Troubleshooting SINAMICS V20 Modbus Control Word 40100 Not

Overview

A common commissioning symptom on the SINAMICS V20 family is that the drive accepts every Modbus request from the PLC except those that target the control word at holding register 40100 (STW1) and the main setpoint at 40101 (HSW). Reads of 40100–40150, reads of fault and warning registers, and writes of ramp times such as 40322 (acceleration) and 40323 (deceleration) all succeed. The drive even acknowledges the write to 40100 with the expected Modbus response, yet it never closes the output stage or applies the speed setpoint. On the same drive, the local Run and Stop keys on the BOP continue to start and stop the motor even though P0700 = 5 (Modbus RTU) is supposed to disable BOP command authority.

This combination of symptoms almost always points to one of three root causes: (a) bit 10 of STW1 is not being set by the PLC, (b) the parameter that maps register 40100 to the drive's internal control word (P2023) has been changed off the default, or (c) the parameter set is internally corrupted, which is why the BOP continues to function despite P0700 = 5. The fastest resolution is a factory reset followed by a clean Modbus commissioning pass.

Affected Hardware, Firmware, and Software Versions

Reference: SINAMICS V20 List Manual (parameter description) and S7-1200 System Manual.

SINAMICS V20 Modbus Register Map (Holding Registers, FC 03/06/16)

The V20 implements a fixed subset of the SINAMICS V-modbus map. Only the registers shown below are relevant to control word and setpoint exchange. Any write outside this subset is answered but ignored.

Reference: SINAMICS V20 Modbus / USS connection guide.

SINAMICS V20 Control Word 1 (STW1) Bit Structure at Register 40100

The drive ignores 40100 unless the bits marked mandatory are set correctly. The two power-on states observed on a healthy drive are:

• 0x047E (1150 dec) – ready, not rotating, bit 10 = 1 (PLC control)
• 0x047F (1151 dec) – ready and rotating (ON/OFF1 = 1)

Required Parameter Configuration

Confirm every parameter in this table before further diagnostics. The defaults shown are the values needed for a Modbus-RTU master on CB1241 at 9600 baud, 8E1.

Root Cause Analysis

From the symptom set reported, the root causes rank as follows by frequency:

1. Bit 10 of STW1 not set (most common). The PLC writes 0x007E instead of 0x047E. Modbus responds with the same ACK as a successful write; the drive simply refuses to interpret the word because the "Control by PLC" bit is missing. This is why manual ramp-time writes to 40322/40323 work – those registers do not require bit 10 to be processed.
2. P2023 / P2024 remapped. A previous integrator changed the source registers of the two PZD words. The drive still ACKs writes to 40100/40101 because the Modbus layer is alive, but the values are not forwarded to the internal control word buffer.
3. Parameter set corruption. The drive ACKs Modbus writes but never applies them; simultaneously, BOP commands continue to work despite P0700 = 5. The internal parameter save area (PSU/EEPROM image) has been written in a way the runtime can no longer apply. This is a known failure mode on field-aged V20 units and on units that lost power mid-parameter-write.
4. Fault F72 present but not visible on BOP. A standing F72 (telegrams lost) inhibits the fieldbus control word. F72 should appear in r0947; if it is present, it must be acknowledged before 40100 takes effect.

Diagnostic Procedure (Step-by-Step)

1. Read 40102 and 40103 with FC 03. Confirm ZSW1 returns a non-zero value that changes when you press the BOP Run key. If it does, the fieldbus link itself is healthy.
2. Read 40150. If it returns a non-zero code, note it. F1 = overcurrent, F2 = overvoltage, F5 = I²t, F72 = USS/Modbus telegram failure, F85 = external fault. Acknowledge any standing fault by toggling bit 7 of STW1.
3. Write STW1 = 0x047E to register 40100 (FC 06) and read it back. The read-back must equal 0x047E. If it returns 0x007E, the PLC is overwriting bit 10 somewhere in the telegram FB before MB_MASTER is called.
4. Read r0700 and r1000 (registers 40200/40201 indirect via PNU access). Confirm r0700 = 5 and r1000 = 5. If r0700 reads back as anything other than 5, the active command source is not Modbus regardless of what you think you wrote.
5. Read r2023 and r2024 directly (P2023 and P2024 read-back). Both must be 40100 and 40101 respectively.
6. Toggle bit 0 only. With STW1 held at 0x047E, write 0x047F and observe r0021 (motor frequency) at register 40103. If frequency does not move while ZSW1 bit 2 (drive running) remains 0, the parameter set is suspect; proceed to factory reset.
7. Check r0947[0] continuously for F72. If F72 appears, raise P2040 or set it to 0 to disable the watchdog.

Factory Reset and Clean Re-Commissioning

A factory reset returns all parameters to factory defaults, clears the parameter set inconsistency that lets BOP override P0700 = 5, and is the documented recovery for any case where the drive is otherwise unresponsive on Modbus. There are two equivalent methods.

Method 1 – via BOP

1. Power-cycle the drive (24 V aux + 400 V main).
2. Within 10 seconds of power-up, hold the FN key for 3 seconds – the BOP will display "P0000" flashing.
3. Press OK to confirm. The drive resets, displays "-----" briefly, and reboots.

