Resolving PROFINET No Cyclic Connection on SINAMICS G120C

Problem Overview

The PROFINET alarm A08526 "PROFINET - No cyclic connection" between a SIMATIC S7-1200 CPU and a SINAMICS G120C drive is one of the most common first-commissioning faults on SINAMICS PROFINET networks. The alarm text originates in the drive firmware and indicates the drive's PROFINET interface has not been able to establish the cyclic I/O exchange channel expected by the configured telegram. The S7-1200 reports the same condition through its PROFINET diagnostics buffer, and both devices surface the fault at the hardware level through a blinking red BF (Bus Fault) LED on the G120C and a blinking red ERROR LED on the CPU.

Three configuration items must remain synchronized for the cyclic channel to come up:

1. IP address - read back at r61001[0..3] on the drive and stored in p8921.
2. PROFINET Name of Station (device name) - read back at r61000[0..5] on the drive and stored in p8920.
3. Active PROFIdrive telegram - selected by p922[0..1] on the drive and matched by the controller's PROFINET slot configuration in TIA Portal.

If any one of these three items is out of sync between the G120C, the TIA Portal hardware configuration, and the installed GSDML, the drive raises A08526 and refuses all cyclic I/O. Diagnosis is systematic: read the drive's parameters, compare them to the TIA Portal configuration, fix the mismatch, and verify by monitoring the PROFINET station status from the S7-1200.

Affected Components and Versions

For drives running firmware V5.x the PROFINET parameters are consolidated under the p8900 group on PROFINET interface 1. The legacy diagnostic parameters r61000 and r61001 still display the active values, so they remain the fastest read-back on any operator panel regardless of firmware version.

Diagnostic Parameters and Read-Back Procedure

Two PROFINET diagnostic parameters on the G120C must be checked first when A08526 is active:

To read these values from the drive panel:

1. Set p0010 = 0 to release the parameter filter (commissioning filter must not be active).
2. Navigate to r61000 or r61001 on the IOP-2, BOP-2, or in StartDrive/Starter's parameter list.
3. Decode the device name from r61000[0..5] by splitting each 16-bit word into two ASCII characters. The result must match the Name of Station configured in TIA Portal exactly.

Note on name encoding: TIA Portal strips non-alphanumeric characters from the device name during hardware catalog entry. A logical name like afd645-lower-drums becomes the encoded string afd645xalowerxadrums5308, where x is a separator character and 5308 is a rolling suffix appended by TIA Portal at the time of device-name assignment. This is normal and the encoded string is what the drive stores in p8920.

LED Diagnostics

Root Cause 1 - Telegram Mismatch

The single most common cause of A08526 after device name and IP are confirmed correct is a PROFIdrive telegram mismatch. The drive and the controller each select a PROFIdrive telegram (Standard Telegram 1, Siemens Telegram 352, etc.). If the controller's hardware configuration sends a different telegram than the drive's p922[0] or p922[1] selection, the drive drops every AR attempt and A08526 remains active.

Verification procedure:

1. In TIA Portal, open the device view for the G120C and inspect Properties > PROFINET interface > Telegram configuration. Note the configured telegram number.
2. Connect StartDrive (or Starter) online to the G120C and read p922[0] and p922[1]. Each index is one PROFIdrive slot.
3. Confirm the values match exactly. A drive configured for Standard Telegram 1 but a controller sending Siemens Telegram 352 will not establish a cyclic connection.

Typical telegram selections for SINAMICS G120C on PROFINET:

Root Cause 2 - Device Name or IP Not Assigned

The Siemens Knowledge Base article on A08526 (Cause of alarm A08526 PROFINET No cyclic connection) documents three primary cause families:

1. PROFINET device name is not assigned.
2. PROFINET device name is incorrect on the device side.
3. Configuration mismatch between TIA Portal hardware configuration and the device.

Recommended assignment procedure for first commissioning:

1. In TIA Portal, right-click the G120C in the device view and select Assign PROFINET device name.
2. Select the target PROFINET interface (Port 1 / Port 2 if dual-port variants are used).
3. Click Assign name. TIA Portal writes the encoded string to the drive's p8920 over the standard PROFINET DCP protocol.
4. From Online > Accessible Nodes, verify the device now reports the assigned name and IP.

Always assign the device name before the IP. The DCP name-assignment is what TIA Portal uses to locate the physical station before it can write the IP through the standard PROFINET Record-Read/Write channel.

Root Cause 3 - S7-1200 Connection Resource Limits

The S7-1200 CPU has a fixed maximum number of connection resources, documented in the SIMATIC S7-1200 System Manual:

Each PROFINET IO device consumes one connection resource for the cyclic IO AR. Each SIMATIC HMI panel consumes multiple resources depending on the configured tag count and area count - a Comfort Panel with 200 tags typically consumes 4 to 6 connection resources.

When the S7-1200 runs out of free resources, the CPU refuses to establish additional PROFINET ARs and surfaces the warning "The maximum number of station resources has been reached" under PLC Properties > Communication > Connection Resources. The symptom in the field is: the first N drives come up cyclically, the (N+1)th drive raises A08526, and reordering drives does not change which one fails. This is the same fingerprint as a telegram mismatch - always check the connection-resource warning before re-cabling the network.

