Configuring SINAMICS G120 to Replace MM420 on Elektronikon MKIV

Configuring a SINAMICS G120 as a Drop-In Replacement for a MICROMASTER 420 on an Atlas Copco Elektronikon MKIV USS Link

Atlas Copco oil-injected screw compressors controlled by an Elektronikon MKIV regulator communicate with the cooling-fan drive through the legacy Siemens USS (Universal Serial Interface) protocol on RS-485. The MKIV firmware assumes the slave drive presents the MICROMASTER 420 (MM420) control-word, status-word, setpoint-scaling, and acyclic parameter-channel (PKW) layout. A direct hardware swap of a failed MM420 for a SINAMICS G120 (CU240B-2 or CU240E-2) will not establish communication unless the G120 is re-parameterized to match the MM420 profile. This reference documents the protocol-level mismatch, the parameter-level fix, the field-commissioning sequence, and the verification tests required to integrate a SINAMICS G120 on an existing Elektronikon MKIV USS link without modifying the controller.

Problem Statement

When an MM420 on the cooling-fan VFD slot fails, field engineers commonly substitute a SINAMICS G120 because the MM420 is end-of-life. The wiring is correct, the G120 powers up, the IGBTs close in local mode, and the BOP-2 shows no fault. The MKIV, however, logs No communication or Drive fault within 200 ms of the first poll cycle. The MM420 replacement part, by contrast, works the moment it is plugged in.

Five independent mismatches stack up on the G120 out of the box:

1. Default protocol: the CU240B-2 ships with Modbus RTU enabled on RS-485 (p2020 = 2). USS is disabled.
2. PZD interconnection: SINAMICS G120 requires explicit BICO mapping of received control-word bits (p2050[]) and transmitted status-word bits (p2051[]). MM420 maps bits automatically.
3. STO/OFF2 hard-wiring: CU240E-2 F-variants tie OFF2 to the safe-torque-off input DI4; if STO is open, OFF2 is forced low and the drive cannot run.
4. Setpoint scaling: MM420 references P2000; G120 references p2000. Both are settable, but the defaults differ (MM420 = 50 Hz, G120 with motor-ID completion = nameplate frequency, often 50/60 Hz).
5. PKW parameter semantics: r0021, r0027, r0052, r0003 numbers are reused but the encoding (scaling, signed/unsigned, 16-bit vs. Float32) differs. The MKIV parser may misinterpret numeric telemetry.

USS Protocol Fundamentals on Siemens Drives

USS is a half-duplex RS-485 master-slave protocol documented in the Siemens application note USS Protocol — Application Description. Both MICROMASTER and SINAMICS implementations share the same telegram structure, but the parameter number space and default interconnect differ.

Frame Structure

Default COM Settings: MM420 vs. G120 vs. MKIV Required

Reference: SINAMICS G120 Parameter Manual, section 4.5 (Communication).

Control Word Bit Mapping: MM420 vs. SINAMICS

The MM420 control word follows the legacy PROFIdrive V1/NORD profile. The SINAMICS G120 control word follows the same bit-numbering convention for bits 0-11, but the factory default for bit 10 (control from PLC) is not auto-routed on the receive side. The status word layout is identical at the bit level, but the source of each status bit must be wired through BICO.

Bits 0-7 are functionally identical. The practical differences are bit 3 (MM420: enable pulse vs. G120: enable operation) and bit 5 (MM420: RFG start vs. G120: unfreeze RFG). On the G120, r899.0 (pulses enabled) only goes high when r52.3 (fault absent) is true AND p8641.0 (modulator enable) is true. The MKIV checks r899.0 implicitly through ZSW1 bit 2 (operation enabled) and trips if it stays low for > 2 s.

Setpoint and Actual-Value Scaling

Both MM420 and G120 normalize the 16-bit main setpoint (HSW) to a percentage of the reference speed defined in p2000 / P2000 (default 50 Hz, normalized 0x4000 = 100 %).

If the G120 is commissioned at 60 Hz (US-built motor) but the MKIV is sending a 50 Hz reference setpoint, the fan runs 20 % over-speed and the high-pressure safety trips within 90 s. Set p2000 = 50.0 explicitly to emulate the MM420 default.

