Resolving Siemens TS Adapter V5.2 ACTIVE LED Not Lighting

Overview of the Problem

The SIMATIC S7 TS Adapter V5.2 (Siemens order number 6ES7 972-0CA34-0XA0) is a protocol converter used to bridge an MPI / PROFIBUS-DP automation network to an RS-232 service interface for remote maintenance (Teleservice) of SIMATIC S7-300 and S7-400 controllers. The adapter sits between a programming device (PG) or PC and the PLC's MPI/DP connector, providing isolation, baud-rate adaptation, and an RS-232 feed for dial-up modems or direct cable connections.

Field reports consistently describe a PWR LED illuminated while the ACTIVE LED remains dark and Step 7 cannot establish an online connection. Because the PWR LED is fed from the MPI bus supply, the symptom proves that the adapter is receiving power from the PLC side; the missing ACTIVE indication is therefore an upstream handshake or configuration problem, not a power fault.

Hardware Identification and Variants

Before troubleshooting, confirm the exact hardware variant. The TS Adapter family has changed terminologies across product generations:

The label on the housing in this case study reads "SIMATIC S7 TS Adapter V5.2 / 6ES7 972 0CA34-0XA0 / S VP U7519473". The "V5.2" suffix is the firmware/hardware revision of the TS Adapter II product, not the Teleservice software version. Verify this against the Siemens Industry Online Support portal before ordering replacements.

LED Status Decoding

The TS Adapter V5.2 has three LED indicators on its housing:

Root Cause Matrix

Diagnostic Procedure

Follow this sequence before opening a hardware return. Each step is non-destructive and can be performed with a multimeter, a known-good serial cable, and the Step 7 V5.4 SPx toolset.

Step 1 — Verify the Teleservice Software Is Installed

The TS Adapter II is not a generic MPI/DP-to-RS-232 bridge. It is a Siemens-proprietary device that depends on the SIMATIC Teleservice (S7-Teleservice) software layer to interpret the PC-side RS-232 traffic. STEP 7 V5.4 alone does not include the Teleservice driver.

1. Open Start → Programs → Siemens Automation → SIMATIC → Teleservice on the PG/PC.
2. If the menu entry is absent, the Teleservice option package is not installed.
3. Install it from the STEP 7 V5.4 option-disk set (TeleService V6.1 or V6.2) and reboot.
4. Confirm the TS Adapter II appears in the SIMATIC Manager → Options → Set PG/PC Interface → Teleservice → TS Adapter selection list.

Step 2 — Confirm a Null-Modem Cable Is Used

The RS-232 port on the TS Adapter II uses a 9-pin sub-D female connector with the following pinout:

A correct PC-to-TS-Adapter cable is a null-modem (crossover) cable, with the minimum crossover of 2↔3, 4↔6, 7↔8 and pin 5 tied straight. A straight-through PC-modem cable will not work. Siemens sells this as part number 6ES7 901-1BF00-0XA0 (RS-232 null-modem cable, 5 m). The user-supplied cable labeled "TDK ELITRONIC-CY LIYCY 4×0.14 TN 2L60184" is a 4-conductor shielded cable that physically fits the connectors but is not guaranteed to be wired as a null-modem. Verify with a continuity tester:

Adapter pin 2 ↔ PC pin 3   (RxD ↔ TxD)
Adapter pin 3 ↔ PC pin 2   (TxD ↔ RxD)
Adapter pin 5 ↔ PC pin 5   (GND ↔ GND)
Adapter pin 4 ↔ PC pin 6   (DTR ↔ DSR)
Adapter pin 7 ↔ PC pin 8   (RTS ↔ CTS)

Step 3 — Verify MPI Power and Token Rotation

1. Confirm the S7-300/400 CPU is in RUN or STOP with no fault.
2. Disconnect the TS Adapter and connect a known-good PG cable (6ES7 901-1BD30-0XA0 for MPI, USB version 6GK1 571-1AA00 for PC-Adapter USB). If this cable establishes an online session, the bus is healthy.
3. Reconnect the TS Adapter and observe the ACTIVE LED while the PC-Adapter USB PG cable is also connected to a different MPI port (i.e., put the adapter in parallel). If the LED still does not flicker when you press "Accessible Nodes" in Step 7, the adapter is not generating bus traffic.

Step 4 — Match the RS-232 Baud Rate

Set the DIP switches on the TS Adapter housing (or the rotary switch on V5.2 units) to the same baud rate that the Teleservice software expects. For the V5.2 firmware the default and recommended rate is 38,400 bit/s. The 9-position switch (S1) selects:

Inside Set PG/PC Interface, the Properties dialog of the TS Adapter driver must show the same value. Mismatched rates produce silent timeouts and a permanently dark ACTIVE LED.

Step 5 — Resolve MPI Address Conflicts

The TS Adapter II occupies MPI address 0 by default, which is also the default PG address. If a second programming device or HMI is already on address 0, the bus will arbitrate incorrectly.

