SIEMENS S5 395 6ES5376-0AA11 CPU Module

The SIEMENS S5 395 6ES5376-0AA11 is a central processing unit (CPU) module for the Simatic S5-395U programmable logic controller platform. This CPU provides the processing power and memory necessary for complex industrial automation tasks, and is part of the widely used S5 series from Siemens. The S5-395U was the high-performance model within the S5 family, offering faster processing and larger memory compared to other S5 CPUs.

Product Overview

The Simatic S5 series was a landmark product line from Siemens that established the company as a leader in industrial automation. The S5-395U CPU, model 6ES5376-0AA11, represents the top-end processor for the S5-395U system, which itself was a modular PLC designed for medium to large-scale automation projects. This CPU is built around a 16-bit processor architecture and executes the STEP 5 programming language (ladder diagram, function block diagram, and statement list). The CPU includes built-in memory for program and data storage, as well as interfaces for programming devices and communication with other S5 modules. The CPU is housed in a double-width module that plugs into the S5-395U rack backplane. The module includes LED indicators for power, run, stop, and fault status. The S5 series is known for its ruggedness and long service life, with many systems still in operation decades after installation.

Technical Specifications

The 6ES5376-0AA11 CPU operates on a 5V DC power supply provided by the S5 backplane. The processor is a 16-bit microcontroller with a typical execution time of 1.6 microseconds for basic operations (bit logic). The user program memory capacity is up to 32 KB of RAM, with additional 32 KB of EPROM or EEPROM for program backup. The data memory includes 1024 bytes of flags (markers), 256 timers, 256 counters, and 2048 bytes of data blocks. The CPU supports up to 1024 digital I/O points (expandable via I/O modules) and up to 128 analog I/O channels. The CPU includes a built-in programming port (TTY or RS-232 depending on variant) for connection to a PG (programming device) running STEP 5 software. An optional second serial port is available for communication with other devices using the AS511 or SINEC protocols. The operating temperature range is 0°C to 60°C. The module dimensions are 233 mm wide, 230 mm high, and 44 mm deep, with a weight of approximately 1.2 kg.

Memory Organization

The S5 395 CPU uses a structured memory model that separates program code from data. The user program is stored in RAM or EPROM. RAM is volatile, requiring a battery backup to retain the program during power loss. The battery backup life is typically 1-2 years. EPROM memory retains the program without battery, but requires a programmer to write. The CPU can execute programs directly from EPROM. The memory is organized into blocks: organization blocks (OB) for cyclic, time-of-day, and interrupt-driven execution; program blocks (PB) for main program code; sequence blocks (SB) for step-by-step sequencing; function blocks (FB) for reusable code with instance data; and data blocks (DB) for structured data storage. The maximum number of blocks is limited by available memory. The S5 395 CPU supports up to 1024 function blocks and 1024 data blocks.

Programming and Software

The 6ES5376-0AA11 is programmed using the STEP 5 programming language, which is available in three representations: ladder diagram (LAD) for electricians, function block diagram (FBD) for engineers, and statement list (STL) for programmers. Programming is performed on a Siemens PG (Programming Device) such as the PG 675 or PG 710, or on a PC running S5 emulation software. The STEP 5 software provides editing, compiling, downloading, and online monitoring capabilities. The CPU supports online testing including single stepping, breakpoints, and forcing of I/O points. The program can be documented using the STEP 5 documentation system. Many third-party software packages also support S5 programming via the AS511 protocol. The CPU is also compatible with the S5-Graph language for sequential control using Grafcet/SFC charts (with additional firmware).

I/O and Expansion

The S5-395U system uses a modular backplane that accepts digital I/O modules (e.g., 6ES5 421, 6ES5 431), analog I/O modules (6ES5 460, 6ES5 466), and function modules (counter, positioning, closed-loop control). The CPU communicates with these modules via the backplane bus. The maximum number of I/O modules depends on the rack configuration; the S5-395U supports up to 4 racks (central and up to 3 expansion racks) via interface modules (IM). The total I/O address space is 1024 bytes for digital inputs and 1024 bytes for digital outputs, and 128 channels each for analog inputs and outputs. The CPU also supports remote I/O via the SINEC L1 or SINEC H1 networks using communication processors (CPs).