PKW Acyclic Channel

The MKIV reads r0021 (output frequency), r0027 (output current), and r0003 (status word 1) over the PKW acyclic channel every 5 s for trending. Both MM420 and G120 expose these parameter numbers, but the encoding differs:

For MKIV firmware ≤ 6.5x the controller only logs these values for trending and does not enforce trip limits. On ≥ 7.0 firmware, r0027 is used for current-based shutdown, and a Float32 response is rejected with PKW invalid. Add a scaling block or change p2051[2..5] mapping to provide the MM420-scaled representation.

Prerequisites for Field Commissioning

• SINAMICS G120 with CU240B-2 PN or CU240E-2 PN F, firmware ≥ V4.7 SP10 (see G120 firmware release notes).
• Connected motor nameplate data: kW, V, A, Hz, rpm, cos φ.
• BOP-2 or IOP-2 handheld, or STARTER / Startdrive ≥ V15.1 with USB cable.
• RS-485 shielded twisted-pair from MKIV TB3 (A+, B-, GND) to CU terminal 29 (P+) / 30 (N-).
• 120 Ω termination at the G120 end (DIP switch S1 = ON); disable on-board termination on the MKIV by removing JP3.
• Risk assessment per IEC 60204-1 if STO is being bypassed.

Step-by-Step Commissioning Procedure

1. Factory reset: p0970 = 1 (resets all parameters to defaults).
2. Quick commissioning start: p0010 = 1, p0003 = 3 (expert access).
3. Enter motor data: p0301 (kW), p0304 (V), p0305 (A), p0307 (Hz), p0311 (rpm), p0308 (cos φ).
4. Run motor identification: p1900 = 1 (standstill ID) or p1900 = 2 (rotating ID, decouple compressor mechanically first).
5. Set reference speed: p2000 = 50.0 (Hz, MM420-compatible).
6. Set limits: p1080 = 5.0 (min), p1082 = 50.0 (max).
7. Set ramps: p1120 = 10.0, p1121 = 10.0.
8. Enable USS protocol: p2020 = 1 (USS on RS-485).
9. Set slave address: p2021[0] = 1 (must match MKIV drive table; cooling fan is typically drive #1).
10. Set baud rate: p2021[1] = 6 (9600 bit/s).
11. Set parity/stop bits: p2030 = 2 (even parity, 2 stop bits).
12. Define PZD length: p2022[0] = 2 (PZD receive words), p2022[1] = 2 (PZD transmit words), p2022[2] = 4 (total PZD).
13. Define PKW length: p2023[1] = 4 (4 PKW words, matching MKIV).
14. Set command source to USS: p0700 = 5.
15. Set setpoint source to USS: p1000 = 5.
16. Wire received STW1 bits: p2050[0] = r2090.0, p2050[1..15] mapped to r2090.1 through r2090.15 by default (BICO convention).
17. Wire transmitted ZSW1 bits (CRITICAL):
    • p2051[0] = r52.0 (ready to switch on)
    • p2051[1] = r52.1 (ready)
    • p2051[2] = r52.2 (operation enabled)
    • p2051[3] = r52.3 (fault)
    • p2051[4] = r899.0 (pulses enabled)
    • p2051[5] = r52.5 (OFF3 inactive)
    • p2051[6] = r52.6 (switch-on inhibited)
    • p2051[7] = r52.7 (alarm)
    • p2051[8] = r2197.7 (speed within tolerance)
    • p2051[9] = r52.9 (control request)
    • p2051[10] = r52.10 (control by PLC — required for MKIV handshake)
    • p2051[11] = r2197.4 (ramp active)
18. Map main setpoint: p2050[2] = r2050[2] (default, PZD receive word 3 → main setpoint).
19. Map actual value: p2051[2] = r21 (output frequency smoothed, in Hz).
20. Configure OFF2 source: p0852[0..2] = 0 (constant 0) only if MKIV does not drive STO and a risk assessment per IEC 60204-1 documents the bypass.
21. Configure OFF3 source: p0856[0..2] = 0 (constant 0, optional).
22. Configure telegram monitoring: p2024 = 100 (ms, USS telegram fail time; extend to 1000 ms if MKIV cycle time is slow).
23. Save parameters: p0971 = 1 (commit RAM → non-volatile).
24. Power-cycle the drive.
25. Verify with STARTER trace or RS-485 sniffer that ZSW1 returns 0xE031 within 200 ms of MKIV STW1 = 0x447E.

