Reading S5-115U Programs: STEP5 Software Upload Guide

Overview: The SIMATIC S5-115U and Its Programming Toolchain

The SIMATIC S5-115U is a modular mid-range PLC from Siemens introduced in the mid-1980s as part of the U-series upgrade to the original S5-110S, S5-130S, and S5-150S lines. Despite sharing the SIMATIC brand with the later S7-300/S7-400 generation, the S5-115U is a completely different hardware and software architecture. It cannot be programmed, read, or commissioned with the STEP 7 (TIA Portal or classic) engineering suite. The only officially supported programming environment for the S5-115U — and the entire S5 family (S5-90U, S5-95U, S5-100U, S5-115U, S5-135U, S5-155U) — is STEP 5.

This article explains why STEP 7 cannot be used with an S5-115U, identifies the STEP 5 software versions and programmer (PG) hardware required, walks through the physical and logical steps to upload (read) a program from the PLC, and documents the firmware, memory module, and CPU-variant constraints that field engineers routinely encounter. Migration paths to S7-300/S7-1500 are also summarized for engineers tasked with modernizing legacy S5-115U cells.

Software Compatibility: Why STEP 7 Cannot Read the S5-115U

STEP 7 and STEP 5 were developed against incompatible instruction sets, file formats, and runtime kernels. The naming convention misled several generations of engineers because both products carry the "STEP" prefix, but the underlying architectures are completely separate.

The S5-115U runs the STEP 5 firmware interpreter. Its program memory stores instructions encoded in the S5 operation set, organized into blocks (OB = Organization Block, PB = Program Block, FB = Function Block, SB = Sequential Block, DB = Data Block). STEP 7 has no decoder for any of these block types and no AS511 / SINEC L1 driver stack to communicate with the S5-115U's serial programming port.

S5-115U CPU Family and Variants

The S5-115U was offered in five standard CPU models and several sub-variants. Each has different memory sizes, instruction execution times, and integrated I/O. The CPU model determines the maximum uploadable program size and the memory module footprint.

Special suffixes identify options and form factors:

• CPU 943-0UA12 / CPU 944-0UA12: Compact form factor for SIMATIC S5-115U compact chassis (no expansion racks).
• CPU 945-...-...B: Enhanced diagnostic buffer and time-of-day clock accuracy.
• -F variants: Fail-safe / safety-related logic, documented in the S5-115F manual set; not interchangeable with the standard -U CPU.
• -C co-processor modules (6ES5 922-...): Plug-in coprocessors for computing-intensive tasks (e.g., closed-loop control, ASCII/3964R protocol conversion).

For a complete matrix of CPU features, scan times, integrated I/O counts, and memory-module slot counts, refer to the official SIMATIC S5-115U Programmable Controller manual (6ES5 998-0UF23) on Siemens Industry Online Support.

Prerequisites: Hardware and Software Required to Read an S5-115U

To read a program from a S5-115U you need four things: a STEP 5 license, a compatible programmer (PG), the correct serial cable, and access to the PLC's programming port.

2.1 STEP 5 Software Versions

Siemens released STEP 5 in several versions. The most common versions still encountered in field service are STEP 5 V6.6 (DOS, English/German) and STEP 5 V7.2 (Windows 3.1/95/98). Both versions use the same AS511 serial protocol to talk to the S5-115U. Versions V7.23 and V7.24 added S5-95U/100U support and minor bug fixes.

2.2 Programmer Hardware

Siemens PGs that natively support AS511 for the S5-115U include:

• PG 730, PG 740, PG 750 (laptop form factor, internal AS511)
• PG 760 (ruggedized)
• PG 685 / PG 750 (legacy)

Any DOS/Windows laptop with a built-in RS-232 (COM1) port and STEP 5 V6.6 / V7.x installed can also read the S5-115U when paired with the correct serial cable (Siemens part number 6ES5 734-1BD20 or 6ES5 734-1BF20, 15-pin sub-D to 25-pin sub-D).

2.3 Required Cable (AS511 Interface)

The S5-115U exposes the AS511 programming interface on a 15-pin sub-D female connector on the CPU front panel. Pin assignments per the S5-115U manual:

Use only a fully shielded cable. AS511 is a current-loop-friendly RS-232 variant with a 20 mA nominal drive; cable lengths beyond 15 m risk signal degradation, particularly in cabinets with VFDs or welding supplies.

Connecting the Programmer to the S5-115U

Follow this sequence to establish a stable AS511 session:

1. Power down the S5-115U chassis. Confirm the PS 951 power supply output LEDs (5 VDC, 24 VDC) are extinguished before plugging the programming cable.
2. Plug the 15-pin connector into the S5 CPU's "PG" or "AS511" socket. On CPUs 941–944 this is on the front faceplate; on the CPU 945 it is on the right-hand side of the front cover.
3. Connect the 25-pin (or 9-pin, depending on cable variant) end to the programmer's COM1.
4. Set the CPU's mode selector to RUN-P (or STOP if you do not need to observe live I/O). In STOP the upload is faster and the PLC will not execute the program during the transfer.
5. Power up the PLC and wait for the CPU's "RUN" / "STOP" / "BASP" LEDs to stabilize.
6. Boot the PG into STEP 5.