Method 2 – via Modbus parameter write

1. Write P0010 = 30 (commissioning filter for factory reset) via Modbus parameter access.
2. Write P0970 = 1 (start factory reset).
3. Wait until r0947 clears or the drive drops off Modbus for 5–10 seconds.

Clean Modbus commissioning sequence (post-reset)

1. Set P0010 = 1 (quick commissioning) and P0100 = 0 (Europe, 50 Hz defaults).
2. Set motor data: P0304, P0305, P0307, P0310, P0311 for your motor nameplate.
3. Set P0700 = 5 and P1000 = 5.
4. Set P2010[0] = 6 (9600 baud), P2011[0] = 1 (station address), P2012[0] = 2 (8E1).
5. Set P2023[0] = 40100 and P2024[0] = 40101 (these are the defaults; explicitly writing them ensures the parameter set is consistent).
6. Set P2040 = 0 to disable fieldbus watchdog during commissioning, or to 1000 ms with active cyclic writes from the PLC.
7. Set P0010 = 0 to exit commissioning.
8. Cycle power once, then write STW1 = 0x047E followed by 0x047F with HSW = 0x4000. The motor must now run at P2000 frequency (default 50 Hz).

TIA Portal V19 – S7-1200 Side Assembly of STW1

The Siemens Modbus example for V20 on TIA Portal V19 expects you to build STW1 in a single network before calling MB_MASTER. The canonical word construction is shown below as structured text.

// Build STW1
"DB_V20".STW1.%X0   := "HMI".CmdStart;          // bit 0  ON/OFF1
"DB_V20".STW1.%X1   := TRUE;                    // bit 1  OFF2
"DB_V20".STW1.%X2   := TRUE;                    // bit 2  OFF3
"DB_V20".STW1.%X3   := TRUE;                    // bit 3  Pulse enable
"DB_V20".STW1.%X4   := TRUE;                    // bit 4  RFG enable
"DB_V20".STW1.%X5   := TRUE;                    // bit 5  RFG freeze
"DB_V20".STW1.%X6   := TRUE;                    // bit 6  Setpoint enable
"DB_V20".STW1.%X7   := "HMI".AckFault;          // bit 7  Fault ack (edge)
"DB_V20".STW1.%X8   := FALSE;                   // bit 8  reserved
"DB_V20".STW1.%X9   := FALSE;                   // bit 9  reserved
"DB_V20".STW1.%X10  := TRUE;                    // bit 10 PLC control <-- REQUIRED
"DB_V20".STW1.%X11  := "HMI".CmdReverse;        // bit 11 Reverse
"DB_V20".STW1.%X12  := FALSE;                   // bit 12 reserved
"DB_V20".STW1.%X13  := FALSE;                   // bit 13 MOP up
"DB_V20".STW1.%X14  := FALSE;                   // bit 14 MOP down
"DB_V20".STW1.%X15  := FALSE;                   // bit 15 CDS bit 0

// Build HSW (scaled to P2000)
"DB_V20".HSW := REAL_TO_INT("HMI".SpeedSetpoint_Pct * 163.84);

// Write STW1/HSW as a single Modbus transaction, FC 16
MB_MASTER_DB(
  REQ       := "Cyclic_100ms".RunFlag,
  MB_DB     := "MB_COMM_LOAD_DB",
  MODE      := 1,                              // RTU master
  DATA_ADDR := 40100,
  DATA_LEN  := 2,
  DATA_PTR  := "DB_V20".STW_HSW_WordArray,    // {STW1, HSW}
  DONE      => "DB_V20".MB_Done,
  BUSY      => "DB_V20".MB_Busy,
  ERROR     => "DB_V20".MB_Error,
  STATUS    => "DB_V20".MB_Status);

Verification

1. With the drive powered and HMI/PLC in control, set HMI start = FALSE and confirm register 40102 (ZSW1) bit 2 (drive running) is 0 and bit 0 (ready) is 1.
2. Set HMI start = TRUE. Confirm 40102 bit 2 transitions to 1 within 100 ms (one Modbus cycle) and 40103 (HIW) ramps to the value requested.
3. Set HMI start = FALSE. Confirm the motor decelerates per P1121 and ZSW1 bit 2 returns to 0.
4. Disconnect the CB1241 cable. After P2040 ms, the drive should fault F72; reconnect, toggle bit 7 of STW1, and confirm fault clears.
5. Set HMI start = TRUE while a digital-input stop (P0702/P0703 etc.) is wired and active; ZSW1 bit 0 must remain 0, confirming the OFF2 path is independent of the fieldbus.

Troubleshooting Matrix

Safety and Commissioning Notes

• Always separate low-voltage signal wiring from line-voltage motor wiring. The CB1241 ground terminal must be tied to the cabinet PE bar at a single point.
• Before any Modbus session, fit an RS485 terminator (120 Ω) at each end of the bus only. The CB1241 ships with selectable termination – leave it OFF if a third-party terminator is fitted.
• After a factory reset, the drive reloads the motor model from P0304–P0311. If your motor was originally matched by the wizard, re-run quick commissioning (P0010 = 1) before resuming production.
• Save the parameter set with P0971 = 1 after every successful commissioning pass. A subsequent corruption can then be cleared by reloading via SD card (V20 parameter clone) rather than a full factory reset.
• Never write P0010 = 30 during a running production cycle. The factory-reset P0970 = 1 cannot be aborted.

FAQ