Configuring Free Telegrams on SINAMICS G120D Drives

Overview: Telegram Types and the SINAMICS G120D

The SINAMICS G120D is a distributed inverter in the SINAMICS family, designed for cabinet-free installation close to the motor and connected over PROFINET or PROFIBUS. Every drive on a PROFINET/PROFIBUS segment exchanges its process data with the controller (S7-1200, S7-1500, ET 200, or third-party PLC) using a fixed-length process-data block called a telegram (or PZD block in PROFIdrive terminology). Siemens offers three categories of telegram for the G120D:

• Standard telegrams (Tel. 1–7): PROFIdrive-compliant, vendor-predefined PZD layouts (e.g. Standard telegram 1 = STW1/ZSW1 + NSOLL_A/NIST_A, Standard telegram 3 = 9/5 words for speed/positioning). They are selected directly in TIA Portal under device configuration → Telegram.
• Siemens telegrams (Tel. 20, 220, 350, 352, 354, 999 base): vendor-specific layouts tuned for use with function blocks such as SinaSpeed, positioning, or safety.
• Free telegram configuration (p922 = 999): a fully user-defined PZD layout where every word is wired through BICO to a parameter of your choice.

The free telegram (telegram 999) is the right answer when a standard layout does not carry the exact signals your application needs, when you must read torque, current, temperature, or process status bits alongside speed, or when you are migrating a legacy drive and want exact bit-for-bit compatibility. Because every PZD word is mapped individually, free telegrams also give full control over the bit layout, which is essential for diagnosing swapped-endian traffic.

Prerequisites and Hardware Identification

Before changing telegram layout, capture the as-delivered configuration and confirm the following items.

The function diagrams referenced throughout this article are documented in the SINAMICS G120/G120D/G120C List Manual, available from the official Siemens Industry Online Support portal at support.industry.siemens.com. The communication function diagrams (FP2410, FP2420, FP2440, FP2450, FP2460, FP2470, FP2480, FP2490) cover telegram selection, PZD extension, status/control words, and free telegram mapping.

Selecting the Right Telegram Strategy

Engineers routinely face three legitimate paths; the right one depends on what you are trying to do.

1. Standard telegram only. Use Tel. 1 (4 PZD) for classic speed control via PROFIdrive. STW1/ZSW1 give command and status, NSOLL_A/NIST_A give setpoint and actual. No additional configuration required.
2. Telegram extension (Tel. 1 + extension). Keep Tel. 1 as the base and append additional PZD words through p2051 and p2061. This is the recommended approach when you want to add a few extra words (e.g. torque actual, fault code) without abandoning the standard profile. TIA Portal exposes this as the "Activate telegram extension" checkbox under Communication → Telegram configuration.
3. Free telegram (Tel. 999). Use when the base layout is wrong, when you need bit-level control over every word, or when interfacing with non-PROFIdrive controllers (third-party PLC, custom protocol bridges).

Most G120D projects start with the standard telegram and extend it. The free telegram becomes necessary when you must select telegram words that the standard layout simply does not expose, or when you are forced to keep the PZD length identical to a non-Siemens drive for swap-out scenarios.

Status Word (ZSW1, r0052) Bit Mapping

The status word ZSW1 is read from parameter r0052 and is mandatory in any PROFIdrive-compliant telegram. In a free telegram you can choose to expose ZSW1 as a whole word or to slice individual bits into separate PZD words — both are common. The standard bit assignments are:

A common question is whether to read ZSW1 from r0052 directly or from a sliced alternative such as r0052.0, r0052.3, etc. The difference is purely one of presentation:

• Whole word (r0052): a 16-bit value is packed into one PZD. The controller unpacks the bits via masking (Status AND 16#0001 for bit 0).
• Bit-sliced (r0052.0, r0052.1, …): individual boolean signals are wired to individual bits of one or more PZD words. This is the recommended approach when a free telegram is being assembled because it lets you combine drive-side booleans (e.g. fault, warning) with integer signals (actual speed, current) in the same compact layout.

Configuring a Free Telegram in STARTER

Open the project in STARTER, double-click the G120D drive, and navigate to Communication → Telegram configuration. The exact menu sequence is: drive tree → right-click → Properties → Communication tab → Telegram configuration. From the drop-down list, select Free telegram (p922 = 999) or pick a specific base telegram and click "Activate telegram extension".