1. Run "Accessible Nodes" from the local PG to enumerate MPI addresses.
2. Reassign the TS Adapter to a free address (e.g., 15) using the rotary switch S2 / address selector.
3. Re-attempt the connection.

Step 6 — Verify the Extension Cable

The user is using an extension cable between the adapter MPI connector and the CPU's MPI port. The original MPI PROFIBUS extension cable from Siemens (6ES7 901-0BF00-0AA0) is a 1:1 extension with no crossover. Aftermarket cables of the LIYCY family are typically 1:1 but can vary by vendor. Continuity-test all six PROFIBUS signals:

Pin 3 (RxD/TxD-P) ↔ Pin 3
Pin 4 (RTS)        ↔ Pin 4
Pin 5 (DGND)       ↔ Pin 5
Pin 6 (VP)         ↔ Pin 6 (+5 V from CPU)
Pin 7 (P24 V)      ↔ Pin 7 (24 V from CPU — not used by some S7-300)
Pin 8 (RxD/TxD-N)  ↔ Pin 8

Note that on a TS Adapter II, the 5 V supply for the adapter electronics is taken from the bus (VP / pin 6). If pin 6 of the extension is open, the adapter may still light PWR if the bus has a second terminator feeding it, but the ACTIVE LED will not flicker because the RS-232 driver IC is starved.

Step-by-Step Resolution

1. Stop the CPU. Disconnect the TS Adapter from the MPI port.
2. Install the SIMATIC Teleservice option package on the engineering PC. Activate the license.
3. Set the RS-232 baud rate selector (S1) to position 2 (38,400 bit/s).
4. Set the MPI address selector (S2) to a value that is not in use on the bus; start with 0 only if you have confirmed no other PG/HMI is on that address.
5. Replace the user-supplied LIYCY serial extension with the Siemens 6ES7 901-1BF00-0XA0 null-modem cable, or rebuild the existing cable to the pinout shown above.
6. Power the CPU and wait for the MPI bus to stabilize (PWR LED of the adapter should be solid green within 2-3 s).
7. Launch SIMATIC Manager → Options → Set PG/PC Interface → Teleservice (TS Adapter).
8. Click Accessible Nodes. The ACTIVE LED must pulse each time Step 7 transmits a frame.
9. Confirm that the target CPU appears in the accessible-nodes list with the correct MPI address.
10. Open the project online. The diagnostic buffer of the CPU should show an entry "Online connection via TS Adapter".

Verification

After the steps above, the following indicators confirm a healthy TS Adapter V5.2:

• PWR LED: solid green while the CPU is powered and the MPI connector is engaged.
• ACTIVE LED: blinks at a 1-2 Hz rate when a PG/PC session is active; goes dark within 5 s of disconnect.
• Step 7 accessible-nodes scan: completes in < 5 s for a CPU at MPI address 2.
• CPU diagnostic buffer: entry "Connection established / removed" appears in chronological order.
• Functional test: read the OB1 cycle time and confirm a numeric value is returned in < 3 s.

Preventive Measures

• Always carry the Siemens RS-232 null-modem cable (6ES7 901-1BF00-0XA0) when commissioning remote-maintenance links. Aftermarket cables of unknown pinout are the single most common source of failed handshakes.
• Document the MPI address of every TS Adapter in the cabinet layout drawing. A duplicate address is silent on PWR but lethal to ACTIVE behavior.
• Label each TS Adapter with the firmware version ("V5.2", "V5.3") because Teleservice option packages are version-keyed. A V6.x Teleservice install is not forward-compatible with all V5.x adapters and vice versa.
• Periodically re-tighten the PROFIBUS D-sub connectors; vibration on cabinet doors is a known cause of intermittent VP loss.
• Keep the Teleservice option-disk set with the project documentation. Replacement PCs that have only STEP 7 V5.4 will not run the adapter out of the box.

Common Pitfalls and Field-Proven Caveats

• Confusing the adapter firmware ("V5.2") with the Teleservice software version. These are independent. The label on the housing is the firmware, not the software release.
• Using a USB-to-RS-232 converter. Many modern PCs lack a native DB-9. Off-the-brand USB converters introduce latency that breaks the 38,400 bit/s handshake. Use an industrial-grade converter based on the FTDI chipset and verify the COM port is set to 38,400 / 8 / N / 1 with hardware flow control.
• Forgetting to stop the CPU before reconnecting the adapter. Hot-plugging a TS Adapter on a live MPI bus can cause token loss. The CPU will then re-initialize the bus, during which the ACTIVE LED will appear to flicker for 10-30 s and then settle.
• Long MPI cables without termination. The bus must be terminated at both ends with PROFIBUS terminators (or built-in CPU terminator on S7-300 CPUs with DB-9). A 6 m LIYCY extension is fine; a 30 m run without termination is the first suspect when ACTIVE remains dark across multiple known-good adapters.
• AC-coupled RS-232 transceivers in low-cost USB converters. They strip the DC bias required by the TS Adapter's opto-isolated input, leaving the ACTIVE LED permanently dark even with perfect wiring.

FAQ