Resolving TDC SC67 USB-RS232 Diagnostic Buffer Access Issues

1. Overview of the SC67 to TDC USB-RS232 Connection Path

The SIMATIC TDC (Technology and Drives Controller) is a Siemens multiprocessor automation platform designed for high-speed, modular drive and process control. It is typically programmed with CFC (Continuous Function Chart) inside SIMATIC Manager and is structurally different from the SIMATIC S7-300/400 PLC family. Because TDC service interfaces still rely on RS-232 signaling, modern engineering stations without native COM ports require a USB-to-RS-232 converter in front of the SC67 serial cable.

The typical physical path is:

1. Engineering PC (USB-A port)
2. USB-to-RS-232 converter cable (Digi 20-151-0178 or equivalent)
3. SC67 point-to-point serial cable (Siemens 6FX2003 series, RS-232, sub-D)
4. TDC CPU module service port (e.g., CP51M1, CP52M1, CP551M1, CP5611 family)
5. SIMATIC Manager V5.x in ONLINE mode targeting the TDC station

When this path is functional, engineers can usually establish an ONLINE connection and monitor CFC blocks in RUN. The most common symptom reported in the field, and the topic of this reference, is that the ONLINE connection succeeds, CFC monitoring works, but the diagnostic buffer of the CPU cannot be read from SIMATIC Manager. The reason is architectural, not cabling.

2. Problem Symptoms

When the SC67 + USB-RS232 path is correctly wired, the following symptoms together indicate the diagnostic-buffer access problem rather than a physical-layer fault:

• SIMATIC Manager reaches the TDC station and displays the CPU object in ONLINE mode.
• CFC online monitoring is functional: chart values, block I/O, and interconnections update.
• Right-clicking the CPU object and selecting PLC → Module Information does not return the expected diagnostic buffer entries, or returns an error/empty result.
• The CPU module front-panel display shows E (letter E) instead of the configured CPU number.
• Reading the same diagnostic data from CFC → CPU → Module Information succeeds.

Any one of the first three items in isolation could be a cabling or driver problem. The combination of (a) successful ONLINE, (b) working CFC monitoring, and (c) failed diagnostic buffer is the diagnostic signature that confirms the issue is the SIMATIC Manager ↔ TDC diagnostic interface, not the USB-RS232 converter.

3. Root Cause: Diagnostic Buffer Access Differs from S7 PLCs

The root cause is that SIMATIC TDC diagnostics are not equivalent to SIMATIC S7 diagnostics. TDC uses a different diagnostic stack, a different CPU operating system, and a different runtime kernel. The SIMATIC S7 diagnostic buffer is exposed through a uniform PG/PC interface and is what the PLC → Module Information dialog queries. TDC's diagnostic information is exposed through a separate channel that is only reachable from the CFC editor while the chart is open ONLINE.

Practical consequences:

Engineers approaching TDC with S7 habits will look for the S7 diagnostic buffer first. The fix is to switch to the CFC-specific path described in Section 5.

4. Required Hardware and Software Stack

Use only hardware and software versions known to be compatible with the target TDC firmware.

4.1 SC67 Cable

The SC67 is the Siemens point-to-point RS-232 service cable for SIMATIC TDC. It is typically referenced as Siemens part number 6FX2003-0XA03 (verify the exact variant against the TDC hardware manual for the specific CPU module family in use, e.g., CP51M1 vs. CP52M1). The cable terminates in a sub-D connector at the PC end and a dedicated service plug at the TDC module end.

4.2 USB-to-RS-232 Converter

Because most modern engineering laptops do not have a physical COM port, a USB-to-RS-232 converter is required between the PC and the SC67 cable. A proven reference part is the Digi International USB-to-RS-232 converter cable, part number 20-151-0178, documented in the Digi USB to RS-232 Converter Cables User's Guide (TN258). The Digi unit presents a virtual COM port on Windows, which SIMATIC Manager can then assign as the TDC PG/PC interface.

