Siemens MM420 A0700 CB Warning 1: Profibus Diagnosis and Fix

Problem Identification

The Siemens MICROMASTER 420 (MM420) raises alarm A0700 ("CB warning 1") the moment a Profibus master (typically a SIMATIC S7 PLC) establishes a cyclic connection to the drive's Communication Board (CBP). The drive remains controllable from the keypad, but the fieldbus process image is rejected and no setpoint/control-word telegrams are accepted.

From the MM420 parameter list, the official description of A0700 is:

CB warning 1 - Communication Board (CB) specific. See CB user manual.

Source: MICROMASTER 420 Parameter List (PDF).

The CB user manual referenced is the operating guide shipped with the optional CBP (Communication Board Profibus) module. It clarifies the actual meaning of A0700 on the MM420:

Parameter or configuration setting transmitted by the Profibus master is invalid. Remedy: correct the Profibus configuration.

This is a configuration-level warning, not a hardware failure. The CBP module has successfully powered up, completed the Profibus handshake, and begun parsing the configuration telegram sent by the DP master. Something inside that configuration string is not acceptable to the MM420 firmware, and the drive signals this with A0700 instead of going into data exchange.

Why the Warning is Reported at All

When a Profibus DP master enters the OPERATE state, it sends a Chk_Cfg (Check Configuration) telegram to the slave. The CBP firmware on the MM420 compares this against:

1. The configured PPO type expected by the master (chosen in the hardware catalog, e.g. PPO 1, PPO 2, PPO 3, PPO 4, PPO 5).
2. The configured Profibus station address set on the MM420.
3. The parameter access slot structure the master intends to use (PKW or word 0 mapping for the MM420 with PPO 1/2/3/4/5).
4. The consistency and length of process data (PZD).

Any mismatch raises A0700. The same CBP module on a Micromaster 440/430 may report a different sub-code; on the MM420 every Profibus configuration anomaly is consolidated into warning A0700, which is why the parameter list refers the reader to the CB manual for the true meaning.

Root Cause Matrix

Prerequisites for Diagnosis

• MM420 drive with installed CBP (Communication Board Profibus) module, catalog number 6SE6400-1PB00-0AA0 (or successor). The CBP slots into the top of the MM420 control board; the address switches are on the front face of the CBP.
• MM420 firmware version must support the CBP you have. CP firmware is stored on the CBP itself; the drive-side compatibility is documented in the CBP operating instructions.
• Correct GSD file installed in the PLC programming tool. The Siemens GSD for the MM420 is SIEM811D.GSD (older releases use SI0008B7.GSD). Always import the one that matches the CBP firmware, not the MM420 firmware.
• STEP 7 / TIA Portal project with the MM420 slave in the DP master system, plus a PG with online access for STARTER or the MM420 IOP/BOP to read parameters.
• Wiring verified: shield grounded at both ends via the grounding bar, termination resistors ON at the two physical ends of the segment only, 24 V on the CBP.

Step-by-Step Resolution

1. Confirm the warning is A0700 and not a fault. On the BOP (Basic Operator Panel) attached to the MM420, press the Fn key and arrow to r0000. Navigate to the warning buffer (r0030 region) or use STARTER online to confirm the buffer shows A0700. The first warning to be latched after a power cycle is the most diagnostic.
2. Verify the CBP module's LED state.
   • Green RDY solid: CBP healthy.
   • Green RDY flashing: CBP in self-test or waiting for configuration.
   • Red BF (Bus Fault) solid: no communication with master (address issue, wiring).
   • Red BF flashing: configuration rejected — this is the state that pairs with A0700.
   A flashing BF is the single most useful indicator: it tells you the master has been seen, the address is correct, and the rejection is at the configuration level.
3. Match the PPO type end-to-end. The MM420 supports PPO 1, PPO 2, PPO 3, PPO 4 and PPO 5. PPO 1 and PPO 3 carry 4 words of PKW (parameter access) plus 2 PZD (process data). PPO 2 and PPO 4 carry 6 PZD and no PKW. PPO 5 carries 2 PKW and 10 PZD. On STEP 7 hardware catalog pick the exact entry; the slot configuration must be identical on the master and what the CBP expects (CBP reads the requested PPO type out of the Chk_Cfg telegram). Mismatches between PPO 1 and PPO 3, or between PPO 2 and PPO 4, are the most common cause of A0700 on an otherwise healthy network.
4. Set the Profibus address correctly. Two sources must agree:
   • The two rotary switches on the CBP face (tens and ones, decimal, range 1 – 99).
   • MM420 parameter P0918 (CBP station address) read-back.
   If the switches and P0918 differ, the CBP uses the switch position. Power-cycle the drive after changing the switches so the new value is latched. The PLC hardware configuration must use the same address.
5. Check the bus timing parameters. On the MM420 these live in the CBP parameter range:

