1. System Overview: S5-150U and S5-150S in the SIMATIC S5 Family
The SIMATIC S5-150U and the closely related S5-150S are high-end members of the first-generation SIMATIC S5 programmable controller family. They were positioned above the S5-130U and below the rack-mounted S5-155U, targeting mid-to-large automation cells that required more program memory, faster instruction execution, and a richer instruction set than the lower-tier S5-110/130/150A/150K models could deliver.
The S5-150U is a compact, modular PLC with a backplane-based rack system. It uses the same I/O bus philosophy as the S5-130U but with a wider instruction set and larger memory capacity. The S5-150S is a special variant, most commonly associated with the SINUMERIK 850 CNC where the same CPU module is used in the machine control cabinet. From a programming, communication, and I/O standpoint the two variants are functionally equivalent; the difference lies in the cabinet system, power supply variant, and the SINUMERIK 850 firmware overlays.
Both systems were phased out of production in the early-to-mid 1990s. The S5-150U was officially discontinued in 1996. Siemens no longer publishes new manuals for these CPUs in the public Siemens Online Support portal, but a large archive of paper manuals, handbooks, and Service Bulletins still exists, and some are mirrored by authorized Siemens distributors and educational institutions.
2. CPU, Memory, and Cycle Architecture
The S5-150U was offered with several CPU variants. The most commonly documented are:
| CPU Module | Program Memory (Statements) | Data Memory | Bit / Word / Timers / Counters | Typical Bit Instruction Time |
|---|---|---|---|---|
| CPU 150U (early) | approx. 24K statements (max via submodule) | depending on submodule | 2048 flags, 256 timers, 256 counters | ~1.6 µs |
| CPU 150U (expanded) | up to 64K statements with memory submodules | depending on submodule | 2048 flags, 256 timers, 256 counters | ~1.6 µs |
| CPU 150S | identical to 150U, SINUMERIK overlays | identical | identical | identical |
The instruction set of the S5-150U is essentially the same as the S5-130U but adds the full set of word operations (addition, subtraction, comparison, logical AND/OR) and supports the complete STEP 5 statement list plus ladder (KOP), function block (FB), and structured text-style sequence block (SB) notations.
3. The IM511 Programming Unit Interface Card
The IM511 is the dedicated programming-unit interface module used in the S5-150S and S5-150U racks. It occupies one slot in the central rack and exposes a 25-pin Sub-D female connector to the outside world. The IM511 implements the physical layer of the Siemens AS511 PG protocol.
Key characteristics of the IM511:
- Slot location: any free I/O slot in the central rack (CC 0U/0S or EU)
- Connector: 25-pin female Sub-D on the front panel
- Electrical layer: 20 mA current loop (TTY) or, on later variants, RS-232 via adapter
- Protocol: Siemens AS511 (active, the IM511 drives the line; the PG is passive)
- LED indicators: PGN, PGS, PE (depending on firmware revision)
The 20 mA current loop is optically isolated inside the IM511 in most revisions, which makes the interface robust against ground potential differences between the PG (laptop) and the rack.
4. The AS511 Protocol
AS511 is Siemens' proprietary point-to-point protocol used between a PG (Programming device) and an S5 CPU through the IM511 / IM308 / CP521 / CP524 family of interfaces. It is a half-duplex, character-oriented protocol running on a current loop.
| Parameter | Value |
|---|---|
| Baud rate | 9600 bit/s (default for IM511) |
| Character frame | 1 start, 8 data, 1 parity (even), 1 stop |
| Parity | Even |
| Handshake | TTY hardware loop, no RTS/CTS |
| Frame type | Fixed-length 26-byte blocks (8-bit control + 16 data + 2 CRC) |
| Maximum PG to CPU distance | ~1000 m on 20 mA current loop (subject to cable spec) |
The IM511 is the active partner. The PG must be wired as a passive device, with its transmit and receive current loops returned externally. This is the reason the cable pinout in Section 5 is non-obvious: the cable must short and cross a number of pins to terminate the PG side of the TTY loop correctly.
5. IM511 (AS511) PG Cable Pinout
The verified cable pinout between a 25-pin male PG connector and the 25-pin female IM511 connector on the S5-150S/150U is:
| Signal Function | PG 25-pin Male | IM511 25-pin Female | Notes |
|---|---|---|---|
| TTY Tx+ / Rx+ (data A) | 10 | 20 | Cross-over |
| TTY Tx- / Rx- (data B) | 22 | 21 | Cross-over |
| TTY return path | 21 | 18 | Cross-over |
| TTY return path | 8 | 19 | Cross-over |
| Common / shield / 0 V | 24, 25 | 24, 25, 1 | Bond shield and 0 V together |
| PG side loop-back | 6, 20 (short) | — | Required at PG connector |
| PG side loop-back | 12, 23 (short) | — | Required at PG connector |
| PG side loop-back | 2, 3, 4, 17 (short) | — | Required at PG connector |
5.1 Building the Cable
- Use a shielded, twisted-pair cable. Two pairs (signal/return) plus shield is the minimum.