Key converter requirements:

• True RS-232 signal levels (±12 V), not RS-232-TTL.
• Windows virtual-COM driver signed for the engineering-station OS.
• Baud rate and protocol matching what SIMATIC Manager expects for TDC ONLINE access (default 19200 bit/s, even parity, 1 stop bit is a common TDC setting — verify against project configuration).
• Hardware or software flow control disabled, or set to match the SC67 pinout (see Section 8).

4.3 SIMATIC Manager and CFC Versions

Use a SIMATIC Manager version that supports both the TDC option package and the CFC editor. TDC projects are typically opened as "S7-compatible" or as native TDC stations depending on the TDC option package version. Confirm the version matrix against the TDC option package release notes before commissioning.

5. Step-by-Step: Accessing the TDC Diagnostic Information

The procedure below restores diagnostic visibility without changing cabling or converter hardware.

1. Open the TDC station in SIMATIC Manager and go ONLINE against the target CPU.
2. Open the CFC chart that is configured to run on the target CPU (any chart bound to the same CPU will do for diagnostic access).
3. In the CFC menu bar, select CPU → Module Information (not PLC → Module Information from the SIMATIC Manager context menu).
4. The CPU Module Information dialog displays the TDC-specific diagnostic information: CPU state, current task assignment, task error counters, and the TDC operating-system event log.
5. Record the diagnostic entries. Pay particular attention to task errors, timeouts, and PSL warnings.
6. Close the dialog. Repeat the operation from any CFC chart bound to the same CPU to confirm consistency.

If CPU → Module Information is greyed out, the CFC is not ONLINE with the same CPU. Open the chart in ONLINE mode first, or check the chart's CPU assignment under Chart → Properties.

6. Reading Task Manager Errors (the "E" Display)

TDC CPU modules have a small front-panel display that normally shows the configured CPU number (for example, 0, 1, 2). When the display shows the letter E, the TDC operating system has detected a task manager error. This is the TDC equivalent of an S7 CPU SF/BF event, but it is a runtime/task-scheduler fault, not a hardware fault.

Common causes of a task manager E:

The E state is the highest-level indicator. The detailed cause is recovered through the CPU → Module Information dialog described in Section 5, and quantified by the cycle task load procedure in Section 7.

7. Monitoring Cycle Task Load (T3, T4, T5) with PSL-FB

TDC schedules runtime work on a fixed set of cycle tasks. The slow, base-rate tasks — typically T3, T4, and T5 — are where non-time-critical CFC charts are placed. Fast tasks (T0, T1, T2) handle closed-loop and time-critical functions. A common TDC pathology is for the slow tasks to be silently overloaded: the system continues to run, but task timeouts eventually trigger a task manager error and the E display.

7.1 PSL-FB Runtime Function Block

The PSL-FB (priority/scheduling layer function block) is a runtime block that exposes the current task load statistics as CFC-visible signals. The exact block name and instance assignment is project-specific; in a TDC project, locate the PSL-FB instance under the chart that owns the task monitoring logic, or insert it from the TDC option package library if it is not present.

Typical PSL-FB outputs to monitor (naming varies by option package):

• Current task run time in microseconds/milliseconds for each task class.
• Configured task cycle time.
• Task load in percent (= run time / cycle time × 100).
• Task timeout counter (number of times the task missed its cycle).
• Current active task index.

7.2 Recommended Threshold Procedure

1. Open the chart containing the PSL-FB instance ONLINE.
2. Monitor the load value of T3 first (T3 is the highest-priority slow task and the first to show overload symptoms).
3. Record the steady-state load under normal operating conditions. A healthy T3 load is typically well below 50%. Above 70% sustained is a warning; above 90% the task is at risk of overrun.
4. Repeat for T4 and T5.
5. Cross-check the timeout counter. A non-zero timeout counter on a slow task confirms that the task has already missed cycles and is the likely cause of the E display.
6. If a slow task is overloaded, identify heavy charts in the CFC project, offload them to a less-loaded task, or extend the cycle time of the affected task in the CPU configuration.