1. Set p922 to 999 via the parameter list, or select "Free telegram" in the dialog. STARTER will then offer editable slots for p923 (drive → controller) and p924 (controller → drive).
2. For each PZD index, type the parameter number whose value should occupy that word. Examples:
   • p923[0] = 52 — PZD 1 send = Status word 1 (r0052)
   • p923[1] = 21 — PZD 2 send = Actual speed smoothed (r0021)
   • p923[2] = 31 — PZD 3 send = Actual torque smoothed (r0031)
   • p924[0] = 3700 — PZD 1 receive = Control word 1 (r0050 source via p2050[0] = 3700)
   • p924[1] = 1150 — PZD 2 receive = Speed setpoint (r1150)
3. If you need bit-level signals, use the indexed bit notation. p923[3] = 52.3 sends bit 3 (FAULT) of the status word into PZD 4. Repeat for as many individual bits as needed.
4. Click Load to drive and then Copy RAM to ROM so the layout survives a power cycle.
5. Run Commissioning → Optimization → Calculate motor data if parameters have been altered before normal operation.

The same parameters can also be edited directly in the expert list:

p922 = 999      ; Free telegram selection
p923.1 = 52      ; PZD 1 send → ZSW1 (r0052)
p923.2 = 21      ; PZD 2 send → n_act (r0021)
p923.3 = 31      ; PZD 3 send → M_act (r0031)
p924.1 = 3700    ; PZD 1 receive → STW1 source
p924.2 = 1150    ; PZD 2 receive → speed setpoint source

The receive side must be wired with a BICO source, not just a parameter number, because the controller's output words are normally fed into the control word 1 (r0050) via p2050, and p1150 for the speed setpoint. The indices of p923/p924 correspond 1:1 with the slot indices in the PROFINET telegram mapping.

BICO Interconnection Principles

SINAMICS uses a BiCo (Binector/Connector) model to route signals between parameters. A connector is a numeric (word or double-word) signal; a binector is a single bit. Free telegram assignment is nothing more than wiring a connector sink (each PZD slot) to a connector source.

The crucial BICO detail for free telegrams is that p923/p924 describe the telegram layout, while p2050/p2051/p2060/p2061 describe the I/O interconnection. Both must agree. A frequent commissioning error is to set p923[2] = 31 (expecting torque) but leave p2061[0] unset, in which case the controller receives zero. Always cross-check p2051/p2061 against p923 after a parameter download.

Endianness, Word Swapping, and Bit Packing

When the controller is a third-party PLC, an OPC UA gateway, or a non-S7 master, the engineer must verify that both ends interpret each 16-bit PZD word in the same byte order. PROFINET transport is big-endian on the wire (per IEEE 802.3 and PROFIdrive profile V4.2), but the bytes inside a 16-bit PZD word are defined as little-endian word — high byte first in PZD order. This means that for a value such as 0x1234, the on-wire representation is 12 34 in PROFINET byte order, but most S7 CPUs view it as word value 0x1234. The discrepancy almost never appears on S7-to-SINAMICS because both use the same convention.

Where it does appear is in user-defined telegrams (UDTs) where the engineer defines a structure with individual byte members:

// Bad: byte-by-byte UDT on a Siemens side
TYPE UDT_ZSW1 :
  STRUCT
    b_low  : BYTE;  // bits 0-7
    b_high : BYTE;  // bits 8-15
  END_STRUCT;
END_TYPE

If the controller then reads the same word into a signed 16-bit INT, the high and low bytes are reversed relative to the status-word convention. The drive has not swapped bytes; the PLC UDT is mis-declared. The fix is either to read the PZD as a single WORD or INT, or to swap the byte order in the UDT (low byte at high address). On a Siemens S7-1200/1500 with PROFINET, the standard I/O accessor always presents a 16-bit PZD in the correct little-endian word format, so byte swapping is unnecessary if the standard I/O access path is used.

Saving, Exporting, and Reusing Telegram Configurations

Two layers of configuration must be backed up:

1. Offline project (TIA Portal / STARTER). Save the project archive (.zap15 or .zip). The telegram layout, hardware configuration, and PROFINET device name are all part of the archive.
2. Drive parameter set. Use Drive → Upload to PG in STARTER to capture the as-running parameters, then Copy RAM to ROM on the drive so the layout is non-volatile on the memory card or internal flash.