Reading (Uploading) the PLC Program with STEP 5

Once STEP 5 is loaded, configure the COM port and start the upload:

1. Start STEP 5 and open an empty project file. Set the PLC interface to "AS511" and the COM port to COM1 (or COM2 if you reassigned the cable).
2. From the STEP 5 main menu, choose File → Read PLC → Program (German: Datei → AG lesen → Programm) or the equivalent in V7.2's Project menu.
3. STEP 5 will perform a handshake with the CPU. If the handshake fails, verify the cable, the mode switch position, and the COM port assignment.
4. Select which blocks to read: organization blocks (OB1, OB21, OB22, OB31, OB34, OB251), program blocks (PB0–PB255), function blocks (FB0–FB255), data blocks (DB0–DB255), and sequential blocks (SB0–SB255). For a full image select all block types.
5. Click Start. STEP 5 will request each block in turn. A 4 KB program on a CPU 943 typically uploads in under 30 seconds on a serial port at 9600 bit/s.
6. Save the result. STEP 5 will offer Save Project As in the working directory (default C:\STEP5\PROJECTS). The resulting file carries the extension .S5D.

2.1 Reading the Program from a Memory Module

If the program resides on an EPROM, EEPROM, or Flash module (e.g., 6ES5 371-0AA11 EPROM 16 KByte, 6ES5 374-1KH21 Flash 256 KByte, 6ES5 372-0AA11 EEPROM 16 KByte), the module must be physically removed and read with a separate memory programmer — for example, the Siemens PG 605 U / PG 610 EPROM writer, a Data I/O 29B/UniSite, or a modern Xeltek SuperPro 610P. The PLC cannot upload a block that lives only in a memory module back to the PG unless the same module is mirrored to internal RAM and the PLC is configured to copy it at restart.

Memory Module and Firmware Constraints

Memory modules for the S5-115U come in two form factors — 32-pin DIP and the later 40-pin ZIP — and three technologies:

The lithium backup battery (typically a 3.6 V, 1.7 Ah primary cell, MLFB 6ES5 974-1AA11 or 6ES7 971-1AA00) must be present and healthy for RAM-resident programs to survive a power loss. A depleted battery is the single most common cause of "lost program" incidents on S5-115Us that have been powered off for weeks or months.

Program File Formats Produced by STEP 5

After upload, the program is stored in STEP 5's native .S5D project container, which holds:

• STL source files (.STL or .AWL): Statement-list listings, human-readable.
• Cross-reference (.XRF): Symbol and operand usage list.
• Assignment list (.ASS): I/Q/M/T/C mapping.
• Symbol table (.SYM): Symbolic names for operands.
• Documentation file (.DOK): Free-form text attached to blocks.

For migration to S7, export the STL source from STEP 5 (File → Generate STL Source) and convert it with the Siemens S5 to S7 Converter (file S7-CONV.EXE) included with STEP 7 classic. The converter handles the syntactic transformation (STEP 5 U → STEP 7 U, etc.) but does not preserve comments or symbols; manual rework is mandatory for production-grade code.

Verification: Confirming a Successful Read

Validate the upload by comparing block-by-block:

1. Open the offline project in STEP 5's Program Display (German: Programm-Anzeige) and check OB1, OB21, OB22 first — these are always present in a working S5-115U and confirm the basic program structure.
2. Verify block checksums. STEP 5 will report an internal block error in the diagnostic buffer (fehlerhafter Bausteinzugriff) if any block failed CRC. Inspect Information → Diagnostic Buffer (German: Diagnose-Puffer) for codes 0x7Fxx.
3. Compare the cross-reference against the expected I/O map. Discrepancies usually point to a partial upload caused by a noisy cable or a CPU that was in RUN (not RUN-P) when the read began.
4. Diff the STL against a previously archived copy if one exists. STEP 5 has no built-in diff tool; an external tool such as fc / n or diff -u works against two text exports.

Troubleshooting Matrix

Migration Considerations: S5-115U to S7

When the read program is destined for a migration to S7, plan the following:

• CPU target: An S7-315-2 DP (MLFB 6ES7 315-2AG10) or S7-315T-2 DP is the typical drop-in replacement for a CPU 943 or 944 in terms of program size and I/O density. For CPU 945 programs (16 KB+), an S7-317-2 DP or S7-319-3 PN/DP is recommended.
• Conversion tool: Use S7-CONV.EXE shipped with STEP 7 V5.5 SP4+. It produces an S7 source file in AWL/STL that you then compile in STEP 7 / TIA Portal.
• Memory module transfer: RAM and EPROM modules are not hardware-compatible with S7 MMC cards. The program must be downloaded to the S7 CPU via Ethernet (PN/PN coupler) or MPI/DP.
• Symbolic names and comments: STEP 5's .SYM symbol table can be imported by the S7 conversion tool, but cross-symbol references and many S5-specific operands (timers T0–T31, counters C0–C31) require manual cleanup.
• Field wiring: S5-115U uses 20-pin front connectors. S7-300 uses 20-pin or 40-pin connectors. Re-termination or pre-wiring adapters (Siemens 6ES7 292-1AB00) are usually required.

For official Siemens conversion documentation and migration guides, refer to the entry point at Siemens Industry Online Support (search term: "S5 to S7 conversion"). The S5-115U manual itself remains the authoritative source for block semantics and instruction set details.

Safety and ESD Precautions

The S5-115U is no longer in production and many spares are obsolete. When handling memory modules and CPUs:

• Power down the rack and wait 60 seconds for capacitor discharge before removing any module.
• Use a grounded ESD wrist strap; CMOS EPROM/EEPROM modules are sensitive to static discharge above 100 V.
• Replace the lithium backup battery only with the PLC under power; otherwise, the RAM-resident program is lost. The recommended battery is 3.6 V, ≥ 1.5 Ah, type AA.
• Never insert or remove the memory submodule while the CPU is in RUN. Always switch to STOP first.

FAQ