Signal & Communication
My DALI signal quality is poor (yellow or red), what are the causes and how do I fix it?
By Lichtvision Engineering · Published
Short answer
Yellow means communication is still possible but outside the preferred limits, investigate before it fails. Red means reliable communication can't be guaranteed. Common causes are weak or faulty devices pulling signal levels down, wiring issues (long cable runs, missing bus termination), or an undersized power supply. Test each device individually to isolate the root cause.
What yellow and red mean
ProbitSite rates DALI signal quality against the timing and level tolerances defined in IEC 62386-101. Green means the line is well inside spec. Yellow means communication still works, but the margins are outside the preferred range, packets occasionally get close to the threshold and intermittent faults become likely as temperatures rise or load changes. Red means the high or low signal levels, or the edge timing, are out of spec, reliable communication can no longer be guaranteed and individual frames will be lost.
Common causes
Several factors push a line out of the green range. A single weak or faulty device, an old EVG or a driver with a degraded opto-coupler, can pull the line low on its own. Wiring issues such as overly long runs (DALI is rated to 300 m at 1.5 mm², shorter at thinner gauge), star-burst topologies, or crosstalk from parallel mains cabling raise capacitance and degrade edges. An undersized or aging bus power supply cannot drive the line during bursts of traffic. And occasionally an installation simply has too many devices on one branch, even if it is below the 64-device address limit, the cumulative input current exceeds what the supply can deliver.
How to isolate the culprit
ProbitSite lets you test each device individually and watch the signal quality change as you connect and disconnect segments, so you can find the specific device or branch responsible instead of replacing parts on suspicion. The recommended sequence is: confirm the bus power supply rating, then disconnect branches one at a time, then re-test with each suspect device on a known-good line. A device that drags a clean line into yellow on its own is almost certainly the one to replace.
Why this matters before handover
A line that reads yellow at commissioning will drift to red over the building's lifetime as devices age, temperatures fluctuate, and new loads are added. Fixing yellow now is much cheaper than diagnosing intermittent control failures from a finished ceiling later.
Diagnose DALI issues like this in seconds
ProbitSite is the DALI-2 certified handheld multimeter built for exactly this kind of on-site fault finding.