Corrective options:

• Reduce HMI tag count or split the HMI into multiple areas with smaller connection footprint.
• Move the HMI to a SIMATIC WinCC Unified Client or to a separate IPC.
• Migrate to an S7-1500 CPU which has a substantially higher connection-resource budget (CPU 1511: 96, CPU 1515: 192, CPU 1518: 256).

Aligning the Telegram with StartDrive

The recommended commissioning path to align the G120C telegram with TIA Portal:

1. Open the TIA Portal project and connect StartDrive online to the G120C over PROFINET.
2. Navigate to Communication > Telegram configuration.
3. Read p922[0] and p922[1]. These are the active PROFIdrive telegrams.
4. Click Change telegram configuration and select the same telegram(s) as configured in the TIA Portal hardware view.
5. Save to the drive's non-volatile memory using Copy RAM to ROM.
6. Power-cycle the drive, or run p0971 = 1 to activate the new telegram selection immediately.

If p2079 (extension of telegram selection) is used for free telegram configuration, the active telegrams are reported under p0922 in StartDrive and the PROFIdrive telegram index must still match the TIA Portal slot.

Aligning the Telegram with Starter (Legacy Tool)

For installed bases still using Starter, the path is equivalent:

1. Open the Starter project and go online to the G120C.
2. Select the drive and open Configuration > Telegram configuration.
3. Choose the matching telegram from the dropdown.
4. Save (RAM to ROM) and power-cycle.

PLC Configuration in TIA Portal

The PLC side of the configuration must be consistent with the drive:

1. Open the device view for the S7-1215.
2. Drag the G120C from the hardware catalog under Drives & Starter > SINAMICS G120C PROFINET.
3. Insert the drive on the PLC's PROFINET subnet.
4. Assign the PROFINET device name to match the encoded string the drive will accept.
5. Set the IP address to match r61001.
6. In the drive's properties, set the same telegram under PROFINET interface > Telegrams.
7. Compile and download the hardware configuration to the CPU.

If the telegram is changed in TIA Portal after the drive has been commissioned, repeat the StartDrive alignment step on the drive and power-cycle the drive before downloading the PLC.

Control Word Handling After Cyclic Recovery

Once the cyclic connection is up, the drive is in PROFINET control mode. The PLC must set bit 10 of control word 1 (STW1.10) to authorise PLC commands. STW1.10 is the master enable from the higher-level controller. If the bit is not set, the drive ignores all setpoints and reports fault F08501 "PROFINET - Sign-of-life failure" once the cyclic watchdog expires.

A robust pattern in TIA Portal (SCL):

// First scan on CPU RUN - hold drive in PLC control mode
IF "FirstScan" THEN
    #driveControlWord.%X10 := TRUE; // STW1.10 = Control by PLC
END_IF;

// On CPU STOP - clear the bit so the drive ramps to standstill
IF "CPU_Stopped" THEN
    #driveControlWord.%X10 := FALSE;
END_IF;

Hold STW1.10 = 1 for the entire interval between CPU RUN and CPU STOP transitions. Never leave the bit floating or relying on the drive's default state; many PROFINET fault scenarios involve a momentary false state on bit 10 leading to a ramp-down or a free-run stop.

Verification Procedure

Apply the fixes in this order and verify after each step:

1. G120C BF LED is off, RDY LED is steady green.
2. S7-1200 ERROR LED is off.
3. Online readback of r61000[0..5] matches the encoded device name assigned by TIA Portal.
4. Online readback of r61001[0..3] matches the IP address.
5. Online readback of p922[0] matches the TIA Portal telegram selection.
6. In TIA Portal, Online > Diagnostics > PROFINET devices, the station status for the G120C is OK.
7. The S7-1200 diagnostics buffer contains no further No cyclic connection entries for the G120C.
8. Using a Monitor & Force Table, write a small speed setpoint (e.g. 10% of p2000 reference) and observe the drive accelerates to the commanded speed. Read back the actual speed in r0021 for confirmation.

Related Alarms and Faults

For the A08526 family, always reference the Siemens KB article Cause of alarm A08526 PROFINET No cyclic connection. The article documents the three root-cause families and the standard escalation path.

Troubleshooting Matrix

Edge Cases and Field Notes

• Re-downloading a TIA Portal project overwrites the device name. If a drive was assigned by another tool, the next download will push the new name and the drive will appear to lose connection until the CPU is restarted.
• Some legacy SINAMICS G120C firmware versions below V4.5 do not support Siemens Telegram 353. Verify the GSDML in TIA Portal matches the firmware version.
• Dual-port PROFINET variants (xAPx) require the device name to be assigned to both ports individually, otherwise the controller sees an AR mismatch on port 2.
• If multiple TIA Portal projects were downloaded against the same drive, the device name may be non-deterministic. Use Online > Accessible Nodes to clear and re-assign the name.

FAQ