- Solder a 25-pin male Sub-D for the PG side and a 25-pin female Sub-D for the IM511 side. Metal hoods must bond the shield 360°.
- Wire the four cross-over connections (10↔20, 22↔21, 21↔18, 8↔19) point-to-point.
- On the PG connector, install the three shorting links between pins 6-20, 12-23, and 2-3-4-17.
- Bond pins 24, 25, and 1 together on the IM511 side. Connect the cable shield only at the IM511 end to chassis ground to avoid ground loops.
- Maximum recommended length: 10 m for laptop-based PGs; up to 1000 m if true current-loop boosters are used.
6. Connecting a Modern Laptop (PG Replacement)
Almost no modern laptop provides a 20 mA current-loop port. Two practical conversion paths exist:
6.1 USB to TTY Active Adapter
Industrial-grade USB-to-TTY adapters with optically isolated 25-pin output (e.g., variants from SIEMENS PG-Adapter family equivalents, third-party brands: ICP DAS I-7561, B&B Electronics USPTL4, Westermo MA-45) can be used. Configure for 9600 bps, 8E1, current loop active, and respect the loop-back jumpers shown above.
6.2 Use a CP521 SI / CP524 as a Protocol Bridge
Insert a CP521 SI module in the S5 rack, connect it to a modern PC via RS-232, and run the original Siemens STEP 5 software. The CP521 SI talks to the CPU over the backplane and exposes a serial AS511-like stream to the PC.
7. STEP 5 Programming Toolchain
| Tool | Use Case | Notes |
|---|---|---|
| STEP 5 (DOS), V6.x | Original PG environment for S5-150U/150S | Runs under MS-DOS or Win9x |
| STEP 5 V7.23 / V7.32 | Latest Windows version, supports S5-150U/S | Requires 16-bit Windows layer or VM |
| PG 685 / PG 750 / PG 720 | Legacy Siemens PGs with native TTY port | Preferred where available |
| S5Tools / S5-Download | Open-source program/DB upload via AS511 | Useful for archival on modern PCs |
Program organization on the S5-150U follows the standard S5 model: OB1 (cycle), OB21/22 (restart), OB31/34 (time-of-day), PB (program blocks), FB (function blocks), DB (data blocks), SB (sequence blocks), and FX (extension blocks for the 928B CPU).
8. I/O System and Signal Modules
The S5-150U accepts the same ET 100U, ET 200U, and 6ES5 digital/analog modules as the S5-130U, with adapter shells (6ES5 491-0... series). Typical signal modules include:
- Digital input 24 V DC: 6ES5 430-4... (16/32 channels)
- Digital output 24 V DC, 0.5 A: 6ES5 454-4...
- Relay output: 6ES5 458-4...
- Analog input ±10 V / 4–20 mA: 6ES5 460-4...
- Analog output ±10 V / 4–20 mA: 6ES5 470-4...
- Counter / position decoder: 6ES5 461-..., 6ES5 463-..., IP 263 / IP 264 / IP 265 / IP 266 / IP 267
The 6ES5 catalog numbering changed several times; always cross-check the order number against the configuration diagram in the S5-150U manual before ordering replacements.
9. Where to Find the Original Documentation
Because the S5-150U has been out of production for nearly three decades, the documentation landscape is fragmented:
- Siemens Online Support (SIOS) — Most S5 product entries were archived, not deleted. Search by order number (e.g., 6ES5 150-3...) in the legacy section. Some manuals remain in PDF form under "SIMATIC S5 / S5-150U".
- Siemens Industry Online Support – Legacy Archive — Several distributors and Siemens regional offices still host authorized PDF copies.
- SINUMERIK 850 Handbook (S5-150 U/S PLC) — The SINUMERIK 850 service manual is one of the few documents that still describes the S5-150S hardware in detail, and most of the I/O, IM511, and CPU details apply 1:1 to the S5-150U.
- Siemens Regional Service — Authorized Siemens service partners can order paper copies of historical manuals for a fee. Lead time is typically 2–4 weeks.
- University and library archives — Many engineering schools still keep bound copies of the S5-150U "Gerätehandbuch" (device handbook) and the S5-150U "Programmierhandbuch" (programming handbook).
