Reading SINAMICS V20 Fault Codes via USS in TIA Portal

Overview

The recurring question from first-time integrators of a SINAMICS V20 with a SIMATIC S7-1200 is: "How do I get the drive to send a fault back to the PLC?" The drive does not initiate communication. On a USS bus (and on any master/slave fieldbus), only the PLC can solicit a response, and the drive can only answer when asked. The fix is therefore not a single drive parameter, it is a coordinated set of: (1) V20 parameter configuration that exposes the status word and fault number, (2) USS_PYM library wiring in TIA Portal that polls the drive on a deterministic cycle, and (3) ladder or structured-text logic that decodes the response.

This reference walks through the entire loop using the official Siemens sample "Speed Control of a V20 with S7-1200 (TIA Portal) via USS® protocol/MODBUS RTU with HMI" (entry ID 63696870) and the underlying SINAMICS V20 ⇄ SIMATIC S7-1200 USS communication application document. After reading it, you should be able to convert any of the V20 Fxxxx faults into a Bool, a Word, and a HMI tag in the S7-1200.

Why the V20 Cannot "Push" Faults to the PLC

USS is a half-duplex, master/slave, point-to-multipoint protocol riding on RS-485. The S7-1200 is the parent; the V20 is the child. The child has no token, no interrupt, and no autonomous transmit rights. The drive only places data on the bus in response to a request that contains its node address (P2011). Forgetting this is the single most common reason a beginner assumes the drive is misconfigured when the wiring and parameters are in fact correct.

Two consequences for fault reporting:

• The status word (ZSW) is delivered automatically with every cyclic PZD poll, so fault bit ZSW.3 is essentially free once USS is running.
• The fault number (r0947 / r2131) is a parameter, not a process value, so it must be read acyclically through the PKW channel of USS or with a separate USS/MODBUS read request.

Prerequisites

• SINAMICS V20, any firmware (the USS behavior is stable across all V20 versions; current branch 6SL3210-5BE27-5VX0 supports it natively).
• SIMATIC S7-1200 CPU with RS-485 onboard (CPU 1211C/1212C/1214C/1215C/1217C) or a CB 1241 RS-485 communication board on a 1200 that lacks the port.
• TIA Portal V15.1 or later (V17/V18 recommended for current USS_PYM revisions).
• Siemens library block USS_PYM_232 from the global library shipped with the V20 sample project, or extracted from the application document attached to entry 63696870.
• Two-wire shielded RS-485 cable, terminated with 120 Ω at both physical ends.
• V20 default-reset to factory settings if you are starting from a used drive: P0010 = 30, P0970 = 1.

SINAMICS V20 Communication Interface

The V20's RS-485 port sits on the lower terminal row of the control board. The default assignment is fixed: pin "P+" = RS-485 data +, pin "N-" = RS-485 data -, and the shield must be bonded to PE at the drive end. The port supports both USS and MODBUS RTU; the active protocol is selected with P2013 (number of PKW words). When P2013 = 4, the protocol is USS; when set to 0, MODBUS RTU is enabled. Setting it to 0 with a PKW length of 0 disables PKW and is the typical Modbus configuration.

Wiring topology

Use a daisy chain, not a star. The S7-1200 is bus master at one physical end, the last V20 is at the other end, both with 120 Ω termination resistors in parallel. Each drive in between has its termination disabled. The total stub length from any node to the trunk should not exceed 1 m.

V20 Parameter Configuration for USS Fault Reporting

The complete minimum parameter set to expose drive health to a S7-1200 over USS is shown below. Numbers are taken from the V20 Operating Instructions and the V20 List Manual, both linked from the Siemens Online Support entry 63696870.

Save with P0971 = 1 (transfer from RAM to EEPROM) before cycling power, or set P0010 = 0 and P0970 = 1 for a full factory reset and re-parameterization.

Status Word (ZSW) and Fault Word Bit Decoding

The V20 returns a 16-bit status word as the first PZD of every USS telegram. Faults and warnings are encoded in fixed bits, identical to the SINAMICS S/G/S120 family. The bit numbering below is the convention used in TIA Portal (bit 0 = LSB).

TIA Portal USS_PYM Library Integration

USS_PYM_232 is a function block published by Siemens specifically to drive a SINAMICS V20 from an S7-1200. The block handles the cyclic PZD, the acyclic PKW (read/write parameter by index), the inter-character timeout, and the bus idle timer. It is imported as a global library: Options → Global libraries → Open library, navigate to the library shipped with the application document and confirm the trust-rights dialog.

Block interface (key I/O)

Drop one USS_PYM_232 instance per V20 on the bus. Wire PORT to the hardware identifier of the RS-485 port, set BAUD to the same code as P2010, set ADDR equal to P2011, and connect the cycle output of a 20-ms cyclic OB to the same DB that builds the cyclic PZD. The PZD exchange is implicit; the REQ edge is only required if you want to read an additional parameter (e.g. r0947 for the fault number).

Reading Fault Numbers Acyclically with PKW

Bit 3 of ZSW tells you a fault is present but not which one. To obtain the actual F-code, read one of the following V20 parameters through the PKW channel:

When indexing r0947 in TIA Portal, address index 0 for the latest fault, index 1 for the previous, up to index 7. The V20 keeps eight history entries; you can extend the PLC code to walk them all on a fault edge if you need an HMI log.

Structured Text Code Example

The snippet below shows the typical S7-1200 logic to flag a fault, read the F-code on a rising edge, and surface both to the HMI. It is intended for the OB that runs the USS_PYM_232 instance and a 50-ms watchdog.

