What is the real difference between 749 and 750, and why power off for both?

750 is the CNC-amplifier handshake failing at power-on; 749 is the link coming up then dropping during run. Both can trace to cable or amp board, but 749 leans toward “contact/noise” and 750 toward “parameters/power/hardware mismatch”. When they won't clear on reset, a full power-off (including the spindle side) and restart is mandatory - a CNC reset alone cannot re-run the handshake, so both require powering off first.

DGN 409 shows SPE=1 telling me to fix the 4000 block, but I have no backup - what now?

The 4000-block serial-spindle parameters (4000, 4001, 4004...) depend on the motor and amplifier model and have no universal values; blind editing makes it worse. Recover them from an identical machine or the FANUC parameter backup first; with no backup, contact the machine builder or Beijing-FANUC and refill from the factory configuration table - do not guess.

The screen shows AL-XX but the amplifier display is dark/unreadable - how do I locate it?

For 751 the authority is the AL-XX on the amplifier display. If the display is dark, the amplifier power (200V input or DC bus) is likely missing - check the amp supply and the emergency-stop chain first. If it shows but is illegible, photograph and zoom to confirm the number, then look up that SPM alarm (AL-12 overcurrent, AL-27 encoder, AL-31 overheat...). The screen number is only relayed - the amplifier is authoritative.

751: Split by Timing, then DGN 409 and Amp Display

cncshare

每天进步一点点

0438

Alarms 749, 750 and 751 are all serial communication alarms between the CNC and the spindle amplifier – not faults of the spindle motor itself. They are often lumped together, but the starting point of diagnosis is completely different: 750 means the power-on handshake never completed, 749 means the link came up then dropped after ready, and 751 means the amplifier raised its own AL-XX, which the CNC merely relays. Telling them apart saves most of the blind board-swapping.

For FANUC 0i-MD / 0i-MF (including 0i Mate) machines with a serial spindle (SPM). Parameter values follow your machine’s manual.

1. The three alarms at a glance

Alarm	Meaning	When it appears	First move
`749` S-SPINDLE LSI ERROR	Serial comms dropped after ready	During run, or won't clear on reset	Full power-off (incl. spindle side), reseat cable
`750` SPINDLE SERIAL LINK START FAULT	Amplifier not ready at power-on	Reported right at power-on	Read DGN 409, read the display
`751` FIRST SPINDLE ALARM DETECTION	Amplifier's own AL-XX	AL-XX shown on screen	Go by the amplifier's AL-XX on its display

In one line: 750 = power-on handshake, 749 = link dropped mid-run, 751 = what the amplifier itself reports.

Three alarms compared: 750 power-on handshake fail / 749 comms dropped after ready / 751 amplifier's own AL-XX, different starting points

2. Split by timing first: 750 at power-on vs 749 after ready

750 at power-on: the CNC-amplifier “handshake” did not finish. Suspect the optical cable, the amplifier 200V input / DC bus, and whether the serial-spindle parameters (4000 block) satisfy startup conditions.
749 after ready: the handshake succeeded, then the link broke later. Suspect a loose optical connector, dirty end-face, a faulty amp board, or drive-line noise during run.
Won’t clear on reset: if 749/750 persists after a CNC reset, you must power the whole system off, including the spindle amplifier side, then back on – a CNC reset alone does nothing.

3. Read diagnostic 409: four bits unpacked

The root cause of 750 almost always sits in diagnostic 409. SYSTEM -> Diagnosis -> type 409 -> search, then read bit by bit:

Bit	Name	=1 means
`#3`	SPE	Serial-spindle params do NOT meet startup conditions (check 4000 block)
`#2`	S2E	Second spindle fault at startup (check 2nd spindle cable/amp)
`#1`	S1E	First spindle fault at startup (check 1st spindle cable/amp)
`#0`	SHE	CNC-side serial comms module fault (main/option board, call service)

Mnemonic: SPE -> parameters, S1E/S2E -> cable & amp, SHE -> CNC board.

Diagnostic 409 four bits: SPE=1 check 4000 block, S1E/S2E=1 check that spindle's cable/amp, SHE=1 is the CNC comms module

4. Read the amplifier 7-segment display

The SPM’s 7-segment display is the second key:

SU-01, AL-24: normal states during power-up, not faults – just wait for init to finish.
A, A1, A2: the amplifier’s own control program did not start, usually a ROM or control-board issue, replace.
Other AL-XX: treat as 751 – the number on the display is authoritative (e.g. AL-12 overcurrent, AL-27 encoder, AL-31 overheat); power-cycle, and if it stays, check the maintenance manual.
The nature of 751 is “the amplifier raised it, the CNC only relays the number”, so the authority is always the amplifier display, not the CNC screen.

Amplifier display read: SU-01/AL-24 normal, A/A1/A2 control-board fault, other AL-XX go by the 751 number on the display

5. Hands-on procedure

Full power-off -> reseat the JA41 (CNC side) and JA7B / JA7A (amplifier side) optical cable ends; check for dirty end-face, loose plug, or a cable pinched by the cabinet door.
Power on -> note the amplifier display, then read DGN 409 S1E / S2E.
Swap in a known-good optical cable -> if 750 remains, check the amplifier 200V input and DC bus.
SPE = 1 -> against a parameter backup, verify the 4000 block serial-spindle parameters (machine-specific, no universal values).
SHE = 1 -> CNC-side comms module fault, stop and call service, do not keep power-cycling.
751 -> go by the display AL-XX, check the motor, encoder, or power line it points to.

The optical cable must not be bent and cannot be replaced by ordinary fiber; clean a dirty end-face with ethanol and a lint-free swab. After swapping cable or amp, a full power-off restart is required – 750 is only generated during the handshake.