DMX512 Timing: Break, MAB, Slots and Refresh Rate
How many times per second a DMX universe refreshes affects everything on stage: whether a fade looks smooth, whether a moving head stutters, whether a camera sees banding on your LEDs. That number is not arbitrary — it falls straight out of the packet structure.
In short
- DMX runs at 250 kbit/s; one bit lasts 4 µs.
- Each slot is 11 bits: 1 start + 8 data + 2 stop = 44 µs.
- Packet: BREAK (≥92 µs) + MAB (≥12 µs) + 513 slots.
- A full universe ≈ 22.7 ms → at most ~44 refreshes per second.
- Sending fewer channels shortens the packet and raises the refresh rate.
- An out-of-spec short BREAK is the classic cause of “some fixtures don’t work”.
Bit time: where everything starts
DMX512 carries 250,000 bits per second. The direct consequence is that a single bit lasts 4 microseconds. Every duration in the packet is a multiple of that 4 µs.
Anatomy of a packet
Every DMX packet arrives in the same order:
| Section | Purpose | Duration |
|---|---|---|
| BREAK | Holds the line low for at least 92 µs, telling receivers a new packet is starting | ≥ 92 µs |
| MAB (Mark After Break) | A short high level marking the end of the break | ≥ 12 µs |
| Slot 0 (START code) | Identifies the packet type; 0x00 means normal lighting data | 44 µs |
| Slots 1–512 | Channel values, each 0–255 | 44 µs each |
Why is a slot 44 µs?
Because each slot is really an 11-bit serial frame:
1 start bit (0) 8 data bits (least significant first) 2 stop bits (1) ----------------------------------- 11 bits x 4 us = 44 microseconds
Maximum refresh rate
A full universe means 513 slots including the START code:
513 slots x 44 us = 22,572 us = 22.572 ms + BREAK (92 us) + MAB ~ 22.7 ms 1 / 0.0227 s ~ 44 packets per second
So a full DMX universe can refresh at most about 44 times per second. However fast your console is, it cannot push data down the wire more often than that. It is the physical ceiling of the protocol.
Shorter packets are faster
The standard permits sending fewer than 512 slots. If you only use 128 channels the transmitter can stop there, cutting packet time to roughly 5.8 ms and pushing the theoretical refresh rate above 150 Hz. On LED walls shot on camera this makes a visible difference — but confirm every device on the line handles short packets correctly.
Gaps between packets
The standard allows anywhere from 0 to 1 second of idle time between slots (MTBS) and between packets (MTBP). That flexibility lets slow transmitters remain compliant, but it also means a receiver can never assume how often data will arrive. Well-designed fixtures therefore latch the last valid value and hold it for a defined period if the data stops.
When refresh rate actually matters
- LEDs on camera. A 44 Hz refresh can show up as banding or flicker on a high-frame-rate camera. The root cause is usually the fixture’s own PWM frequency, but a low DMX refresh makes it visible.
- Long, slow fades. A 30-second fade from 0 to 255 on an 8-bit channel steps through 256 values and visibly stair-steps at the bottom end. The fix is not faster DMX but 16-bit channels.
- Fast-moving heads. Position data updates 44 times per second; during quick moves the intermediate positions come from the fixture’s own smoothing algorithm.
- Cheap equipment. Transmitters producing an out-of-spec short BREAK, or receivers with narrow tolerance, cause random faults in a rig that looks compliant on paper.
Checking timing when troubleshooting
If you have a DMX tester or a scope, measuring the BREAK duration and packet interval tells you a lot. A BREAK shorter than 92 µs makes some receivers miss the packet entirely — and that is the classic cause of the maddening “some fixtures work, some don’t” fault.
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