Troubleshooting Siemens OP27 HMI Communication with ProTool 6.0

The Siemens OP27 is a text-based operator panel from the SIMATIC HMI product line, designed for monitoring and control of SIMATIC S5 and S7 PLCs. Although officially declared obsolete by Siemens, many OP27 units remain in service in legacy systems across water/wastewater, HVAC, packaging, and machine-building applications. This article addresses one of the most common service issues: failure to establish a download/upload connection between a PC running ProTool/CS 6.0 SP3 (or ProTool/Pro) and the OP27 over the serial (RS-232) configuration port.

1. Problem Description

Symptom cluster reported in the field:

• OP27 powers up and the operator can navigate to the system menu, which shows the four standard entries: ONLINE, OFFLINE, DOWNLOAD, and PASS THROUGH.
• Selecting DOWNLOAD from the system menu, or holding ESC + UP arrow at power-up, displays the prompt TRANS MODE: Press ESC for load mode with the words TRANS MODE highlighted.
• ProTool 6.0 SP3 is launched on the PC, the COM port is set to COM1, the baud rate is set to 19200, and the operator clicks Connect or Transfer.
• Within approximately 2 seconds ProTool returns a generic error such as "Check cables, settings, COM port configuration…". No project is transferred.
• Same failure occurs on an OP25 panel tested with the same cable and software, suggesting a systematic issue with the PC-side configuration or the cable, not the panel itself.

2. OP27 Communication Architecture

The OP27 uses Siemens' serial point-to-point protocol (also called the ProTool/Pro RT/AS511 family for AS511-attached S5, or MPI/Profibus-DP for S7 depending on the OP27 variant). The configuration port on the back of the unit is an RS-232C (25-pin male SUB-D) connector labeled IF1 or PtP, with the following signal set:

The PC side is a DB-9 male (COM1) connector. The OP27 expects full hardware handshaking (RTS/CTS and DTR/DSR); ProTool's protocol engine requires this for a successful link. This is the single most common root cause of "instantaneous" connection failure: the cable loops TX/RX but does not provide the required handshake loopbacks.

3. Root Cause Analysis

Five root causes account for the majority of OP27/ProTool connection failures in the field:

3.1 Missing or incorrect handshake loopbacks

The pinout published on many third-party sites (and reproduced by the original poster — pins 3-3, 4-2, 5-8, 10-7, 15-5) routes data lines but does not satisfy the OP27's RTS/CTS and DTR/DSR requirements. The OP27 will not start the protocol handshake until it sees CTS=HIGH and DSR=HIGH. A standard Siemens 6XV1 440-… cable pre-wires these; a hand-made cable must replicate them.

3.2 Wrong COM port selected in ProTool

ProTool 6.0's transfer dialog allows only COM1–COM4, and there is no auto-detect. If Windows device manager shows the USB-to-serial adapter as COM3 or higher, ProTool will silently fail.

3.3 Baud-rate mismatch

The OP27 default transfer rate is 19200 bit/s, 8 data bits, no parity, 1 stop bit (19200,8,N,1). If the OP27 project was last saved with a different rate (9600, 38400, 57600), the unit will only respond at that rate. ProTool will not auto-baud — the user must match it.

3.4 CTS/RTS polarity swap

Some clone cables and pinout charts cross RTS of the OP27 to CTS of the PC directly (i.e., pin 4 of the OP27 to pin 7 of the DB-9 on the PC, where pin 7 is RTS on the PC side). The Siemens-correct wiring is RTS-to-CTS and CTS-to-RTS (crossed). If the cable is wired straight-through on these two pairs, handshaking will be inverted and the link will fail.

3.5 Transfer mode not properly entered

If the operator is on the ONLINE or PASS THROUGH system-menu entry, the OP27's serial port is owned by the runtime application, not the loader. The user must either select DOWNLOAD from the system menu, or boot the OP27 with ESC + UP arrow held until the message TRANS MODE: appears with the words TRANS MODE highlighted (this is the loader, not the runtime).

4. Solution: Build a Compliant PC-to-OP27 Cable

The Siemens-original PC/PtP cable is part number 6XV1 440-2A (also sold as 6ES7 901-1BF00-0XA0 on some legacy order codes). For a workshop-built replacement, use the wiring below. This is the corrected Siemens-issued pinout for OP27 DB-25 ↔ PC DB-9:

The single most important change from the original poster's pinout is pin 4 (RTS) of the OP27 going to pin 8 (CTS) of the PC DB-9, and pin 5 (CTS) of the OP27 going to pin 7 (RTS) of the PC DB-9. The original poster's chart had OP27 pin 4 to PC pin 2 and OP27 pin 5 to PC pin 8 — both wrong. The original poster's pin 15 → PC pin 5 (signal ground) is also incorrect; signal ground on a DB-9 is pin 5, but on the OP27 DB-25 it is pin 7, not pin 15.

