System Overview
The Siemens SIMATIC S5-150S is a multi-board CPU system in which four processor cards (CPU 924, CPU 925, CPU 926, CPU 927) work as a tightly coupled set inside a single CR / ER rack. The communication processor 6ES5511-5AA12 is a 25-pin serial interface module that exposes the AS511 protocol on the front panel. Failures to establish a PG/PC link, or the appearance of a yellow "cold restart required" LED after any card is removed from the rack, are the two most common field incidents with this hardware. This document covers the pin-level TTY cable, the cold-restart sequence on CPU 926/927, and the memory-recovery workflow for the 6ES5340-3KB41 32 KB RAM card.
Hardware Identification
Confirm the catalog numbers on the front of each card before applying any procedure. The behavior of the rack depends on which of the four CPU boards is currently installed and whether the 6ES5511-5AA12 is the 15-pin or 25-pin variant.
| Catalog Number | Function | Field Notes |
|---|---|---|
| 6ES5924-3SA11 | CPU 924 (lower processor) | Handles bit / byte / word operations; must remain seated during restart. |
| 6ES5925-3SA11 | CPU 925 (arithmetic) | Floating point and accumulator tasks; seat firmly before applying power. |
| 6ES5926-3SA11 | CPU 926 (coordinator) | Carries the RUN/STOP toggle and the Warm Restart button. Owns the 30-second memory-clear watchdog. |
| 6ES5927-3SA11 | CPU 927 (I/O coordinator) | Removal of this card forces the CPU set into cold-restart state. |
| 6ES5511-3AA1x | CP 511, 15-pin TTY | Original communications module. |
| 6ES5511-5AA12 | CP 511, 25-pin TTY | Replacement variant; pinout is mechanically different but electrically identical. |
| 6ES5340-3KB41 | 32 KB RAM memory card | Volatile; loses contents when unseated. Backed up by rack battery. |
| 6ES5350-x | EPROM memory card | Non-volatile; used to archive STL programs. |
Communication Problem: AS511 Link Fails
The 6ES5511-5AA12 exposes the SIMATIC AS511 protocol over a 20 mA current loop (TTY). The PC or PG must terminate this loop with the correct active/passive adapter, and the cable must source the 20 mA from a port that provides it. The symptoms that show up on the bench are:
- STEP 5 reports "No connection to AG" or "Timeout on online function".
- CP 511 front-panel LED stays dark; no echo of keystrokes from STEP 5.
- Link works on the 15-pin version of the card but fails after swap to the 25-pin version.
The root cause is almost always a wiring mismatch, not a card failure. The 25-pin variant uses a D-sub pinout that is not pin-compatible with the 15-pin variant, so a straight adapter or null-modem is required.
TTY 15-Pin to 25-Pin Adapter Wiring
Build a passive adapter that maps each signal between the 15-pin CP 511 (when the older 6ES5511-3AA1x is in the rack) and the 25-pin CP 511 (6ES5511-5AA12). Use a metal-shell hood and keep the cable shorter than 30 m for noise immunity; 20 m is the conservative limit for AS511 over TTY.
| Signal | 15-pin D-sub (CP 511 side) | 25-pin D-sub (PC/PG side) | Direction |
|---|---|---|---|
| TTY TX+ (20 mA source) | Pin 1 | Pin 4 | CP → PC |
| TTY TX- (20 mA return) | Pin 2 | Pin 20 | CP → PC |
| TTY RX+ (20 mA sink) | Pin 3 | Pin 5 | PC → CP |
| TTY RX- (20 mA return) | Pin 4 | Pin 6 / 22 | PC → CP |
| RTS (out, optional) | Pin 5 | Pin 4 (PC RTS) | Handshake |
| CTS (in, optional) | Pin 6 | Pin 5 (PC CTS) | Handshake |
| Shield | Housing | Housing | Drain to PG ground at one end only |
If you are interfacing a 25-pin CP 511 directly to a 9-pin PC COM port, use a Siemens 6ES5731-0BF00 PG-to-AS511 cable in reverse, or build a custom breakout using the pin map below:
| CP 511 25-pin TTY | PC DB-9 RS-232 | Function |
|---|---|---|
| Pin 4 (TX+) | Pin 2 (RD) | Data from CP to PC (use TTY/20 mA isolator) |
| Pin 20 (TX-) | Pin 2 (RD ground) | Return |
| Pin 5 (RX+) | Pin 3 (TD) | Data from PC to CP |
| Pin 6 / 22 (RX-) | Pin 7 (RTS) / 4 (DTR) | Source the 20 mA from PC side |
| Pin 7 (GND) | Pin 5 (SGND) | Signal ground |
CPU Status LEDs and Cold Restart Trigger
After any combination of the four CPU boards or the 6ES5340-3KB41 RAM card has been removed and re-inserted, the S5-150S enters an undefined state. The rack front-panel LED pattern is the only ground truth on the bench:
- Yellow LED steady — Cold restart required. The CPU set has lost its memory image or its inter-board handshake. The user program is not running.
