Resolving Conv.Format Fail 52 on MicroMaster 420

1. Problem Statement

When a Siemens MicroMaster 420 (MM420) or MicroMaster 440 (MM440) is integrated with an Atlas Copco Elektronikon Mk IV / Mk V compressor controller over a USS link on RS485, two failure modes are commonly reported and frequently confused:

• Drive-side: The drive trips on F0052 — Converter fault, USS: incorrect converter format (Conv.Format Fail 52). On power-up or run command, the drive either refuses to start or starts and immediately decelerates to a fixed low frequency (~38 Hz on 50 Hz mains) for one minute, then repeats the cycle.
• Controller-side: The Elektronikon logs "Converter Status Failures" and either locks out the compressor, refuses to issue a run command, or displays "No Communication" with the converter, even though the drive's COM LED flashes and the bus appears alive on an oscilloscope.

Both symptoms are nearly always the result of a USS parameter handshake mismatch — specifically, the PKW (Parameter-Kennung-Wert) channel of the USS telegram is sized or interpreted differently by the master (Elektronikon) and the slave (MM420). The link carries something, but the parameter word block fails integrity checks, so the drive either rejects the request, falls back to its last valid setpoint, or declares the telegram corrupt.

This reference covers the complete diagnostic, parameter, wiring, and commissioning workflow to clear F0052, restore the USS link to the Elektronikon, and prevent recurrence.

2. System Architecture

The standard Atlas Copco GA-series compressor integration with the Elektronikon Mk IV uses a two-wire RS485 multidrop bus (typically daisy-chained between the Elektronikon, the MM420, and sometimes a secondary VFD or soft-starter). The Elektronikon acts as the USS master and the MM420 as the USS slave.

2.1 Logical layers

2.2 USS telegram anatomy

Each USS frame from the Elektronikon looks like:

STX | LGE | ADR | PKE | IND | PWE1..PWE4 | PZD1..PZDn | BCC
 02h  14h   01h   0000h 0000h 4 words        2 words      XOR

With PKW length = 4 (the Elektronikon's default expectation) and PZD length = 2, the total payload is 14 bytes (LGE = 14), producing a 17-byte frame including STX, LGE, and BCC.

3. The F0052 Fault — Definition and Decode

F0052 belongs to the MM4 fault group converter/inverter protection in the Siemens fault code table. The drive's internal firmware raises it when the COM link telegram structure does not match the configured P2012/P2013 lengths or when the parity/format of received frames is inconsistent with the drive's RS485 settings.

3.1 Trigger conditions (from MM4 firmware behavior)

• P2013[0] = 0 and the master transmits PKW words → PKW overflow.
• P2013[0] = 3 and the master transmits a fixed-length PKW request → length conflict.
• P2012[0] ≠ 2 and the master expects 2 PZD words (control word + setpoint) → PZD count mismatch.
• BCC checksum failure on consecutive received frames (more than P2014 telegram-off time).
• PKW request to a parameter that the drive cannot serve (read/write on a read-only or non-existent parameter).

3.2 Observable symptoms

• Drive starts on previous analog/terminal setpoint but stays at a frozen frequency (commonly reported at ~38 Hz on 50 Hz systems — typically the last setpoint captured before the bus failed, or the P1080 minimum frequency).
• After ~60 s, drive ramps to stop, the FAULT LED illuminates, and the display shows F0052.
• Elektronikon records a converter fault and locks out the compressor until a manual reset at the controller.

4. Root Cause Analysis

When a service engineer replaces a working compressor's MM4 parameters into a non-working unit and the fault persists, the cause is almost always one of the following five classes:

4.1 Class A — PKW length mismatch

The most common root cause. The Elektronikon Mk IV expects PKW = 4. A drive cloned from a unit that previously ran on a Profibus option (PROFIdrive) or that was last configured for a different controller will retain P2013[0] = 0 or 3.

4.2 Class B — Command source misrouting

The drive's run command is sourced from the wrong input. With P0700 = 2 (terminals), the Elektronikon's control word through the COM link is ignored; the drive will free-run on the last setpoint whenever DIN1 is closed. With P0700 = 5, the COM link is the active command source. The Elektronikon integration requires P0700 = 5.

4.3 Class C — Setpoint source misrouting

Analogously, P1000 = 2 (analog input) ignores the USS setpoint. The Elektronikon expects P1000 = 5 (USS via COM link).