Verification Tests

BOP-2 Local Override

1. Temporarily set p0700 = 1 (BOP-2 command source).
2. Press RUN, observe drive accelerates to 25 Hz.
3. Stop, set p0700 = 5 (USS).
4. Restart MKIV, observe ZSW1 transitions 0xE031 → 0xE033 → 0xE037 as MKIV walks the state machine.

USS Frame Capture

Connect a USB-to-RS-485 sniffer on terminal 29/30. The captured frames should match:

Master → G120 (Addr 1):
02 0E 01 7E 44 00 00 00 00 00 00 00 00 00 25
                       ^STW1=0x447E  ^HSW=0x0000

G120 → Master:
02 0E 01 31 E0 00 00 00 00 00 00 00 00 00 D1
                       ^ZSW1=0xE031

If ZSW1 returns 0x0000: USS receive is not enabled — check p2020 and p0700.
If ZSW1 returns 0xE031 but the drive does not run on STW1 = 0x447F: check p1000 and p2050[1] (OFF2).
If F30002 or F07220 appears: check p0852 wiring and p2024 timeout.

State-Machine Walk

Fault Code Mapping

Fault codes per SINAMICS G120 List Manual, fault and alarm chapter.

Troubleshooting Matrix

Edge Cases and Field Notes

CU240E-2 F (Fail-Safe) Variants

The fail-safe CU variant ties OFF2 to the integrated STO input (DI4). If STO is open, the drive will never enable, regardless of USS commands. Field remedy:

1. Verify DI4 (terminal 16 on CU240E-2 F) is high (+24 V).
2. If the MKIV does not have a dedicated STO output, install a jumper between terminal 16 and 24 V internally on the CU.
3. Document the bypass in the safety file per ISO 13849-1.

Modbus-Default CU240B-2

CU240B-2 ships with Modbus RTU active on RS-485. The Modbus register map is wholly different from USS PZD/PKW, so even valid MKIV frames will be parsed as garbage. Always confirm r2020 = 1 before further commissioning.

IOP-2 Plug-and-Play Interference

If an IOP-2 is plugged in during commissioning, the drive defaults to the IOP command source (p0700 = 4) regardless of USS. Remove the IOP-2 or explicitly set p0700 = 5 after IOP-2 use.

MKIV Firmware Variants

MKIV firmware versions 6.x and 7.x differ in their PKW parser strictness. 7.x enforces parameter-number validation; if you have changed r0027 to a Float32 readout and the MKIV expects a 16-bit integer, the request will fail and the controller may interpret it as "current out of range". Workaround: route ZSW2 / HIW word to a free function block that scales r27 to MM420 16-bit format.

Alternate Controllers

If a third-party controller (not MKIV) is involved, the same procedure applies with different default telegram numbers:

Pre-Commissioning Checklist

• [ ] Motor nameplate captured (kW, V, A, Hz, rpm, cos φ).
• [ ] Mechanical coupling verified (compressor fan free to rotate).
• [ ] USS cable A+/B-/GND correctly wired; shield grounded at MKIV end only.
• [ ] 120 Ω termination active at G120 end (DIP S1 = ON); MKIV on-board termination disabled (JP3 removed).
• [ ] 24 V SELV supply grounded per IEC 61131-2.
• [ ] p2020 = 1 confirmed.
• [ ] p2021[1] = 6 (9600 baud).
• [ ] p0700 = 5, p1000 = 5 confirmed.
• [ ] p2051[10] = r52.10 confirmed.

Post-Commissioning Verification

1. Confirm BOP-2 displays run state within 1 s of MKIV issuing STW1 = 0x447F.
2. Confirm ZSW1 transitions through bits 0, 1, 2, 3 in the expected order (LED blink pattern on BOP-2).
3. Confirm r21 (output frequency) tracks HSW setpoint within ± 0.5 Hz.
4. Confirm r0027 (output current) on MKIV display tracks measured clamp-meter current.
5. Confirm compressor high-pressure switch does not trip within 10 minutes of continuous run.
6. Confirm emergency-stop on MKIV trips the G120 (OFF1=0 → coast → ZSW1 bit 1 = 0).
7. Confirm power restoration restarts the drive without MKIV intervention.

FAQ