SIMATIC 545-1103 Port 1 RS-232 Communication Troubleshooting

Problem Description

The SIMATIC 545-1103 CPU exposes two onboard serial ports on its faceplate: a multi-mode Port 1 and a fixed-function Port 2. Field reports describe a recurring condition in which Port 2 establishes programming sessions with 505-Workshop or TISOFT, but Port 1 times out, returns no carrier detect, or fails to echo characters even when the same null-modem cable is moved between the two connectors. Because the same physical cable and the same PC serial port succeed on Port 2, the failure is internal to the Port 1 hardware path, the DIP-switched personality, or the firmware handling of the Port 1 UART.

This article consolidates the verified Port 1 commissioning procedure from the SIMATIC 545/555/575 System Manual, the legacy 505-Workshop cable reference, and field-replicated diagnostics that isolate the failure to one of three layers: physical layer (cable, connector, signal levels), configuration layer (DIP switch SW2 position, baud rate, parity, echo), or component layer (UART, port driver, optocoupler).

SIMATIC 545-1103 Port Architecture

The 545-1103 faceplate carries two 9-pin D-sub male connectors labelled PORT 1 and PORT 2. Port 2 is hard-wired as an RS-232 DTE programming port that always supports the TISOFT / 505-Workshop protocol stack. Port 1 is multi-mode and is reconfigured at power-up by reading the on-board DIP switch bank. The supported Port 1 personalities are:

• RS-232 programming — full TISOFT / 505-Workshop protocol, DTE pinout.
• RS-422 programming — differential TISOFT protocol for long runs.
• RS-485 multi-drop — peer-to-peer data acquisition.
• Printer / terminal port — ASCII output only, no programming handshake.

The Port 1 UART and line drivers are physically separate from Port 2, which is why a Port 2 success does not imply Port 1 integrity. The two ports share the CPU's main 16-bit data bus and the +5 V logic supply, but each has its own isolated RS-232 charge-pump and its own DB-9 shell ground. A failure on Port 1's RS-232 transceiver (historically an SP3232 or MAX202-equivalent) will not affect Port 2.

DIP Switch Configuration for Port 1

The 545-1103 carries a 10-position DIP switch accessible through a small cut-out in the front bezel. Switches 1–8 control port personalities, port modes, and CPU options. Switches 9–10 control memory protection and battery back-up behaviour. The exact position of the switch (left = open, right = closed) is read only at power-up; changing the switch while the CPU is running has no effect until the next power cycle or warm restart.

After changing SW2, cycle power on the CPU and wait at least 5 seconds for the boot diagnostics to complete before attempting a programming session. A rapid power-cycle <2 s may not re-read the DIP bank on early 545-1103 firmware revisions.

RS-232 Cable Pinout and Wiring

The Port 1 and Port 2 connectors on the 545-1103 are wired as DTE devices. A null-modem (crossover) cable is required to connect to a PC COM port, which is also DTE. The wiring is identical for both ports when Port 1 is in RS-232 mode. Reference the Siemens RS-232/RS-422/RS-485 communication interface specification for the electrical characteristics of the signalling standard.

The minimum three-wire null-modem cable connects pin 2 to pin 3 (swap), pin 3 to pin 2 (swap), and pin 5 to pin 5 (straight). For TISOFT to complete the handshake, pins 4, 6, 7, and 8 must also be looped back: 4↔6 and 7↔8 on the PC end. The 505-Workshop programming cable sold by Siemens (part number 505-W1234 or equivalent) implements this loop-back internally and is the reference for any field-built cable.

A common failure is a cable that omits the handshake loop-backs. Such a cable may function with a PC that asserts DTR unconditionally but fails on a USB-to-serial adapter that follows strict DTE/DCE conventions. Always verify with a breakout box or a continuity tester that the four handshake lines (4, 6, 7, 8) are properly crossed at the PC end of the cable.

Baud Rate and Communication Parameters

The 545-1103 auto-detects the baud rate on Port 1 at session start. The supported auto-baud set is:

Parity must be set to EVEN when the CPU is in legacy TISOFT mode (SW6 = LEFT). When SW6 is RIGHT (Modbus RTU mode), parity is forced to NONE with 8-N-1 framing. TISOFT will not establish a session on Port 1 if SW6 is in the Modbus position, even though the RS-232 transceiver is healthy.

Systematic Diagnostic Procedure

Follow this sequence in order. Do not skip steps; each step eliminates one failure layer.

