SIMATIC S5-115 CPU: Replacing the 232PCLR Current Loop Converter with a Siemens 6ES5734-1BD20 Programming Cable

The SIMATIC S5-115 CPU exposes its programming and online diagnostic port (SI1 / PG port) as a 20 mA TTY current loop on a 15-pin sub-D connector. The interface does not natively speak RS-232, RS-422, or USB. To attach a PC, an active converter is required between the PLC's TTY port and the PC's serial (or USB-to-serial) port. Field experience with third-party passive 232PCLR current-loop adapters on S5-115 hardware shows recurring failure modes: blown transmit/receive opto-isolators, intermittent handshaking, and silent loss of communication after a few hours of operation. The replacement path of choice is the Siemens 6ES5734-1BD20 programming cable, which integrates the TTY converter into the cable assembly itself.

1. Problem Definition and Communication Stack

The S5-115 uses the AS511 protocol on its serial interface (SI1) for programming, online status, and block upload/download. The lower layers are:

The TTY loop cannot be connected directly to an RS-232 port. An active converter — either a standalone box (232PCLR, Phoenix Contact PSM-ME-RSRS232/TTY-P) or an integrated cable (Siemens 6ES5734-1BD20, Classic Automation 10203) — is mandatory.

2. Siemens 6ES5734-1BD20 — The Canonical Replacement

The 6ES5734-1BD20 (also written 6ES5 734-1BD20) is the Siemens factory programming cable for the S5 family. It has the TTY converter built into the cable assembly, so no external converter box is required.

3. Third-Party Replacement Cables (Plug-Compatible)

Two drop-in replacements for the 6ES5734-1BD20 are documented in field service:

The 10203USB variant is preferred on modern hardware because no separate USB-to-serial adapter is required and the cable length is bounded. The legacy 10203 (DB-25) variant is appropriate only when paired with a known-good USB-to-serial adapter based on the FTDI FT232 or Prolific PL2303 chipset.

4. Phoenix Contact PSM-ME-RSRS232/TTY-P — Modular Alternative

For environments where the S5 must communicate with a modern operator panel (OP) or touch panel (TP) — rather than a programming PC — Phoenix Contact's PSM-ME-RSRS232/TTY-P converter module is the documented industrial solution. It supports both active and passive TTY operating modes and is DIN-rail mountable.

5. CP 524 with RS-422 Com Module — When You Need More Than the SI1 Port

If the application requires RS-422 instead of RS-232 or TTY (longer distance, multi-drop, or noise immunity in a plant environment), an alternative is to install a CP 524 communications processor in the S5-115 rack and fit it with the RS-422 Com Module. The CP 524 handles serial protocols independently of the CPU and exposes either RS-232 or RS-422 depending on the inserted Com Module.

6. Adapter Strategy: DB-25 to DB-9 and USB-to-Serial

The 6ES5734-1BD20 and the 10203 variants terminate at a DB-25 male on the PC side. Modern PCs no longer expose DB-9 RS-232 ports, and most no longer have DB-25 either. The bridging strategy is:

1. DB-25 to DB-9 adapter: A passive gender-moulded adapter on the PC end of the cable, followed by a USB-to-serial adapter on the DB-9. Use only adapters with full shield bonding (the backshell must continue the cable shield through the adapter body).
2. Direct USB cable: The 10203USB cable integrates both the DB-25-to-USB conversion and the RS-232-to-TTY conversion into one assembly. This is the preferred modern option.
3. USB-to-serial chipset selection: Choose adapters based on FTDI FT232, Prolific PL2303, or Silicon Labs CP2102/CP2104. Avoid unbranded CH340-based adapters when working with STEP 5 under DOS or older Windows versions; they often lack proper DTR/RTS handshake buffering.

7. Cable Shielding and Length Constraints

The S5-115's TTY port is sensitive to both shield integrity and cable length. Two field-proven constraints apply:

• Shielding must be continuous end-to-end. A cable that loses shield continuity through a passive DB-25-to-DB-9 adapter (because the adapter's backshell is plastic) introduces common-mode noise on the current loop, which manifests as intermittent communication dropouts after a few hours.
• Cable runs should not exceed 3 m to 5 m for reliable TTY operation, particularly in panels with VFDs or other switching noise sources. The Siemens 6ES5734-1BD20 ships at 3.2 m for this reason.

8. Host Software Configuration

The PC-side host software must be configured to match the AS511 protocol layer:

For STEP 5 operation, prefer running the host software under native DOS rather than inside a Windows DOS box. The original Siemens drivers were written against direct COM-port access; a virtualized DOS window can mishandle the timing-critical AS511 handshaking and produce spurious timeouts.

9. Step-by-Step Replacement Procedure

1. Power down the S5-115 rack before touching the SI1 port. Although the TTY loop is current-limited, hot-plugging the loop can damage the optocouplers on the CPU's serial port.
2. Disconnect and remove the 232PCLR converter and its associated cable from the S5-115 SI1 (DB-15) port.
3. Inspect the S5-115 DB-15 socket for bent pins, contamination, or signs of overheating. Clean if necessary with isopropyl alcohol; do not use contact cleaners that leave residue.
4. Connect the 6ES5734-1BD20 (or 10203 / 10203USB) cable's DB-15 end to the S5-115 SI1 port. Tighten the connector screws.
5. Connect the PC-side end:
   • If using the DB-25 variant: attach a DB-25 to DB-9 adapter (shielded backshell) and then a USB-to-serial adapter based on FTDI/PL2303/CP2102.
   • If using the 10203USB variant: plug the USB end directly into the PC.
6. Power up the S5-115 rack and confirm the CPU is in RUN or STOP as expected.
7. Map the COM port: In Windows Device Manager, note the COM port number assigned to the USB-to-serial adapter.
8. Configure STEP 5 / PG-2000 / S5W to that COM port at 9 600, 8N1.
9. Initiate an online connection. The host software should report the CPU type (CPU 941, CPU 942, CPU 943, CPU 944, or CPU 945) and the program status.
10. Verify block transfer by reading a known FB/DB and confirming the checksum or block header matches.

10. Verification and Commissioning Tests

After installing the replacement cable, run the following checks before declaring the workstation operational:

• Cold-start loop test: Power-cycle the S5-115 rack three times in succession, re-establishing the online connection each time. Verify zero timeouts.
• Block upload/download test: Upload all FBs and DBs, then download them back. Checksum must match exactly.
• Force table test: Set and reset a known output via the force table and verify it propagates to the physical output module.
• Soak test: Leave the connection idle but online for at least 4 hours. The 232PCLR failure mode was thermally triggered after hours of operation; if the replacement has not dropped by then, the failure mode is unlikely to recur.
• Cable flex test: Gently flex the cable near both connectors during an online session. The connection should not drop; if it does, the connector backshell is not bonding the shield properly.

11. Long-Term Migration Path — S5-115 to S7-1500

If the underlying concern is broader fleet modernization, Siemens documents a serial-coupling path between a SIMATIC S5 and a SIMATIC S7-1500 via the S5's serial port. Refer to the official Siemens application example Serial communication between SIMATIC S5 and SIMATIC S7-1500 for the topology, block configuration, and parameterization. The same 6ES5734-1BD20 / 10203 cable is used on the S5 side, while the S7-1500 connects via its CM (communications module) RS-232 / RS-422 interface. This allows the S5-115 to remain operational while data is forwarded to a modern controller for SCADA or historian purposes.

12. Troubleshooting Matrix

13. Selection Decision Matrix

Frequently Asked Questions