   Parameter	Meaning	Typical value
   P2041	PKW length	0 = no PKW, 4 = 4 words (matches PPO)
   P2042	CBP station address (read-only mirror of switches)	1–125
   P2043	Watchdog time in ms	200 ms default; raise to 3× PLC OB1 cycle
   P2044	Setpoint scaling factor	1.0

   If the PLC OB1 cycle is, say, 50 ms and the watchdog is left at 20 ms, the CBP will drop the link and you will see A0700 followed quickly by F0070 (bus link loss).
6. Route the control and setpoint to the CBP. Even with a correct Profibus configuration, the drive will not act on the master if P0700 (command source) and P1000 (setpoint source) still point at terminals or the BOP. Set:
   • P0700 = 6 — command source = CBP.
   • P1000 = 6 — setpoint source = CBP.
   Without these, the warning may still appear because the master is writing PZD that the drive refuses to bind, but the drive's behaviour is unchanged.
7. Verify the GSD file in the PLC project. Open the slave properties in STEP 7 / TIA, confirm the device identifier matches the CBP variant you have, and re-compile the hardware configuration. A common oversight is importing the MM440 GSD by accident; the MM420 and MM440 GSDs are similar but not interchangeable.
8. Re-cycle the drive. MM420 CB warnings are not latched the same way as faults. Some configuration errors only clear on a full power-off/power-on of the MM420, because the CBP negotiates once on bus initialisation.
9. Re-test. With the PLC in RUN, watch r0000 / r0030 and the BF LED. If both go quiet and the CBP RDY LED is solid green with the BF LED off, A0700 will not reappear.

Verification Checklist

Parameter Reference (P2041, P2042, P2043, P2044)

These four parameters are the only ones you should normally touch when chasing an A0700 on a CBP-equipped MM420. Treat them as a unit:

• P2041 PKW length — must mirror the PPO type chosen in the PLC. Set to 0 if you are using a PPO that has no parameter channel, and 4 if you are using PPO 1/3/5. A value of 4 with a PPO 2/4 master is the second most common cause of A0700.
• P2042 station address — read-only on the MM420; tells you what the CBP sees. If this disagrees with the PLC's configured address, the bus will not enter data exchange.
• P2043 watchdog — a value of 0 disables the watchdog, which is rarely correct in production. 200 ms is a safe default for a 100 ms OB1 cycle.
• P2044 scaling — leave at 1.0 unless you have a specific reason to scale the PZD setpoint to a different engineering range.

Related MM420 Warnings and Faults

Cross-reference: MICROMASTER 420 Parameter List and the Siemens entry 6586565 (CB documentation).

PLC-Side Configuration Audit

Because A0700 is reported by the drive in response to what the master sends, half the work is on the PLC. Walk the project once and confirm:

1. The MM420 slave object sits in the correct DP master system and is not a stray copy from another project.
2. The slave's slot configuration shows the PPO type you intend, and you did not edit the I/Q addresses by hand so they overlap with another slave's PZD range.
3. The PKW / PZD lengths in the slave properties are not greyed out (they will be locked once the GSD is installed; if you can edit them you are on the wrong GSD).
4. The OB1 cycle time (or OB82 for isochronous) gives the watchdog P2043 at least a 3× margin.
5. Diagnostic interrupts are enabled on the slave — without them, a configuration fault is logged only as a generic bus error in the PLC and you will not see the rejected slot.

Field-Proven Edge Cases

• A0700 right after a firmware update on the CBP. A newer CBP firmware may default to a different PPO. Re-confirm PPO on the master and check the CBP parameter P2041; the drive's stored value may no longer be valid.
• A0700 on a cold start, clear on warm start. Almost always a watchdog-related issue. The PLC boots faster than the MM420 finishes its precharge and the first few telegrams are lost, tripping the CBP watchdog. Raise P2043 or add a startup delay in OB100.
• A0700 only when the SFC14/SFC15 DP read/write blocks are used for consistency over 4 words. PPO 1/2 use 10 bytes of PZD (2 words). SFC14/SFC15 must be used with a length of 4 only if the master is configured for 8-byte consistency; otherwise the slave interprets the second half of the telegram as a parameter write attempt.
• A0700 plus duplicate station address on the segment. Two CBP modules set to the same address will both answer and the master will only see one of them. The dropped slave logs A0700 and the surviving slave may still come up.
• A0700 after a Profibus connector replacement. Check that the terminating resistor is OFF in the middle-of-segment connectors. With an extra terminator the bus reflections corrupt the Chk_Cfg.

Safety and Reset Considerations

Summary Flow

1. Read r0030 — confirm A0700.
2. Read CBP LED state — flashing BF means configuration rejected, solid BF means address/wiring issue.
3. Verify GSD file (SIEM811D.GSD) and PPO type in the PLC match the CBP.
4. Verify address: rotary switches = P0918 = PLC configured address.
5. Verify P2041, P2043, P0700 = 6, P1000 = 6.
6. Power-cycle the drive.
7. Confirm r0030 empty and BF LED OFF.

FAQ