Communication Capabilities

The S5 395 CPU includes a built-in programming interface that supports the AS511 protocol at 9600 baud. This interface can also be used for communication with operator panels (OPs) and simple HMIs. For more advanced networking, the CPU can be equipped with communication processors (CP) such as the CP 525 for SINEC L1 (serial bus) or CP 535 for SINEC H1 (Ethernet). These CPs plug into the same backplane and handle the communication protocol, offloading the CPU. The CPU supports peer-to-peer communication with other S5 CPUs using the SINEC L2 bus (Profibus FMS). The CPU's data blocks can be accessed by network devices for data exchange. The communication configuration is done using the COM 525 or COM 535 software packages.

Battery and Clock Functions

The 6ES5376-0AA11 requires a lithium battery to retain the user program and data memory during power loss. The battery is typically a 3.6V lithium cell (Siemens part number 6ES5 980-0AA11 or equivalent). A low battery alarm is indicated by an LED on the front panel and can be read by the program. The CPU includes a real-time clock (RTC) that is also battery-backed. The RTC provides time-of-day and date information for time-stamping events and executing time-based control (e.g., scheduled starts). The clock accuracy is approximately ±2 seconds per day. The RTC can be set and read from the program using system function calls. The battery life under normal conditions is approximately 1-2 years. When the battery is low, the program and data should be backed up to EPROM or a PC before replacing the battery.

Diagnostic and Fault Handling

The S5 395 CPU includes comprehensive diagnostic capabilities to assist in troubleshooting. The front panel LEDs indicate the operating mode (RUN/STOP), power status, battery condition, and internal faults. The CPU generates diagnostic messages that can be read by the programming device. The CPU can be configured to automatically stop or continue operation when specific faults occur (e.g., I/O access errors, program execution errors). The user program can include error handling OBs (e.g., OB 32 for I/O access error) that execute when a fault is detected, allowing the program to take corrective action or log the error. The CPU also supports system diagnostics using the S5-DIAG function. A built-in watchdog timer monitors the program scan time; if the scan exceeds the set maximum (typically 100-200 ms), the CPU will stop or trigger a fault.

Applications

The S5 395 CPU was widely used in industrial applications requiring high performance and reliability. Typical applications include automotive assembly lines where multiple robots and conveyors are coordinated; material handling systems such as automated warehouses and sortation systems; process control in chemical and pharmaceutical plants (with analog I/O and closed-loop control); power generation and distribution control; water and wastewater treatment facilities; and machine tools such as CNC machining centers. Many of these systems are still in operation today due to the longevity of S5 hardware. The CPU's processing power and memory were sufficient for complex interlocking, sequencing, and data handling tasks typical of the 1980s-1990s industrial automation era.

Maintenance and Legacy Support

While the S5 series has been discontinued and replaced by the S7 series (S7-300, S7-400, S7-1200, S7-1500), many S5 systems remain in service. Maintenance of the 6ES5376-0AA11 includes periodic battery replacement, cleaning of modules and racks, and verification of proper operation. Spare parts are available through specialized suppliers and the used/refurbished market. Siemens also offers migration solutions, including the S5 to S7 conversion software and adapter modules that allow S5 I/O to be used with S7 CPUs during a phased migration. For continued support, third-party service companies offer repair and testing of S5 modules. The CPU's firmware is not field-upgradable; the module must be replaced with a unit containing newer firmware if required. The S5 series is well-documented, with manuals available from Siemens and online archives.

Replacement and Migration Options

For users seeking to replace the 6ES5376-0AA11 with modern automation hardware, Siemens recommends migrating to the S7-1500 series for new installations. For existing S5 systems, migration can be done by replacing the CPU with an S7-300 or S7-400 CPU and using an adapter to retain S5 I/O, or by full system replacement. The STEP 5 program can be converted to STEP 7 (TIA Portal) using Siemens' conversion tools, though manual adjustments are often required due to differences in memory organization and instruction sets. Alternatively, several third-party companies offer replacement CPUs that emulate the S5 395 function using modern hardware, allowing the existing program to run without modification. These replacement units are plug-compatible with the S5 rack and I/O.