4.4 Class D — P0719 override conflict

P0719 is a bit-field override that takes precedence over P0700 and P1000. A value of P0719.0 = 11 or P0719.1 = 11 (or = 13, depending on Elektronikon firmware generation) routes the command and/or setpoint away from the COM link. The Elektronikon expects P0719.x = 0 so that the P0700 = 5 / P1000 = 5 sources are honored.

4.5 Class E — Bus address collision

If two slaves share USS address P2011[0] = 0 (the default), only one of them responds. The other silently times out. With Elektronikon Mk IV the default slave address is 1 (sometimes 2 on dual-VFD skids). Verify the unique address of the failing drive.

5. Required Parameter Set for Elektronikon Integration

The values below are derived from the working compressor configuration and the USS/Profibus parameter list shipped with the MM4 firmware. Always cross-check against the current Siemens parameter list for the firmware version installed on the drive.

6. USS Telegram Structure in Detail

To diagnose F0052, an engineer must understand what the Elektronikon transmits and what the MM420 expects.

6.1 PKW (parameter channel) layout — 4-word mode (P2013 = 4)

With P2013 = 3, only PKE + IND + PWE1 are present (3 words). With P2013 = 0, no PKW words are present. The Elektronikon always sends 4 PKW words for its parameter read/write cycle (e.g. reading r0021, writing to control word), so P2013 = 0 or 3 is incompatible.

6.2 PZD (process data) layout — 2-word mode (P2012 = 2)

7. RS485 Wiring and Termination

Physical-layer errors can also produce F0052, especially in compressor rooms with long cable runs, VFD-induced noise, and shared cable trays.

7.1 MM420 RS485 terminal pinout

7.2 Termination rules

• Place a 120 Ω, 0.25 W resistor between terminals 29 and 30 at both ends of the bus only.
• Do not terminate intermediate nodes — only the two physical end devices.
• On the Elektronikon end, the bus terminator is typically a jumper or DIP switch on the controller PCB. On the MM420, termination is a hardware switch or solder-link on the COM board — confirm in the MM420 operating instructions.
• Use a shielded twisted pair (e.g. Belden 3106A or equivalent), shield grounded at one end only (typically the Elektronikon cabinet ground).
• Maximum cable length at 19200 baud: 1200 m. At 38400 baud: 800 m. Compressor rooms usually sit well within this, but a long daisy-chain through the plant can approach the limit.

7.3 Address uniqueness

Each USS slave on the bus must have a unique P2011[0] value. Typical Atlas Copco compressor installations:

• Single compressor: P2011[0] = 1
• Dual compressor skid (lead/lag): MM420 #1 = 1, MM420 #2 = 2
• Compressor + separate fan VFD: compressor = 1, fan = 3

8. Step-by-Step Resolution Procedure

Use this procedure when the Elektronikon reports "Converter Status Failures" and the drive shows F0052. It assumes the drive is powered, the BOP (Basic Operator Panel) is accessible, and the compressor can be safely de-energized.

1. Isolate the compressor. Lock out and tag out per site procedure. Open the drive cabinet, verify zero voltage at the line side with a meter, then close the disconnect to power the MM420 control electronics only (typical Atlas Copco service procedure: line disconnect open, control fuse pulled, or service position switch on the cabinet).
2. Record current parameter values. Navigate parameter-by-parameter on the BOP and note the existing values of P0700, P1000, P0719, P0731, P2010, P2011, P2012, P2013, P2014, P1080, P1120, P1121. This gives you a baseline and a recovery point if a misstep occurs.
3. Factory reset the drive. Set P0010 = 30 (factory settings), then P0970 = 1 to begin the reset. Wait for the display to read "----" and the drive to return to run display. This typically takes 30–60 s.
4. Set the user access level. Set P0003 = 3 to allow access to the standard USS parameters (otherwise the BOP will hide some indices in default access level 1).
5. Configure the command source. Set P0700 = 5 (USS on COM link). The drive will automatically rewrite P0701–P0708 to the COM-link default. Do not change P0701–P0708 manually.
6. Configure the setpoint source. Set P1000 = 5 (USS via COM link). The drive will automatically rewrite P1001–P1005 to the COM-link default.
7. Disable P0719 override. Set P0719[0] = 0 and P0719[1] = 0. This ensures P0700 and P1000 are not bypassed.
8. Configure USS bus parameters.