8. Cable Pinout and Signal Verification

Even when the diagnostic signature points to the S7-vs-TDC interface mismatch, it is good practice to verify the SC67 + USB-RS232 link is electrically clean. The most common physical-layer issues that mimic the "empty diagnostic buffer" symptom are driver-level flow control mismatches and incorrect COM port assignment.

8.1 Confirm the Virtual COM Port

1. Open Windows Device Manager and expand Ports (COM & LPT).
2. Confirm the USB-RS232 converter enumerates as a COM port (e.g., COM3).
3. In SIMATIC Manager, open Options → Set PG/PC Interface and assign the matching COM port and the TDC-specific access point (commonly S7ONLINE or the TDC option package's own access point).

8.2 Flow Control and Handshaking

SC67 is a point-to-point service cable. TDC does not require hardware handshaking for ONLINE access. Configure the virtual COM port with:

• RTS/CTS: disabled (or "off")
• DTR/DSR: disabled
• XON/XOFF: disabled
• Data bits: 8
• Parity: even (verify against project)
• Stop bits: 1
• Baud rate: as configured in the TDC project (19200 bit/s is a common default; some projects use 9600 or 38400)

8.3 Link-Layer Sanity Test

The single fastest link-layer test is the CFC ONLINE monitoring capability. If CFC values update in RUN, the link is fully functional. If they do not, the issue is not the diagnostic buffer; it is the link. Recheck the COM port, the access point, the cable seating, and the TDC module's service-port enable state.

9. Verification Procedure

After applying the CFC diagnostic path and resolving any task overload, verify the fix end-to-end:

1. Open SIMATIC Manager and go ONLINE with the TDC station.
2. Open the project CFC chart and confirm CPU → Module Information is accessible and non-empty.
3. Confirm the CPU front-panel display no longer shows E. It should show the configured CPU number.
4. Open the PSL-FB instance ONLINE and confirm: T3/T4/T5 load values are within the healthy range (well below 70% sustained), and all task timeout counters are at zero.
5. Force a controlled CPU restart from SIMATIC Manager and re-verify all four items above.
6. Save the diagnostic screenshots, the PSL-FB load values, and the post-restart display state as part of the commissioning dossier.

10. Troubleshooting Matrix

11. Field-Proven Caveats and Common Pitfalls

• Engineers with S7-only background will look for the S7 diagnostic buffer first. This is the single most common wasted-time step in TDC commissioning. Always start from the CFC path.
• The USB-RS232 converter must be RS-232-level, not 3.3 V or 5 V logic. A TTL-level cable will link to SC67 electrically but produce intermittent or completely failed communications.
• The TDC option package version must match the SIMATIC Manager version. A version skew can leave CPU → Module Information present but functionally empty.
• Slow task overload is silent until the timeout counter trips. PSL-FB monitoring is the only way to see the leading indicator before the E display appears.
• The Digi 20-151-0178 is a general-purpose USB-RS232 converter. The Digi TN258 user's guide describes the converter's electrical and driver characteristics; the same converter is used in many TDC and SIMADYN D installations because of its robust RS-232 line drivers.
• When the CPU display shows E after a project download, the most common cause is a chart bound to a task that no longer exists, or a renamed CPU that the chart's CPU assignment does not follow. Reassign the chart's CPU and re-download.

12. Summary

The SC67 + USB-RS232 path to a SIMATIC TDC is electrically simple and reliable when the converter is RS-232-level and the COM port is correctly assigned. The diagnostic-buffer confusion arises from assuming TDC behaves like S7. TDC diagnostics live in the CFC editor's CPU → Module Information dialog, not in the SIMATIC Manager's PLC → Module Information dialog. When the CPU display shows E, the operating system has detected a task manager error; the immediate action is to monitor T3, T4, T5 load with the PSL-FB runtime block, beginning from T3. Resolving slow-task overload and using the CFC-native diagnostic path together restore full visibility and bring the system back to a clean operational state.

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