10. Modern Replacement and Migration Paths
For new installations, no direct 1:1 replacement exists. The most common migration path is to a SIMATIC S7-300 or ET 200S with a STEP 7 conversion utility. Forcing the migration to S7-1500 is possible but requires significant rewiring because the S5-150U's connector pitch and I/O pinout differ from the S7-1500's front connector. The SINUMERIK 850 is typically upgraded to a SINUMERIK 840D sl with a separate SIMATIC S7 PLC for auxiliary functions.
| Migration Direction | Effort | Typical Use |
|---|---|---|
| S5-150U → S7-300 | Medium — STEP 5 to STEP 7 converter; rewiring likely | Most cost-effective modern replacement |
| S5-150U → S7-1500 | High — full rewire, I/O remap, program rewrite | Long-term plants, TIA Portal alignment |
| S5-150S (SINUMERIK 850) → SINUMERIK 840D sl | High — controller, drives, I/O, HMI all swap | Machine tool retrofits |
11. Troubleshooting Matrix
| Symptom | Likely Cause | Verification Step | Remedy |
|---|---|---|---|
| STEP 5 "Online / Connect to PLC" times out | Missing PG-side loop-back jumpers (pins 6-20, 12-23, 2-3-4-17) | Open the PG connector, measure continuity on jumpers | Install the three shorting links |
| IM511 PGN LED off, PGS LED on | Loop not closed, current loop broken | Measure 20 mA on the line with a clamp meter | Check shield, return wire, connector pin engagement |
| Frequent CRC errors during program upload | EMI on the cable, ground loop, or cable > 10 m on unisolated PG | Check shield bonding at IM511 end only | Use a shielded, twisted cable; bond shield at IM511 only |
| PG sees PLC but cannot read DBs | Wrong parity / baud — STEP 5 set to 8N1 instead of 8E1 | Verify COM port: 9600, 8, E, 1 | Set COM to 9600 8E1 |
| Laptop with USB-TTY adapter: PG not detected | USB adapter in RS-232 mode, not TTY mode | Loop back the TTY side and check current | Use a TTY-capable USB adapter (active 20 mA) |
| CPU in "Stop" with IM511 plugged in but PG works | IM511 slot conflict with another intelligent module | Check slot assignment in CPU configuration | Move IM511 to a different free slot |
| PG transmits, no response from PLC | Cable wired as straight-through, no cross-over | Verify 10↔20, 22↔21, 21↔18, 8↔19 | Rewire cross-overs |
12. Field-Proven Engineering Notes
- The IM511 is electrically robust but its front-panel 25-pin connector has poor retention. When the cable is unplugged, always re-tighten the locking screws. A loose connector on a vibrating cabinet will produce intermittent "PG connection lost" faults that look like CPU crashes.
- For SINUMERIK 850 systems still running S5-150S, keep at least one fully programmed PG 685/PG 750 with a working TTY port. Modern laptops can read and archive programs but cannot initialize a corrupt EPROM submodule on the S5-150S without the original PG.
- The 20 mA current loop is polarity-sensitive on the cross-over pairs. If the IM511 LED indicates "PG active" but STEP 5 cannot connect, swap the cross-over pair 10/22 with 20/21 — the convention A/B may be reversed on a third-party IM511 clone.
- Always keep a known-good PG-IM511 cable stored near the cabinet. The cable failure rate on these systems is the single largest cause of "CPU is dead" misdiagnoses.
- When ordering replacement EPROM/EEPROM submodules for the S5-150U, use only Siemens original 6ES5 375-... series. Third-party submodules are reported to work but may fail Siemens' checksum verification on cold start.
What is the IM511 used for in the Siemens S5-150U and S5-150S?
The IM511 is the dedicated programming-unit interface module for the S5-150U and S5-150S PLCs. It occupies a slot in the central rack, terminates the AS511 protocol on a 25-pin Sub-D connector, and provides a 20 mA current-loop (TTY) physical interface between the CPU and a Siemens PG or laptop running STEP 5.
Which three jumpers must be installed on the PG side of an AS511 cable?
The PG side of the AS511 cable requires three shorting links: pins 6–20, pins 12–23, and pins 2–3–4–17. These close the 20 mA current loop on the passive PG end; without them the IM511 reports a "PG not connected" fault and STEP 5 online functions time out.
Can a modern Windows 10/11 laptop connect to an S5-150U via IM511?
Yes, but it requires a USB-to-TTY (20 mA) active adapter, STEP 5 V7.23 or later running inside a virtual machine with a 16-bit Windows layer, and the IM511 cable wired as shown in Section 5 with the PG-side jumpers installed. The COM port must be configured for 9600 bit/s, 8 data bits, even parity, 1 stop bit.
When was the S5-150U discontinued and where can I still get documentation?
The S5-150U was officially discontinued in 1996. Documentation is no longer published in the current Siemens Online Support main catalog, but historical manuals can still be obtained through the Siemens legacy archive, authorized Siemens regional service partners, the SINUMERIK 850 service manual, and university engineering library collections.
What is the maximum distance between PG and S5-150U on the IM511/AS511 link?
On a properly shielded twisted-pair cable, the AS511 current-loop interface supports up to 1000 m between the PG and the IM511 when true current-loop boosters are used. With a standard laptop and a passive USB-to-TTY adapter, keep the cable under 10 m to avoid CRC errors and ground-loop issues.