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| Place of Origin | GERMANY |
| Brand Name | SIMENS |
| Certification | CE RoHS |
| Model Number | 6ES5948-3UA11 |
The Siemens 6ES5948-3UA11 is the CPU 948 — the central processing unit that drove the SIMATIC S5-155U, Siemens's largest and most capable programmable controller from the 1980s and 1990s.
To understand the CPU 948's significance, you have to understand what the S5-155U was: not a compact PLC for machine control, but a full-scale process automation controller — the ancestor, in capability and application, of what the S7-400 would later become.
The S5-155U with CPU 948 was specified for power plant distributed control, chemical reactor management, large-scale water treatment automation, and continuous process applications where the programme was measured in hundreds of kilobytes and the I/O count in the hundreds of points.
The 6ES5948-3UA11 provides 640KB of internal RAM — a number that requires context. In the early 1990s, when the CPU 948 was designed and deployed, 640KB was a substantial memory allocation for an industrial controller.
The S5-155U's programme memory was measured in STEP 5 instruction words, and 640KB of RAM supported complex process control programmes with extensive PID loop management, alarm processing, and data handling that were ambitious automation tasks for the era.
Today, the CPU 948 is not a product for new installations — it is a maintenance component. The S5-155U systems it was designed for are still operating in legacy process plants, often in applications where the cost and disruption of migration to modern platforms have not yet been justified.
When a CPU 948 fails in such an installation, finding a replacement is the fastest and least disruptive path to restoring production.
| Parameter | Value |
|---|---|
| Internal RAM | 640 KB |
| External Memory | Up to 4,096 KB (Flash EPROM) |
| Programming Interface | 20mA TTY, 15-pin Cannon |
| PG Interface Slot | 1 (pluggable) |
| Memory Submodule Slot | 1 |
| Module Format | Double-width |
| Weight | 0.526 kg |
| Compatible | S5-135U, S5-155U |
| Status | Discontinued spare |
The S5-155U was a modular rack-based controller designed to handle the largest automation tasks of its generation. The system used a common bus rack (CR) that could accommodate the CPU module, multiple I/O modules, and communication processors.
The CPU 948 is a double-width module — it occupies two physical slot positions in the rack.
This double width was driven by the complexity of the CPU's internal electronics: the 640KB RAM, the memory submodule socket, the PG interface socket, the current loop TTY communication circuits, and the processor and bus interface electronics all occupy a PCB assembly that could not fit within the standard single-slot width.
The rack's backplane bus connected the CPU 948 to the I/O modules to its right, carrying both address/data bus signals for I/O access and power.
The 20mA TTY (current loop) interface on the CPU 948's front panel served as the programming and HMI connection — the same physical interface used by Siemens operator panels (OP5, OP15, OP25, OP35) and the STEP 5 programming devices of the era, from handheld programmers to PC-based PG 730 and PG 740 engineering stations.
The CPU 948's memory architecture reflects the engineering realities of late 1980s and early 1990s semiconductor economics. The 640KB internal RAM provided the working memory for the user programme and data — but even 640KB had its limits in large-scale process automation.
The memory submodule slot addressed this limitation: a pluggable Flash EPROM submodule could extend the total addressable programme/data memory to up to 4MB, providing enough space for complex multitasking process control programmes with hundreds of programme blocks, extensive PID control libraries, and large data structures for alarm management.
In practice, the Flash EPROM submodule served as load memory — the non-volatile store from which the programme was loaded into working RAM on startup.
A power-up cycle caused the CPU 948 to copy the programme from the EPROM submodule into working RAM before beginning execution — a process that took several seconds for large programmes, which is why S5-155U systems are remembered for their longer restart times compared to modern controllers.
The 20mA current loop (TTY) interface on the CPU 948 predates RS-485 as the standard for PLC programming and HMI connections. Rather than voltage-based signalling, the current loop uses 20mA flowing current to represent a logical '1' and 0mA for logical '0'.
This current-based encoding provides inherent noise immunity — industrial environments with large motors and long cable runs generate substantial electromagnetic interference, and the current loop's differential current sensing is far less susceptible to this noise than single-ended voltage signalling.
Programming the S5-155U with CPU 948 required a compatible 20mA TTY interface on the programming device.
Connecting a modern laptop requires a USB-to-TTY adapter with STEP 5 software — typically STEP 5 V7.2 or later.
The S5-155U installations that rely on CPU 948 units were built for multi-decade service lives.
A petrochemical plant control system installed in 1992 with S5-155U hardware might remain on the original hardware platform well into the 2020s — the replacement cost is enormous, the production risk is real, and as long as the hardware functions, the economic case for migration is difficult to make.
When a CPU 948 fails, the immediate response is to source a replacement unit from the industrial spare parts market and restore production as quickly as possible.
Q1: What are the differences between CPU 948 variants UA11, UA12, and UA13?
UA11 (this unit) has 640KB RAM, one TTY programming interface, and one memory submodule slot. UA12 adds a second interface socket, allowing simultaneous connection of a programming terminal and an operator panel. UA13 reflects a later firmware revision.
All variants are mechanically compatible with the S5-135U/155U rack and electrically compatible with the S5 backplane; the programme loaded on the memory submodule operates on any variant, subject to firmware version compatibility.
Q2: How do you connect a modern PC to the CPU 948 for diagnostics or programming?
The CPU 948 uses a 15-pin Cannon TTY (20mA current loop) interface. Connecting a modern PC requires a USB-to-TTY adapter compatible with the AS511 protocol, plus Siemens STEP 5 V7.2 or later installed on a Windows XP system (or virtual machine).
No direct connection via USB cable to a modern PC running STEP 7 or TIA Portal is possible.
Q3: How is the programme restored after replacing a failed CPU 948?
If the programme is stored on the Flash EPROM memory submodule, remove the submodule from the failed CPU and insert it into the replacement unit.
The CPU loads the programme from the submodule automatically at power-up — no programming station needed.
If no submodule is present, the programme must be downloaded from a STEP 5 archive via a TTY-connected programming terminal.
This is why storing the programme on an EPROM submodule is strongly recommended for all production installations.
Q4: Can I/O modules be added to the S5-155U rack while the CPU 948 is running?
Limited online modification is supported. The CPU 948 monitors I/O module slots and can recognise newly inserted modules during operation, but assigning addresses and integrating new modules into the programme typically requires a STEP 5 programme modification and download.
For safety-critical plants, any hardware or programme change should follow the site's change management procedure and be executed during a planned maintenance window.
Q5: What migration options exist for an S5-155U/CPU 948 system?
The standard migration target is the SIMATIC S7-400, with the STEP 5-to-STEP 7 conversion tool assisting programme restructuring — though manual validation is always required.
A phased approach using coupling modules allows incremental S5-to-S7 migration while the plant continues running on remaining S5 sections.
Full migration of a large S5-155U process control system is typically a multi-month capital project planned around a plant shutdown.
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