   P2010[0] = 6   ; 19200 baud (or 7 = 38400, match Elektronikon)
   P2011[0] = 1   ; unique USS address (not 0)
   P2012[0] = 2   ; PZD length = 2 words
   P2013[0] = 4   ; PKW length = 4 words (full)
   P2014[0] = 0   ; telegram monitoring off (or 1000 ms)

9. Configure the digital output for fault feedback (optional but recommended). Set P0731 = 52.3. DO1 will close when status word 1, bit 3 (fault active) is true.
10. Set compressor-specific ramp and limits.

    P1080 = 0       ; min frequency, Hz
    P1082 = 50      ; max frequency, Hz (match motor nameplate)
    P1120 = 10      ; ramp-up, s
    P1121 = 10      ; ramp-down, s
    P2000 = 50      ; reference frequency for 100 % setpoint, Hz

11. Save parameters. Briefly press the P button on the BOP, or send USS write to P0971 = 1. The display should flash and return to run display.
12. Re-energize the line and test the bus. Close the line disconnect. Watch the COM LED on the MM420 — it should flicker at the configured baud rate. On the Elektronikon, navigate to the converter status screen and confirm communication is now "OK".
13. Issue a controlled test run. From the Elektronikon service menu, command a load/unload cycle. Verify the drive ramps from 0 → 50 Hz on the HSW setpoint and that the HIW actual frequency tracks within 1–2 %.
14. Monitor ZSW1. Use the BOP to read r0052 (status word 1). Bit 0 (ready) and bit 2 (running) should toggle as expected. Bit 3 (fault) should stay 0. Bit 6 (switching on inhibited) should clear when the Elektronikon issues the correct control word sequence.

9. Parameter Verification Matrix

After the procedure, verify every row in this matrix. Any deviation is a potential F0052 trigger.

10. Atlas Copco Elektronikon Mk IV Specifics

The Elektronikon Mk IV expects a fixed USS profile. From the controller's service menu (Service → Communication → USS) the integrator can read the active baud rate, expected PKW length, and expected PZD length. These must match the MM4 settings. On Mk IV firmware revisions ≥ 3.0, the controller may auto-detect PKW length; on earlier revisions, mismatched PKW length is the only error and produces exactly the symptoms described above.

10.1 Elektronikon-side checks

1. Service → Communication: confirm the converter's USS address matches P2011[0].
2. Service → Communication: confirm baud rate matches P2010[0].
3. Service → I/O: if the fault input is wired to MM420 DO1 (terminal 19/20), confirm the Elektronikon is configured for active-high fault (the MM420 DO1 closes on fault with P0731 = 52.3 and the factory DO type set in P0725).
4. Service → Reset converter: if a converter fault is latched in the Elektronikon, clear it before retrying the bus. The Elektronikon will not poll the drive while a latched converter fault is pending.

10.2 Status word mirror at the Elektronikon

The Elektronikon displays the drive's status word as a bit field. Expected bits in normal running state:

• ZSW1.0 (Ready) = 1
• ZSW1.1 (Ready to run) = 1
• ZSW1.2 (Running) = 1 when loading
• ZSW1.3 (Fault) = 0
• ZSW1.6 (Switching on inhibited) = 0 (the Elektronikon must issue the OFF → ON sequence; a stuck bit 1 means the controller's previous OFF1 was not acknowledged)

11. Field-Proven Caveats and Edge Cases

11.1 P0700 side-effect on P0703–P2236

As noted, changing P0700 to any non-zero value rewrites downstream parameters. Always re-verify P0731 and P0719 after a P0700 change. The reverse is also true: if you previously set P0703 to a custom digital-input mapping, it will be lost the moment P0700 is touched.

11.2 The "38 Hz freeze" symptom

When the bus handshake fails, the MM420 often retains the last valid setpoint in HSW and runs at that frequency indefinitely. On 50 Hz mains the typical captured value is ~38 Hz (corresponding to ~76 % of P2000 = 50 Hz, or HSW = 12452 decimal). This is the default Atlas Copco load setpoint expressed in the 0–16384 HSW scaling. The 60 s before stop is governed by the bus-off reaction set in P2014; with P2014 = 0 the drive will not stop on its own, so a stop is only seen if P2014 ≥ 1 s. If your drive stops in exactly 60 s, you have P2014 ≈ 1000.

