Drive cooling fan
Common heat-related VFD and servo-drive failure point.
NUM CNC Alarm E28 — What It Means
Alarm E28 on NUM CNC controls (NUM 1060, 1080, and Axium series) indicates a drive communication error — the NUM control lost communication with one or more of the servo drive modules. NUM uses a high-speed serial link between the NCU (Numerical Control Unit) and drive amplifiers; E28 fires when this communication is interrupted or a drive stops responding.
Common Causes
- Communication cable fault — The serial link cable between the NUM control and the drive cabinet has been damaged or disconnected. High vibration and heat exposure can degrade these cables over time.
- Drive module power supply fault — If the DC bus or 24V supply to a drive module fails, the drive stops communicating and E28 is generated for that axis.
- Drive module failure — A failed servo amplifier module stops participating in the communication bus, causing E28 for the affected axis.
- Control board (NCU) communication port failure — The NUM NCU’s communication output circuit has failed.
Step-by-Step Fix
- Full power cycle — Turn off main disconnect, wait 60 seconds, power up. A clean power cycle resolves transient communication faults in a significant number of cases.
- Check drive module status LEDs — Each NUM drive module has status indicators. A drive showing a fault LED is the one that’s not communicating.
- Inspect communication cables — Check the serial link cabling between the NUM NCU and drive cabinet for visible damage and secure connections at both ends.
- Check 24V power supply to drives — Verify 24V control power to the drive modules is within spec (±5%). A sagging 24V supply causes communication dropouts.
- Contact NUM service — E28 that doesn’t clear after power cycle requires NUM-trained service with NUM diagnostic software for drive communication diagnostics.
Parts Often Needed
| Part | Notes |
|---|---|
| Serial link communication cable | NUM-specific — replace if damaged |
| 24V power supply | Replace if voltage is marginal |
| NUM servo drive module | Replace if module has internal fault |
Need Replacement Parts?
Before ordering, match the machine-control part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
When to Call a Pro
NUM CNC systems are less common in the US than Fanuc or Siemens; NUM-trained service engineers are specialized. Contact NUM directly for authorized service referrals.