- Red STOP LED steady — CPU set is initialized but a fault is pending (e.g. I/O not plugged, OB not loaded).
- Green RUN LED flashing — Warm restart in progress.
- Green RUN LED steady — User program executing.
The condition is triggered automatically when CPU 927 (6ES5927-3SA11) is reseated, when the 6ES5340-3KB41 RAM card is reseated without battery, or when the rack battery has been disconnected for more than the RAM retention window (typically days to weeks, depending on temperature).
Cold Restart Procedure on CPU 926 / CPU 927
The cold-restart sequence wipes the volatile user program and re-initializes the CPU set. The 30-second watchdog on CPU 926 means the operator must act quickly once the prompt is given by STEP 5.
- Confirm all four CPU cards (6ES5924, 6ES5925, 6ES5926, 6ES5927) and the 6ES5511-5AA12 CP are fully seated in the rack with the backplane screws torqued.
- Verify the rack battery is connected and has open-circuit voltage > 3.4 V (two-cell NiCd / NiMH, ~2.4 V nominal).
- Connect the PG to the CP 511 with the TTY cable built in the previous section. Power the rack.
- Open STEP 5, go to PLC > Memory > Clear PLC. The PG transmits an initialization frame to the CP.
- Within 30 seconds of receiving the prompt, flip the RUN/STOP switch on CPU 926 to STOP, then to RUN, while holding the Warm Restart button. The button must be held until the green RUN LED begins to flash.
- Release the button. The yellow "cold restart" LED should extinguish and the rack should report PLC in STOP in STEP 5 with the message buffer cleared.
- Re-transfer the program from EPROM card, backup file, or re-type. The S5-150S will not auto-start in RUN after a cold restart; an operator action or STEP 5 PLC > Cold Restart command is required.
Warm Restart Procedure
A warm restart retains the user program and is appropriate when the CPU set has been disturbed but the RAM card is still good and the battery is healthy.
- Set the RUN/STOP switch on CPU 926 to STOP. Wait for the STOP LED to settle.
- Press and hold the Warm Restart button on CPU 926.
- While holding the button, flip the RUN/STOP switch from STOP to RUN.
- Continue to hold the button until the green RUN LED begins to flash (typically 1–3 s).
- Release the button. The CPU set resumes execution at OB 22 (warm restart OB). Process-image retentive flags (M 0.0–M 199.7) are preserved; timers and counters re-initialize from their loaded values.
If the warm restart returns the CPU to STOP with a yellow "cold restart" LED, the memory image is no longer trusted — proceed to the cold restart sequence above.
Memory Architecture and Battery Backup
The S5-150S uses a hierarchical memory map spread across the four CPU cards and the plug-in memory sub-module:
| Region | Carrier | Volatile? | Backed up by |
|---|---|---|---|
| System memory (AS / BS / SS) | CPU 926 / 927 onboard | No | None required; EPROM-like |
| User program (OB / PB / SB / FB / DB) | 6ES5340-3KB41 (32 KB RAM) or 6ES5350 EPROM | RAM: yes / EPROM: no | Rack battery |
| Process image (PII / PIQ) | CPU 927 onboard | Yes | Rack battery, ~30 days typical |
| Retentive flags (M 0.0–M 199.7) | CPU 926 onboard | Yes | Rack battery |
| Timers / Counters | CPU 924 / 925 | Yes | Rack battery |
The rack battery is the cylindrical NiCd / NiMH cell clipped into the right-hand side of the CR rack. Its open-circuit voltage should be measured with the rack powered down for at least 10 minutes to avoid the on-board charger masking a dead cell. A reading below 2.6 V on a two-cell stack means the RAM card was without backup during the previous outage and the program is likely lost.
Memory Recovery Strategy
When the program appears to be lost, work through the following decision tree before declaring it gone:
- Is the 6ES5350 EPROM card present and seated? If yes, the STL is non-volatile. Power up, run a cold restart, and use STEP 5 PLC > EPROM > PLC > RAM to copy the program into the 6ES5340-3KB41 RAM card.
- Is the 6ES5340-3KB41 RAM card present and is the battery good? Attempt an online directory read with STEP 5: PLC > Directory. If the block list returns, the program survived — issue a warm restart and back it up immediately to an EPROM card or to disk via File > Save As.
- Is the directory empty but the LEDs indicate a healthy CPU set? The RAM card lost its image. The only recovery path is the EPROM card (option 1) or a paper / PDF listing typed back in via STEP 5 STL editor.
- Is the 6ES5340-3KB41 missing or in a different rack? The 32 KB image is tied to the card. Reinserting a different blank card will not recover the program; it must come from EPROM, backup, or a manual re-entry.