5. Step-by-Step Configuration Procedure

1. Verify the PC COM port. Open Windows Device Manager → Ports (COM & LPT) → expand and confirm that Communications Port (COM1) exists. If using a USB-to-serial adapter, note the assigned COM number (e.g., USB Serial Port (COM3)).
2. Validate the COM port with a loopback plug. Short pins 2 and 3 on a DB-9 female, then use a terminal program (e.g., PuTTY, HyperTerminal) at 19200,8,N,1. Each typed character must echo. If it does, the COM port and UART are working.
3. Power up the OP27 in transfer mode. Disconnect the OP27 from the PLC. Press and hold ESC and the UP arrow simultaneously while applying power. Continue holding until the screen shows TRANS MODE: Press ESC for load mode with the words TRANS MODE highlighted. Release the keys.
4. Set ProTool 6.0 SP3 transfer parameters. In ProTool, go to File → Transfer → Settings. Set: COM Port = COM1 (or the actual COM number from step 1), Baud rate = 19200. ProTool 6.0 SP3 does not expose parity/stop-bit settings in the GUI; they are hard-coded to 8,N,1. Do not change them in any underlying INI file.
5. Initiate the transfer. In ProTool, click Transfer → Download to OP. ProTool will display a status dialog (typically <5 s on a successful link) and the OP27 will display RECEIVING… with a percentage counter.
6. If transfer fails immediately: First, swap the cable with a known-good Siemens 6XV1 440-2A. Second, verify the OP27 is in loader mode (not runtime) by observing the TRANS MODE highlight. Third, try the second OP27 system-menu entry — exit DOWNLOAD, then re-enter it. Fourth, re-check that ProTool's COM port matches the active Windows port.

6. Verification

After a successful transfer, perform the following validation steps:

• OP27 displays RECEIVING… followed by 100% and a REBOOT prompt.
• OP27 reboots automatically into the new runtime project; the configured start screen appears within 10 s.
• In ProTool, the transfer log records a Transfer completed successfully entry with the project CRC.
• Reconnect the OP27 to the controlled PLC and verify the on-screen process values update against the PLC tag database (use the OP27 system menu's DIAG → PLC STATUS option if available).

If step 1 of validation fails (OP27 stays in TRANS MODE: indefinitely and ProTool still reports a generic error), the issue is the cable. Re-verify the pinout table in section 4 against the actual physical wiring with a continuity tester before assuming a panel fault.

7. Common Error Codes and Their Meaning

ProTool 6.0 SP3 returns non-specific error text on most transfer failures, but the underlying error codes (visible in protlog.txt under C:\Program Files\Siemens\ProTool) follow Siemens' standard numbering:

For background reference on the OP27 product status, see the official Siemens SiePortal entry: OP27 Mono — SiePortal. Siemens has officially declared the OP27 obsolete; spare-parts and project support are handled by the SIMATIC migration team.

8. Alternate Transfer Path: Ethernet via PASS THROUGH

If the OP27 system menu shows a PASS THROUGH entry (this is firmware-version dependent — OP27 firmware < V3.0 generally does not support it), the OP27 can act as a transparent serial-to-MPI/Profibus bridge for the attached S7 PLC. This is useful for online diagnostics with STEP 7, but it does not enable ProTool transfers over Ethernet. ProTool 6.0 SP3 is strictly serial for the OP27 product line.

9. Field-Proven Diagnostics Checklist

Use this matrix as a rapid triage tool when an OP27 will not talk to ProTool:

10. Safety and ESD Notes

• Always power down the OP27 before inserting or removing the DB-25 configuration cable. Hot-plugging the IF1 port can latch up the OP27's UART and require a full power cycle to recover.
• The OP27 internal battery (CR2032 or equivalent, depending on variant) backs up the real-time clock and retentive tags. If the battery is dead, the OP27 will still boot, but the project CRC may fail to validate and ProTool will refuse to start runtime. Replace the battery before transferring a new project.
• If the panel is in a hazardous (Ex) area, the PC used for the transfer must be outside the hazardous area, or be rated for the area. The serial port itself is not intrinsically safe.