HAAS® Alarm 791 — COMM. FAILURE WITH MOCON2
Part of: our HAAS® alarm reference · covering 60 of the most common alarms.
What it means
On machines with two motion control boards (more axes, twin spindle, complex 5-axis), the comms link to the second MOCON has failed.
Common causes
- Cable between boards
- Failed MOCON2 board
- Power issue
What to check first
- Same approach as 101 — power-cycle once, then call for service if persistent
Prevention
Communications and parameter-integrity faults are rarely caused by anything the operator did. They're typically end-of-life components or the result of a bad shutdown. The single most useful preventative step is to replace the control's backup battery on schedule (every 3-5 years) and to ensure parameter and offset backups are taken regularly.
If you’re still stuck
If you’ve worked through the checks above and the alarm hasn’t cleared — or if it cleared and came back — that’s the point to call. Persistent alarms after a reset usually mean a real component fault, and pushing through them risks turning a simple repair into a bigger one. Keith covers HAAS® breakdowns across the UK with same-day phone diagnosis.
A note on this guide. Alarm wording and numbering can vary by HAAS® machine model and software revision. The descriptions here are written in plain English to help you understand what the machine is reporting and what to check first — they’re not a substitute for the official HAAS® operator manual or for a qualified service engineer’s diagnosis. Always confirm the exact alarm message on your screen and consult your machine’s documentation before attempting any repair.