SINAMICS G120C: Masking F8501 PROFINET Communication Fault

1. Problem Statement

A SINAMICS G120C Profinet (PN) drive trips with fault F08501 the moment the PROFINET controller drops the connection or the PLC transitions to STOP / CLEAR. The default reaction is OFF1 (controlled ramp stop via the ramp-function generator defined by p1121), which halts the motor. In processes where the application requires the drive to keep running from terminal commands when the fieldbus link is lost, the controller must be reconfigured so that F08501 no longer stops the drive and so that a Command Data Set (CDS) switchover moves control from PROFINET back to digital inputs. The same approach applies to the G120C DP (PROFIBUS) variant using F01910 and F07220.

The behavior matches what is already familiar from the legacy MICROMASTER 4 (MM440), where P2040 could be set to 0 to disable the fieldbus watchdog. The SINAMICS platform does not expose a single P2040-style toggle; instead, the same effect is achieved by remapping the fault reaction through the P2100 / P2101 pair.

2. Affected Drives and Fault Codes

Note: the user-supplied notation "F8501" in the field is the common short form of F08501 as displayed in older STARTER / Startdrive alarm windows. The fault number is identical. The same applies to F1920 ↔ F01910 and F7220 ↔ F07220.

3. Root Cause Analysis

When the G120C receives a PROFINET frame, the on-board PROFINET interface expects another valid frame within the configured sign-of-life fail-safe time. Three conditions raise F08501:

1. The PROFINET controller enters the CLEAR state (PLC goes to STOP, OB1 is no longer scheduled, or the AR is aborted).
2. The watchdog timer expires. The timeout is set by the controller in the PROFINET AR and is typically 3 missed cycles (default ~30-50 ms depending on the send clock).
3. The physical link is broken (cable removed, switch port disabled, EMC-induced link flap).

By default, the SINAMICS fault handler in P2100 / P2101 reacts to F08501 with the standard reaction defined in the firmware: OFF1. The drive decelerates along the ramp defined by p1121, coasts if the fieldbus also carried the OFF2 command, and the process stops. There is no P2040-style "disable comm monitoring" parameter on G120C; instead, the fault reaction itself must be remapped.

4. Solution: Masking F08501 with P2100 / P2101

The P2100 / P2101 pair is the standard SINAMICS mechanism for overriding the reaction of any internal fault or alarm. It is not vendor-locked; the same mechanism masks F30003 (DC-link undervoltage), F07900 (motor blocked), and any other Fxxxxx in the firmware.

4.1 Parameter Semantics

4.2 G120C PN — Mask F08501

Use the BOP-2, IOP, STARTER, or Startdrive commissioning tool to enter the following values. This example assumes index 0 is free:

1. Set p2100[0] = 8501 (PROFINET communication failure).
2. Set p2101[0] = 0 (reaction = NONE, i.e. alarm only, no stop).
3. Save with p0971 = 1 or use the tool's "Copy RAM to ROM" button.

After this change, when the PROFINET connection drops, the drive will issue A08501 (an alarm, not a fault) and continue running with the last valid setpoint. The bit r2139.0 (fault present) and r2138.7 (alarm present) update accordingly; the drive status word will still show the Operation enabled bit set as long as the contactor close / enable bits remain on.

4.3 G120C DP — Mask F01910 and F07220

On the PROFIBUS variant the failure is reported by two faults, both of which should be masked together:

1. Set p2100[0] = 1910, p2101[0] = 0.
2. Set p2100[1] = 7220, p2101[1] = 0.
3. Save with p0971 = 1.

With both reactions set to NONE, losing the PROFIBUS connection no longer stops the drive. The process is now identical to the G120C PN configuration described above.

5. Masking F30003 (DC-Link Undervoltage) by the Same Method

The same p2100 / p2101 pair masks F30003. The source confirms:

"Set P2100[x] = 30003 & p2101[x] = 0. You need to set p2100 index [x] value which is already 0 & corresponding p2101 index [x] to 0 e.g. if p2100[4] is 0, modify it to 30003 & p2101[4] to be set 0."

1. Set p2100[4] = 30003.
2. Set p2101[4] = 0.
3. Save with p0971 = 1.

6. PLC Integration: CDS Switchover Logic

Once the fault reaction is NONE, the drive will keep running with the last PROFINET setpoint forever. That is not what most applications want; the correct topology is:

1. Define two Command Data Sets on the drive:
   • CDS0 = control from digital inputs (terminals), reference from analog input 1.
   • CDS1 = control from PROFINET, reference from PROFINET PZD2.
2. Set p0170 = 2 to enable two CDS.
3. Wire p0820 and p0821 (CDS selection bit 0 and bit 1) to a free digital output or to a control word bit that the PLC can write via PZD1.
4. In the PLC, monitor ZSW1 (Status Word 1, PZD1 received from the drive). When the connection is lost the fieldbus driver clears the input image, the status word reads zero, and the PLC must switch to CDS0.

6.1 Status Word (ZSW1) Bit Map

6.2 TIA Portal SCL — PROFINET Link-Down Detection and CDS Switchover

Sample code block for a SIMATIC S7-1200/S7-1500 with PROFINET to the G120C PN. Replace the data block / IO addresses with the actual configuration from the device configuration.

