Siemens 6ES5948-3UR23 | S5-155H Redundant CPU

Description

Product Introduction

In an S5-155H installation, the CPU is not simply another rack module. It is one half of the redundant control architecture, executing the same user program as the paired controller and participating in event-synchronized operation. The Siemens 6ES5948-3UR23 is the CPU 948R central processing unit specified for the S5-155H platform, with 1600 KB RAM. Siemens documentation describes the S5-155H as a fault-tolerant system using two CPUs in a 1-out-of-2 arrangement.

For maintenance teams keeping an older S5 installation operating, the Siemens 6ES5948-3UR23 is primarily a compatibility-sensitive replacement part rather than a generic PLC processor. The CPU must match the redundant partner, installed memory configuration, system software, interfaces, and existing STEP 5 application. Procurement records identify this exact article as cancelled, so the physical part number, hardware revision, memory specification, and condition should be checked before committing it to an S5-155H repair or legacy PLC replacement project.

Key Technical Specifications

The published technical records identify 1600 KB RAM, 256 timers and 256 counters for this exact article. The maximum I/O figures are system-level capacity figures and should not be interpreted as I/O physically integrated into the CPU.

Application Scenarios & Pain Points

The typical problem is not finding an S5 CPU that physically fits the rack. It is finding one that behaves correctly inside the existing redundant controller pair.

• S5-155H process control systems: Used as the central processor in high-availability S5-155H installations where redundant CPUs maintain controller availability. Siemens documentation describes synchronized master/reserve operation.
  

  6ES5948‑3UR23

• Legacy process automation: Appropriate for plants retaining established S5 architecture because a direct CPU replacement can avoid an immediate full control-system migration.
• Emergency maintenance stock: A spare CPU can reduce restoration time when an installed CPU develops memory, communication, or processor faults.
• S5 migration preparation: The unit can also be relevant during staged migration projects, where the existing S5-155H must remain operational while application logic, I/O assignments, and system interfaces are documented.

Field note: Before removing a failed CPU, capture the complete hardware identification, memory configuration, redundant-pair arrangement, program backup, and communication configuration. With an S5-155H, replacing only the processor while ignoring the reserve/master relationship is asking for commissioning trouble.

Related Products

• Siemens 6ES5948-3UR21 — CPU 948R variant for the S5-155H with a different memory specification; do not treat it as automatically interchangeable with the 3UR23.
• Siemens 6ES5948-3UR22 — Another CPU 948R S5-155H variant; memory and revision differences should be checked before substitution.
• Siemens 6ES5948-3UR13 — CPU 948R variant associated with a 640 KB RAM configuration.
• Siemens 6ES5948-3UA23 — CPU 948 for the S5-155U rather than the redundant S5-155H architecture.
• Siemens 6ES5947-3UR21 — Earlier CPU 947R family member for S5-155H systems.
• Siemens 6ES5300-series interface modules — S5 system interface hardware used according to the particular rack and system architecture.
• Siemens 6ES5530-3LA12 — CP530 communication processor used in legacy S5 installations; application-specific communication compatibility must be checked.
• Siemens 6ES5955-series power supplies — S5 power-supply hardware used with applicable controller configurations.

The important distinction is between the CPU model and the surrounding S5-155H infrastructure. A similar-looking CPU 948R does not automatically constitute a drop-in replacement; the exact article number and system configuration matter.

Engineer’s Guide — Field Pitfalls to Avoid

1. Do not confuse CPU 948 with CPU 948R

The R designation matters. CPU 948R belongs to the redundant S5-155H architecture, while conventional CPU 948 variants are associated with S5-155U systems. Siemens documentation specifically describes CPU 948R/948RL in the S5-155H.

Avoidance: Match the complete Siemens article number against the existing CPU before ordering.

2. Do not substitute based only on memory capacity

Several CPU 948R variants exist with different memory specifications. The fact that another CPU has a similar processor designation does not establish software or hardware interchangeability.

Avoidance: Record the complete part number and hardware identification from the installed CPU. Confirm the memory configuration before installation.

3. Protect the redundant-controller configuration

The S5-155H uses a master/reserve arrangement and event-synchronized execution. Both controllers run the same user program, with system functions handling synchronization between them.

Avoidance: Back up the STEP 5 project and document the redundant pair before replacing either CPU.

4. Check the programming and service environment

This is an S5-generation processor, not an S7 CPU. Modern Siemens engineering assumptions should not be applied to it automatically.

Avoidance: Confirm the available STEP 5 programming/service equipment and the existing project format before planning commissioning work.

5. Treat lifecycle status as a procurement risk

The 6ES5948-3UR23 is listed as a cancelled product, with one catalog record giving a cancellation date of October 1, 2016.

Avoidance: For a replacement purchase, inspect the actual nameplate, revision, memory identification, connectors, and physical condition. Do not rely on a stock photograph or a generic “CPU 948R” description.

FAQ

What is Siemens 6ES5948-3UR23?

The Siemens 6ES5948-3UR23 is a SIMATIC S5 CPU 948R central processing unit for the S5-155H programmable controller. Published product records specify 1600 KB RAM for this exact article.

Is 6ES5948-3UR23 compatible with an S5-155H?

Yes. The part is specifically identified as a CPU 948R for the S5-155H. The S5-155H documentation covers CPU 948R and 948RL hardware and describes its redundant controller architecture.

Is the 6ES5948-3UR23 the same as a standard CPU 948?

No. CPU 948R is the redundant-controller version intended for the S5-155H. Standard CPU 948 variants belong to the S5-155U family. The distinction is important during replacement planning.

Can I replace a 6ES5948-3UR23 with another CPU 948R?

Not automatically. CPU 948R variants exist with different article numbers and memory configurations. The redundant partner, installed software, hardware revision, interfaces, and application program all need to be considered before substitution.

Is Siemens 6ES5948-3UR23 still a current product?

Available Siemens catalog information identifies the article as cancelled, with a PLM cancellation date of October 1, 2016. It should therefore be treated as a legacy component for procurement and lifecycle planning.

What should I verify before purchasing a replacement 6ES5948-3UR23?

At minimum, verify the complete 6ES5948-3UR23 article number, hardware revision, memory specification, physical condition, interface configuration, and compatibility with the existing S5-155H redundant pair. For a used or refurbished CPU, functional testing is also worth requiring because a CPU that powers up is not necessarily a reliable production spare.

Can this CPU be used as part of an S5-to-S7 migration?

It can remain useful during a staged migration, particularly where the existing S5-155H must continue controlling the process while the replacement architecture is engineered. It should not, however, be treated as a modern migration target by itself. The practical approach is to document the STEP 5 application, I/O structure, communications, timing behavior, and redundancy requirements before mapping them into the new platform.