AS511 Cable Selection Guide
| Cable | For | Works with 6ES5511-5AA12? |
|---|---|---|
| 6ES5731-1BD20 | PG 730/750 laptop TTY port | No — needs +5 V supply from PG TTY port, not present on AS511 |
| 6ES5731-0BF00 | Standard PG 6ES5 685 to AS511 | Yes — designed for the 25-pin AS511 |
| Custom 15→25 adapter | Retrofit 6ES5511-3AA1x to 6ES5511-5AA12 | Yes — see pinout table above |
| Standard RS-232 null-modem | Any RS-232 PC | No — TTY current loop ≠ RS-232 voltages |
Verification Checklist
Run these checks after the cable and the cold restart have been applied, in this order:
- With the rack powered, the CP 511 front-panel ACTIVE LED should light within 2 s of PG connection.
- From STEP 5, PLC > Status must return the rack type, CPU set version, and a non-empty memory configuration.
- PLC > Directory must list all OB/PB/SB/FB/DB blocks. If empty, the cold restart is incomplete — repeat the 30-second watchdog step.
- Toggle a known output (e.g. Q 0.0) from Force in STEP 5 with the PLC in STOP. The output module LED should change within one scan cycle.
- Run a full warm restart. Process-image and retentive flags should match the last saved values.
- Immediately back up to EPROM card and to disk. Never leave the only copy on the 6ES5340-3KB41 RAM card.
Troubleshooting Matrix
| Symptom | Likely Cause | Action |
|---|---|---|
| CP 511 LED dark, STEP 5 timeout | Wrong cable, no TTY/RS-232 converter | Verify 6ES5731-0BF00 or TTY converter in line |
| Yellow LED on after reseating CPU 927 | CPU set initialization required | Cold restart sequence, 30 s watchdog |
| Warm restart returns to STOP | RAM image corrupt or missing | Pull from EPROM, reload from backup |
| STEP 5 reports empty directory | RAM card unbacked, battery dead | Check battery voltage, restore from EPROM |
| Link drops during long upload | Cable > 30 m, EMI on TTY loop | Shorten cable, add shielding, isolate from VFD cables |
| PG laptop cable 6ES5731-1BD20 will not link | Cable expects +5 V from PG port | Replace with 6ES5731-0BF00 or TTY/232 converter |
| CPU 926 toggle ignored after STEP 5 Clear PLC | 30-second watchdog elapsed | Reissue Clear PLC, toggle within 30 s |
Field-Commissioning Notes
- Always power the rack from a UPS during any card swap — a brown-out mid-extract can corrupt the 6ES5340-3KB41 even with the battery fitted.
- Label every cable on both ends. The 6ES5511-5AA12 TTY loop polarity is not standardised across all third-party converters, and a reversed pair will appear to work at low baud but fail on the first AS511 handshake retry.
- Keep at least one verified EPROM (6ES5350) image per machine in the panel drawer. The 30-second watchdog is easy to miss on a hot afternoon in a noisy plant.
- Document the warm-restart and cold-restart sequence on a laminated card inside the cabinet door. The 6ES5926-3SA11 button positions are not self-evident to a night-shift operator.
Why does the S5-150S show the yellow "cold restart" LED after I simply reseated the CPU 927?
CPU 927 (6ES5927-3SA11) is the I/O coordinator; its seat interrupt forces the four-board CPU set to re-initialize, which is signaled by the yellow LED. Run the cold-restart sequence on CPU 926 within 30 seconds of the STEP 5 Clear PLC command to clear the LED.
Can I use the 6ES5731-1BD20 laptop cable on the 6ES5511-5AA12 AS511 port?
No. The 6ES5731-1BD20 expects +5 V DC from the PG/PC TTY port to power its line drivers, and the AS511 connector does not supply it. Use 6ES5731-0BF00 or a TTY ↔ RS-232 converter instead.
How do I read a program that is still on the 6ES5340-3KB41 RAM card?
Connect the PG with the TTY cable, open STEP 5, and run PLC > Directory. If the block list comes back, immediately back up with File > Save As to disk and to a 6ES5350 EPROM card. Do not power-cycle the rack before the backup is taken.
What is the difference between warm and cold restart on a 6ES5926-3SA11?
A warm restart (RUN/STOP → STOP, hold button → toggle to RUN) retains the RAM image and resumes at OB 22. A cold restart (Clear PLC in STEP 5, then toggle within 30 s) wipes the user program and re-initializes the CPU set. Use warm first; only fall back to cold if warm returns to STOP.
How do I know if the rack battery is healthy enough to retain the program?
Power the rack down for at least 10 minutes, then measure the open-circuit voltage of the NiCd / NiMH cell on the right side of the CR rack. A two-cell stack below 2.6 V will not retain the 6ES5340-3KB41 image for more than a few days.