to a current-generation controller, this guide covers the compatibility, wiring, communications, and installation considerations you need to make the right procurement decision.

Compatibility Comparison Table

Seamless Upgrade Solutions

A successful migration or replacement involving the 6ES5947-3UR21 rarely involves the CPU alone. In most S5-115U installations, the CPU 947 operates alongside a coordinated set of modules that must be evaluated for compatibility or replaced in parallel.

The PS 951 (6ES5951-7LB21) and PS 955 (6ES5955-3LC41) power supply modules are the standard rack power sources for the EU 183U and EU 185U backplanes. These units remain fully compatible with the CPU 947 and typically do not require replacement unless they show signs of capacitor aging or output voltage drift — a common issue in systems that have been in continuous service for more than 15 years.

For I/O expansion, the SM 321 digital input modules (such as the 6ES5321-8MA12 for 32-channel 24 VDC inputs) and SM 322 digital output modules (such as the 6ES5322-8BF11 for relay outputs) connect directly to the S5 parallel bus and are addressed by the CPU 947 without any configuration change. Similarly, SM 331 analog input modules and SM 332 analog output modules retain their existing hardware configuration when the CPU is swapped.

Communication continuity is a critical concern in any S5 retrofit. If your system uses PROFIBUS-DP for distributed I/O, the CP 5431 FMS/DP (6GK1543-1AA01) communication processor plugs into the S5 rack and maintains the existing DP master configuration. For sites using Industrial Ethernet, the CP 1430 TF (6GK1143-0TA01) provides TCP/IP and ISO transport connectivity without requiring changes to the network topology.

Programming and commissioning the replacement CPU 947 requires a STEP 5 programming cable — typically the 6ES5734-1BD20 (TTY to RS-232) or a USB-to-TTY adapter for modern laptops. The existing STEP 5 project file, including all organization blocks (OB1, OB13, OB21), function blocks, and data blocks, can be downloaded directly to the replacement CPU without modification, provided the memory configuration matches.

For facilities planning a longer-term migration from S5 to SIMATIC S7-300 or S7-400, the CPU 947 replacement serves as a stable interim measure. The S7-400 CPU 414-3 PN/DP (6ES7414-3XM07-0AB0) is a common target platform for S5-115U migrations, offering backward-compatible I/O addressing through the S5/S7 converter toolchain and supporting the same PROFIBUS and Industrial Ethernet infrastructure already installed in the control cabinet.

Where signal isolation is required between legacy S5 I/O wiring and new instrumentation, signal isolators and terminal modules such as the Phoenix Contact MINI Analog series or Weidmüller ACT20P isolators are frequently specified alongside the CPU replacement to protect the new module from field-side transients common in older installations.

Retrofit Guidance FAQ

Q: Can I replace the 6ES5947-3UR21 without rewriting my STEP 5 program?
A: Yes. The CPU 947 uses the same STEP 5 instruction set as earlier S5-115U CPUs. Your existing AWL (Statement List), FUP (Function Block Diagram), or KOP (Ladder Diagram) program can be downloaded directly to the replacement unit using STEP 5 v7.x on Windows. No code conversion is required for a like-for-like CPU swap.

Q: Will my existing field wiring need to be changed?
A: No. The 6ES5947-3UR21 connects to the S5 backplane bus — field wiring terminates at the I/O modules (SM 321, SM 322, SM 331, SM 332), not at the CPU itself. A CPU replacement does not require any changes to terminal blocks, cable routing, or field device connections.

Q: Is the CPU 947 compatible with my existing PROFIBUS or Ethernet CP modules?
A: Yes. Communication processor modules such as the CP 5431 FMS/DP and CP 1430 TF are configured independently of the CPU and retain their existing parameter sets after a CPU swap. The DP master configuration stored in the CP module is not affected by replacing the CPU.

Q: My control cabinet has limited space. Does the 6ES5947-3UR21 fit in the same slot as my current CPU?
A: Yes. The CPU 947 is a standard double-width S5 module and occupies the same physical slot in the EU 183U or EU 185U rack as other S5-115U CPU variants. No mechanical modification to the cabinet or rack is required.

Q: What programming tool do I need to commission the replacement CPU?
A: You need STEP 5 v7.x (S5 for Windows) and a compatible programming cable — typically the 6ES5734-1BD20 (TTY/RS-232) or a USB-TTY adapter. The replacement CPU ships without a user program; your existing project file must be downloaded from your programming device before the system is returned to service.

Q: How do I verify the replacement CPU is functioning correctly before returning the system to production?
A: After downloading the program, verify the CPU enters RUN mode without fault LEDs (BASP, STOP, or SF). Check the diagnostic buffer (DB 0 in STEP 5) for any startup errors. Confirm all I/O modules are recognized and that process values on SM 331 analog inputs match expected field readings before releasing the system.

Warranty Assurance

Every 6ES5947-3UR21 unit shipped by TOPNLMS is covered by a 12-month warranty from the date of shipment. Prior to dispatch, each unit undergoes a functional verification test including power-on self-test (POST) confirmation, memory read/write verification, and bus interface check. A test report is available upon request.

In the event of a warranty claim, TOPNLMS provides a replacement unit or full refund within the warranty period. Our after-sales response target is 24 hours for initial assessment and 72 hours for replacement dispatch. We support customers through the commissioning process with technical guidance on STEP 5 program download, CPU configuration, and diagnostic interpretation.

Stock is maintained in Xiamen, China, with express shipping available to most industrial regions globally. Standard lead time for in-stock units is 3–5 business days for international destinations. Expedited shipping options are available for urgent breakdown situations.

For procurement inquiries, volume pricing, or technical pre-sales support, contact our team directly.