11.3 P0719.1 = 11 vs 13

Different Elektronikon firmware generations use different P0719 override bit encodings. The values 11 and 13 both appear in field reports. Set P0719[0] = 0 and P0719[1] = 0 to remove the override and let P0700/P1000 govern. Do not try to use the override value from the working compressor blindly — verify the Elektronikon firmware version first.

11.4 Cloned parameters from a Profibus-driven unit

If the "working" compressor was previously controlled via a Profibus option module (e.g. 6SE6400-1PB00-0AA0), P2012 and P2013 may be set to 0 (no PKW/PZD on COM link) because the option board was the active interface. Cloning those values to an Elektronikon-driven unit will produce F0052 immediately. Always start from a factory reset, not from a clone.

11.5 Profibus option present, USS inactive

The MM420 has only one COM link channel. If a Profibus option is fitted, USS on terminals 29/30 is still available, but P0700 = 5 must be set explicitly. Some option modules change default P2010–P2014 values on insertion. If the compressor was previously Profibus-controlled and the option has been removed, the COM-link defaults may still be the option's defaults.

11.6 BOP "--- " after factory reset

After P0970 = 1 the BOP may show "----" for 10–30 s while the drive reinitializes. Do not power-cycle during this window. Wait for the run display to return before continuing.

11.7 Elektronikon controller firmware revisions

Elektronikon Mk IV controllers with firmware < 2.4 do not support PKW = 4 in some installation profiles and may expect P2012 = 4 instead of 2. Verify with the controller's "Converter profile" parameter before assuming P2012 = 2. The Atlas Copco service tool (or the Elektronikon's diagnostics page) reports the active profile.

11.8 Galvanic isolation

The MM420 RS485 interface is not isolated from the line side. If the Elektronikon is on a different ground reference, fit an RS485 isolator between the two cabinets to avoid ground-loop noise that can produce intermittent F0052 trips.

12. Verification and Commissioning Tests

After resolving F0052, perform the following before returning the compressor to service:

1. Read r0052 (status word 1) on the BOP. Confirm bit 0 (ready) = 1 at idle. The drive should display "Ready to Run" status.
2. Read r0021 (actual frequency) at idle. Should be 0.00 Hz.
3. Issue a run command from the Elektronikon. Observe the drive accelerate at the P1120 ramp to the setpoint. The BOP should show r0021 climbing from 0 to the target frequency.
4. Confirm ZSW1 toggles correctly. Bit 2 (running) = 1 during rotation, bit 0 (ready) = 1, bit 3 (fault) = 0.
5. Issue a stop command from the Elektronikon. The drive should ramp down at P1121 and ZSW1.2 should return to 0.
6. Simulate a bus break. Disconnect the RS485 cable at the MM420 end. With P2014 > 0, the drive should trip F0072 (USS telegram off) after the timeout. With P2014 = 0, the drive continues on the last setpoint. Document the chosen behavior for the site.
7. Force a fault and observe the Elektronikon response. Trigger a controlled fault (e.g. set P1210 = 0 and break motor cable). The MM420 should raise F0001, DO1 should close (P0731 = 52.3), and the Elektronikon should display a converter fault within 1 s.
8. Document the final parameter set. Save a parameter dump via DriveMonitor, STARTER, or the BOP printout. Keep the file with the compressor service record.

13. Related Fault Codes and Cross-Reference

14. Diagnostic Flowchart

Use this sequence when troubleshooting from scratch:

1. Is the COM LED on the MM420 blinking at the configured baud rate? If not → check wiring, terminator, address.
2. Is P2013[0] = 4? If not → set to 4, save, power cycle.
3. Is P2012[0] = 2? If not → set to 2, save, power cycle.
4. Is P0700 = 5 and P1000 = 5? If not → set both, save, power cycle.
5. Is P0719[0] = 0 and P0719[1] = 0? If not → reset to 0.
6. Is the USS address unique and non-zero? If not → assign unique P2011[0].
7. Is the Elektronikon service menu reporting the same address? If not → change the Elektronikon setting to match the drive.
8. Issue a run from the Elektronikon. Does r0052 bit 6 (switching on inhibited) clear? If not → check the Elektronikon control word sequence; the controller must issue OFF1 then ON in the correct USS control word bit pattern.
9. If all above are correct and F0052 still occurs → swap MM420 for a known-good unit; the COM board may be damaged.

Frequently Asked Questions