// DB "V20_Fault_DB"
// Inputs from USS_PYM_232
iNodeAddr      : INT  := 1;       // matches P2011
iBaud          : INT  := 6;       // 6 = 9600 baud
wPZD_OUT       : ARRAY[0..1] OF WORD;  // ZSW, NIST
wPKW_RESULT    : WORD;            // result from last PKW read
bUSS_Done      : BOOL;
bUSS_Error     : BOOL;
wUSS_Status    : WORD;

// Derived flags
bFaultActive   : BOOL;            // mirrored from ZSW bit 3
bWarningActive : BOOL;            // mirrored from ZSW bit 7
wFaultCode     : WORD;            // F-code from r0947[0]
dFaultTime     : DWORD;           // r0948[0] in ms
bNewFault      : BOOL;            // edge memory

// 1. Decode ZSW from the cyclic PZD (auto-delivered)
bFaultActive   := wPZD_OUT[0].%X3;   // ZSW bit 3 = FAULT
bWarningActive := wPZD_OUT[0].%X7;   // ZSW bit 7 = ALARM

// 2. On rising edge of bFaultActive, request a PKW read of r0947
IF bFaultActive AND NOT bNewFault THEN
    bNewFault   := TRUE;
    bPKW_Req    := TRUE;          // connect to USS_PYM_232.REQ
    wPKW_Param  := 947;           // r0947
    bPKW_Write  := FALSE;         // read mode
END_IF;

// 3. On completion, latch the fault code
IF bPKW_Req AND bUSS_Done AND NOT bUSS_Error THEN
    wFaultCode  := wPKW_RESULT;
    bPKW_Req    := FALSE;
END_IF;

// 4. Fault clears on ZSW.3 going low and user ack on STW.7
IF NOT bFaultActive THEN
    bNewFault   := FALSE;
    // optional: also clear wFaultCode after HMI has read it
END_IF;

// 5. Optional: HMI mapping
//   HMI tag "DriveFaultActive"  <- DB.bFaultActive
//   HMI tag "DriveFaultCode"    <- DB.wFaultCode
//   HMI tag "DriveWarning"      <- DB.bWarningActive

SINAMICS V20 Fault Code Reference

The most common V20 fault numbers reported in the field. Full list is in the V20 List Manual on Siemens Online Support (entry ID 109479590).

Commissioning Verification Procedure

1. Power up the V20 with the RS-485 cable disconnected. Navigate to P0010 = 0 → P2010 = 6 (9600 baud) → P2011 = 1 (node 1) → P2012 = 2 → P2013 = 4 → P2014 = 500 ms. Save with P0971 = 1.
2. Set command source: P0700 = 5, setpoint source: P1000 = 5.
3. Power down, connect the RS-485 cable between the S7-1200 and the V20, and enable termination only on the drive (P2023 = 2) and the S7-1200 side (DIP switch on the CPU or CB 1241).
4. In TIA Portal, import the global library containing USS_PYM_232. Place a single instance DB in the project tree, wire PORT to the RS-485 hardware identifier.
5. Insert the instance call inside a 20 ms cyclic OB. Compile and download to the S7-1200.
6. Go online and monitor the instance STATUS output. A value of 16#0000 means the bus is healthy. Anything in the 16#8xxx range is a bus or timeout error.
7. Force the ZSW word in the watch table. At idle, ZSW = 16#0C31 (bits 0, 1, 4, 5 set = ready and inhibited). If you see 16#0C30 for more than a second, your bus is dropping.
8. From the S7-1200, raise STW.0, then STW.1, then STW.3 in that order. ZSW bits 0, 1, 2 must follow. The drive is now in Operation enabled.
9. Trigger a fault (e.g. unplug the motor). Verify that within one telegram cycle, ZSW.3 rises in the PLC, and the read of r0947 returns the F-code for "motor overload".
10. Acknowledge with STW.7 edge 0→1. ZSW.3 must clear within one cycle.

Troubleshooting Matrix

Edge Cases and Field-Proven Caveats

PKW index handling: USS_PYM_232 always reads the value of the parameter, not the parameter descriptor. To walk the fault history, request parameter 947 with index 0, then 1, etc. The drive returns 0 once the history is exhausted. Do not confuse this with the parameter number itself.

Multi-drive bus: Multiple V20s on the same RS-485 port is supported up to 31 nodes. Each drive must have a unique P2011, and each PLC application should instantiate one USS_PYM_232 per drive. Sharing a single instance between drives will not work because the function block holds the bus for the duration of the transaction.

Long cables and baud: At 9600 baud, the V20 supports up to 1000 m of cable. At 115200 baud, drop to 50 m. If you must go further or faster, move to PROFIBUS with a SINAMICS V20 + PROFIBUS option (which the V20 does not have natively) or upgrade the drive to a SINAMICS V90/G120 with PROFINET.

Drive fault buffer reset: Reading r0947 does not clear the fault. To clear, the PLC must (a) drop STW.3, (b) drop STW.0, then (c) raise STW.7 as a pulse after ZSW.3 has fallen. Skipping step (b) leaves the drive in the "inhibit" state.

Fault code 0 vs. no fault: A return value of 0 from r0947[0] does not mean communication is fine. It means the fault buffer is empty. If you trigger a read while ZSW.3 = 1 and you get 0, the bus is probably answering with the wrong index. Check the index parameter of the PKW request and confirm P2013 is non-zero.

FAQ