To reuse a free-telegram configuration on another G120D, three viable approaches exist:

• Project duplication. Copy the G120D device in the project tree, then change the PROFINET device name and IP. Download to the new drive. This is the cleanest method.
• STARTER script file. Export a script that writes p922, p923, p924, p2050, and p2051 on the new drive. Scripts can be executed from Tools → Scripts → Execute or invoked at project start.
• Parameter upload/download with the expert list. Use Drive → Parameter assignment → Parameter comparison to identify deltas between the source and target drive.

STARTER Script Files for Automation

A STARTER script file is a plain-text file containing one parameter assignment per line. Scripts are executed from Tools → Scripts in STARTER or from the command-line variant STARTER.exe -s script.txt. A typical G120D free-telegram script is:

; G120D_free_telegram.script
; --- Telegram selection ---
p922 = 999                ; Free telegram
; --- Drive → Controller (p923 send) ---
p923[0] = 52              ; ZSW1
p923[1] = 21              ; n_act smoothed
p923[2] = 31              ; M_act smoothed
p923[3] = 52.3            ; ZSW1 bit 3 (FAULT)
p923[4] = 2139            ; Fault code (r2139)
; --- Controller → Drive (p924 receive) ---
p924[0] = 3700            ; STW1
p924[1] = 1150            ; Speed setpoint source
; --- I/O interconnection (must agree with p923/p924) ---
p2051[0] = 52
p2051[1] = 21
p2051[2] = 31
p2061[0] = 52.3
p2061[1] = 2139
p2050[0] = 3700
p2050[1] = 1150
; --- Commit ---
p971 = 1                  ; Save parameter changes non-volatile

Script files are particularly useful when commissioning many drives with identical layouts — a common requirement on conveyor sections, AGVs, or modular machine cells. Siemens documents the script syntax in the STARTER online help and at Siemens Support entry 75163088.

PROFINET / PROFIBUS Integration and PLC Programming

On the controller side, the G120D is integrated as a PROFINET IO device with a slot structure that mirrors the telegram. For a free telegram with five send and two receive words, TIA Portal shows:

Inside the PLC, the input/output addresses are assigned automatically (or can be fixed with the "I/O address" property). For an S7-1500 with a free telegram of 5 input words and 2 output words starting at input address 256 / output address 256, the symbolic names would be:

// SCL example for S7-1500 PROFINET free telegram
#drive_status_word  := %IW256;     // ZSW1
#drive_speed_act    := %IW258;     // n_act (RPM, scaled per p2000)
#drive_torque_act   := %IW260;     // M_act (Nm, scaled per p2003)
#drive_fault_bit    := %IX264.3;   // ZSW1 bit 3 — FAULT
#drive_fault_code   := %IW266;     // r2139 — active fault number

#plc_control_word   := %QW256;     // STW1
#plc_speed_setpoint := %QW258;     // speed setpoint (RPM, scaled per p2000)

// Bit-level interpretation
IF #drive_status_word.2 THEN        // RDY_REF (operation enabled)
    // … machine logic
END_IF;

Note that the scaling of n_act follows p2000 (reference speed). If p2000 = 1500 RPM, then a value of 16384 (0x4000) on the wire equals 1500 RPM. The conversion is:

For PROFIBUS, the procedure is identical but the slot indices are fixed by the GSD file and the slot assignment is performed in HW Config or TIA Portal device configuration.

Verification and Commissioning

After loading the free-telegram configuration, run the following verification sequence before enabling the drive:

1. Parameter readout. Connect STARTER online and check r2050[0..4] and r2051[0..4] — these are the connectors actually being sent on the wire. They must match p923/p924 numerically.
2. Sign-of-life check. With the controller in stop, drive parameter r2050[0] should be 0. Toggle a single bit in the controller and confirm it appears in r2050[0] within one PROFINET cycle (typically 1 ms).
3. Loopback test. In STARTER, set p2051[1] = r2050[1] (echo the controller's speed setpoint back to the controller's input address). Watch the input word in the controller to confirm wire integrity before applying actual speed commands.
4. Scaling check. Command 50 % of p2000 on the setpoint word. Confirm r0021 matches the expected value within 1 % (allowing for ramp-up time).
5. Fault simulation. Trigger a non-latching alarm (e.g. by raising the load) and confirm bit 7 of ZSW1 is set in r0052 and propagated to the controller's input word.

Troubleshooting Matrix

FAQ