// FB: G120C_CommLossHandler
// Inputs
//  i_hwIdDrive    : HW_IO  Hardware identifier of the drive's input slot
//  i_dbDrive      : BLOCK_DB  Symbolic DB mapped onto the drive PZDs
//  i_bProfiOK     : BOOL  TRUE while the PROFINET AR is established
// Outputs
//  q_bUseTerminal : BOOL  TRUE -> drive should follow CDS0 (terminals)
//  q_bAlarm       : BOOL  TRUE -> raise operator alarm

// Drive status word is at i_dbDrive.%DBW0 (ZSW1, PZD1)
// Drive control word is at i_dbDrive.%DBW2 (STW1, PZD1 outgoing)

// Detect comm loss:
// 1. PROFINET AR aborted -> i_bProfiOK FALSE
// 2. ZSW1 == 0          -> fieldbus input image is zeroed by the controller
#iZSW1 := WORD_TO_INT(i_dbDrive.%DBW0);

IF (NOT i_bProfiOK) OR (iZSW1 = 0) THEN
    // Communication lost -> force CDS0 (terminal control)
    // CDS bit 0 = 0 selects CDS0; CDS bit 0 = 1 selects CDS1
    i_dbDrive.%DBX4.0 := FALSE;   // p0820 = CDS bit 0 = 0

    // Issue controlled ramp stop via the control word
    // STW1 bit 0 = ON/OFF1, bit 1 = OFF2, bit 2 = OFF3
    // Set bit 0 = FALSE to ramp down with p1121
    i_dbDrive.%DBX3.0 := FALSE;   // OFF1
    i_dbDrive.%DBX3.1 := TRUE;    // No OFF2 (keep torque available for regen)
    i_dbDrive.%DBX3.2 := TRUE;    // No OFF3

    q_bUseTerminal := TRUE;
    q_bAlarm       := TRUE;
ELSE
    // Link is up: PLC controls the drive via PROFINET (CDS1)
    i_dbDrive.%DBX4.0 := TRUE;    // p0820 = CDS bit 0 = 1 -> CDS1
    q_bUseTerminal := FALSE;
    q_bAlarm       := FALSE;
END_IF;

When i_bProfiOK drops, the S7-1500 also zeros the entire process input image for the missing AR. The S7-1200 CPU behaves the same way. The PLC must not only remap the CDS — it must also stop the drive gracefully, because the masking means the drive itself will not stop on comm loss.

6.3 Ladder Alternative

// Rung 1: comm-loss detector
|  NOT  i_bProfiOK            |
|  OR   (i_dbDrive.DBW0 == 0) |
|------( )q_bUseTerminal------|

// Rung 2: pick CDS bit
|  q_bUseTerminal  |
|-----------------(/)---+---( )i_dbDrive.DBX4.0---|
|  NOT q_bUseTerminal |
|---( )i_dbDrive.DBX4.0---|

// Rung 3: ON/OFF1 (STW1 bit 0)
|  q_bUseTerminal  |
|---(/)i_dbDrive.DBX3.0---|

7. Complete Parameter Reference

8. Verification Procedure

Verify the masking on a real test rig before going into production. The order matters: confirm fault reaction override first, then verify the PLC's comm-loss response.

1. Baseline. With default parameterization, unplug the PROFINET cable. Drive should ramp down on F08501 with default reaction OFF1. Record p1121 and the deceleration time.
2. Apply the masking. Write p2100[0] = 8501, p2101[0] = 0, and p0971 = 1. Power cycle the drive so the new parameters load from ROM.
3. Repeat the cable pull. Disconnect the PROFINET cable while the drive is running at 50% speed. The drive should continue at 50% speed and the HMI should show A08501 (alarm) instead of F08501 (fault). Status word r0052.0 (drive running) should remain TRUE.
4. PLC verification. With the drive still at 50% speed and the cable disconnected, observe the S7 input image. It should zero out, i_bProfiOK should go FALSE, and the PLC should drive STW1.0 FALSE to command OFF1 on the drive. Verify the drive ramps down on p1121.
5. Restoration. Reconnect the cable. The PLC should detect i_bProfiOK = TRUE, set CDS1, and resume PROFINET control. Verify the drive accelerates back to the new PROFINET setpoint.
6. Reset faults. If you want to clear the alarm history after a test, use p0952 = 0 (no fault case counter) or trigger OFF1 then OFF1 on the control word. The CDS switchover in CDS0 should remain in place until the PLC explicitly switches back.

9. Alternative Approaches

Masking the fault reaction is one of several methods to deal with comm loss. Evaluate the alternatives against the safety and process requirements before adopting any of them.

10. Field-Commissioning Caveats

• Firmware version: p2100 indexing on G120C has varied across firmware versions. On older V4.4 SP3 the array length was 4 indices; on V4.7 it is still 4 in the standard dataset, but the list manual may show more for newer G120C variants. Always read p2100 from the drive to confirm free indices before writing.
• Indexing reset: Writing p2100[x] = 0 does not free the index for re-use on all firmware. If you change the fault number on a used index, the previous fault's reaction is restored. Always write to a known-free index.
• Cold start vs. warm start: On power-up the masked reaction loads from ROM. If the masking was only done in RAM (p0971 not executed), a power cycle will restore the default reaction.
• BOP-2 navigation: On the BOP-2, the index is set with the up/down arrow on the parameter number — easy to overlook. The BOP-2 displays the index as a small superscript; STARTER / Startdrive displays the array index in the inspector window.
• CDS bit source: If p0820 is wired to a digital input, the PLC cannot switch the CDS from PROFINET. Use a control-word bit mapped through r2090 if you need PLC-driven selection.

11. Troubleshooting Matrix

12. Frequently Asked Questions