DMX512 Protocol Explained: Timing & Frames

DMX512 is a 250 kbps, RS‑485 based protocol that sends repeating frames made of a break, MAB, a start code, and up to 512 data slots. This guide explains how the frame works and how timing affects real‑world behavior.

What a DMX frame actually contains

Short answer: a DMX frame is a timed sequence that starts with a break and then streams channel values in order.

• Break: signals the start of a new frame
• Mark After Break (MAB): the idle time after break
• Start code: usually 0 for standard DMX data
• Slots: 1–512 channel values in sequence

In practice, receivers read channel values by counting slots after the start code. This makes addressing deterministic, which is why clean timing matters.

How refresh rate really works

Short answer: the more slots you transmit, the slower the refresh rate.

Full 512‑slot frames refresh at around 44 Hz; shorter frames can update faster. This is why large rigs feel slower even when the controller is fine.

Related guides: DMX timing and refresh rate and Live show DMX timing checklist.

Why DMX can feel “laggy” even when latency is low

Short answer: refresh rate and fixture processing create perceived lag.

• Long frames reduce refresh rate.
• Some fixtures smooth or buffer channel changes.
• Poor cabling adds errors that look like jitter.

For deeper diagnostics, see DMX latency and jitter and Latency vs jitter vs packet loss.

Timing fixes that actually help

Short answer: send fewer slots, keep the line clean, and eliminate reflections.

• Only transmit the channels you use.
• Use true DMX cable (120 Ω twisted pair).
• Terminate the last fixture with 120 Ω.
• Verify your XLR pinout before blaming timing.

Practical references: DMX cables in practice, DMX cable & termination field guide, and 3‑pin vs 5‑pin XLR for DMX.

When you need to care about the start code

Short answer: most rigs use start code 0, but RDM and special data use other codes.

Start code 0 is standard DMX. Other start codes are used for bidirectional or special data. If a fixture behaves strangely on shared lines, check whether RDM is enabled on the controller.

See: RDM explained.

In practice: how to sanity‑check a DMX frame

Short answer: verify frame size, refresh rate, and end‑of‑line termination before you replace hardware.

1. Confirm the controller is not sending unused channels.
2. Measure refresh rate against your channel count.
3. Inspect cable type and termination.

If the rig still behaves oddly, a DMX tester can reveal intermittent framing errors.