1. Power down the 545-1103 rack and remove the CPU module from its slot. Set the module on an ESD-safe mat.
2. Inspect the DB-9 shells on both Port 1 and Port 2 with a magnifier. Look for bent pins, broken solder joints at the connector shroud, or contamination in the pin field. A bent pin 2 or pin 3 will kill RS-232 communication without affecting the UART.
3. Verify the DIP switch SW2 is in the LEFT (open) position. Verify SW1 is in the position required by 505-Workshop (typically LEFT for echo off in programming mode). Verify SW6 is in the LEFT (TISOFT) position. Photograph the switch bank for the maintenance record.
4. Verify the cable on a known-good 545-1104 or 545-1105. The cable that works on Port 2 of the suspect unit MUST also work on Port 1 of a known-good unit when SW2 is LEFT. If it does not, the cable is faulty and the CPU is innocent.
5. Re-seat the CPU in its slot and apply power. Wait 5 seconds for boot completion. Confirm the CPU OK LED is steady green and the COMM LED on Port 1 is dark (idle).
6. Launch 505-Workshop on the PC. Configure the COM port for 1200-8-E-1. In the connection dialog, select Port 1 (RS-232) explicitly. Press F4 (Go Online) twice with a 3-second pause. The first attempt trains the auto-baud detector; the second attempt establishes the session.
7. If Go Online times out, drop the baud rate to 300-8-E-1 and repeat. If 300 baud also times out, the Port 1 UART is suspect.
8. Check CPU fault log from 505-Workshop after a Port 2 session is established. Look for fault code F045 (Port 1 framing error), F046 (Port 1 overrun), or F047 (Port 1 hardware fault). These codes do not appear on Port 2 faults.
9. Swap with a known-good CPU of the same part number to isolate whether the fault follows the CPU or the rack backplane. A backplane with a corroded pin on the Port 1 UART interrupt line will produce identical symptoms to a CPU failure.

Common Failure Modes

Field-Proven Cross-Diagnostics

The fastest field confirmation of a healthy Port 1 is the "back-to-back" test:

1. Connect the suspect 545-1103 Port 1 to a known-good 545-1104 Port 1 with the same null-modem cable.
2. On the known-good CPU, launch 505-Workshop and select Port 1 on the COM port connected to the suspect CPU.
3. Press F4 twice. If the session establishes, the suspect CPU's Port 1 is alive. If it times out, the suspect CPU's Port 1 is dead.
4. Reverse the test: from the suspect CPU, try to go online against the known-good CPU. This catches a one-sided port failure (e.g., a blown TX driver only).

Documented in legacy field records, the "first attempt times out, second attempt connects" pattern on Port 1 is a known behaviour of early 545-1103 firmware revisions when the auto-baud detector is cold. A persistent timeout on both attempts is the genuine failure indicator.

Replacement and Repair Considerations

For a 545-1103 with confirmed dead Port 1, three recovery paths exist:

1. Use Port 2 only. Port 2 has identical programming capability. Re-cable the workstation to Port 2 and document the change on the panel drawing. This is the lowest-risk path for production systems.
2. Refurbish the CPU module. The Port 1 RS-232 transceiver, the UART clock crystal, and the DB-9 connector are all field-replaceable with through-hole equivalents. Replacement requires a hot-air rework station and a clean bench, but the part cost is under $5 USD. A refurbished module must pass the back-to-back test before redeployment.
3. Replace the CPU module. Refurbished 545-1103 modules are available on the secondary market. Verify the seller has performed a back-to-back test on both ports and provides a 30-day warranty. New-old-stock modules are rare and command a premium.

Verification Procedure

After any corrective action, validate Port 1 with the following checklist:

1. Power up the rack and confirm the CPU OK LED is steady green within 5 seconds.
2. Verify DIP switch SW2 is LEFT, SW6 is LEFT, SW1 is in the position required by the programming tool.
3. Connect the 505-Workshop PC to Port 1 with a known-good null-modem cable.
4. Launch 505-Workshop, select Port 1 at 1200-8-E-1, press F4 twice.
5. Confirm the session establishes and the program directory loads within 10 seconds.
6. Read the CPU fault log and confirm no Port 1 fault codes (F045, F046, F047) are present.
7. Perform a program read from the CPU to disk and compare the file checksum to the last known good backup.
8. Disconnect, wait 30 seconds, reconnect, and confirm the session re-establishes on the first attempt (warm auto-baud test).