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Electronic Theatre Controls Inc

DMX Speed

DMX-512 has a number of timing variables. The information presented here will probably be useful for only a small proportion of users, but can assist in getting troublesome DMX equipment to work.

If you need further information, the book " Recommended Practice for DMX512: A guide for users and installers" is highly recommended and available from The Esta Foundation.

DMX Timing.png

There are several areas where DMX timing can vary in a DMX packet. Each packet follows the format shown above.

First, a break occurs telling the receiver that this is the start of a new packet. Then the line is driven high for the Mark after Break (MAB) time. Then follows the start code, followed by the "post start" time. Then there follows the channel data, during which the line can be high (idle) for a variable amount of time between channels, known as the interbyte time. Finally after the last channel the line goes high (idle) for the interpacket delay time until the break of the next packet.

The following are the minimum values allowable by the ESTA DMX-512A standard :

Timing Parameter  Minimum Maximum
Break 92μs Note 1
Mark-After-Break 12μs Note 1
Post-Start 8μs (Two stop bits)  Note 1
Interbyte 8μs (Two stop bits)  Note 1
Interpacket 8μs (Two stop bits) 

Note 1

Note: These timings can be any value to add up to make a packet length of up to one second.

As you can see a very wide variety of timings is permissible. For reference, here are the timings for some different ETC products.

Eos, Congo and Cobalt Family Consoles

  Post Start (μs) Break (μs) MAB (μs) Packet (ms) Interbyte (μs) Interpacket Idle (μs)
Maximum 36 142 128 24 12 12
Fast 36 142 128 27 18 16
Medium 36 142 128 34 30 30
Slow 36 142 128 42 46 44

Smartfade, Software Version 3.0.0

  Post Start (μs) Break (μs) MAB (μs) Packet (ms) Interbyte (μs) Interpacket Idle (μs)
Maximum 0 176 16 22 0 0
Fast 12 176 44 27 4 2000
Medium 16 220 100 34 16 4000
Slow 36 264 100 50 36 8000

Express, Software Version 3.1

  Post Start (μs) Break (μs) MAB (μs) Packet (ms) Interbyte (μs) Interpacket Idle (μs)
Maximum 32 94 26 26 12 1160
Fast 32 138 28 26 12 1160
Medium 32 184 32 26 12 2120
Slow 32 270 50 31 12 6960

4 Port Net3 Gateway and Response DMX/RDM Gateway

Software Version 5.1.0+, ACN Mode

PORTS 1&2 Post Start (μs) Break (μs) MAB (μs) Packet (ms) Interbyte (μs) Interpacket Idle (μs)
Maximum 8 176 12 22.8 8 180 - 432
Fast 12 192 60  25.3 12 2320
Medium 12 240 60 25.3 12 4260
Slow 12  288 60 25 12 8320

 

PORTS 3&4 Post Start (μs) Break (μs) MAB (μs) Packet (ms) Interbyte (μs) Interpacket Idle (μs)
Maximum 8 194 12 23.9 8 150-400
Fast 12 188 48 25.2  12 2260
Medium 12  240 64 25.3  12 4140
Slow 12 272 64 45  48 8560

4 Port Net3 Gateway, Software Version 4.0.5.9.0.36, Net2 Mode

Observations

  • Variable DMX output speed is not implemented
  Post Start (μs) Break (μs) MAB (μs) Packet (ms) Interbyte (μs) Interpacket Idle (μs)
Any 8 126-132 56 24 8 784-1260

2 Port Net3 Gateway, Software Version 4.0.5.9.0.36, Net2 Mode

Observations

  • Variable DMX output speed is not implemented
  Post Start (μs) Break (μs) MAB (μs) Packet (ms) Interbyte (μs) Interpacket Idle (μs)
Any 8 176 12 23 8 1260

2 Port Net3 Gateway, Software Version 1.0.0.9.0.83, ACN Mode

  Post Start (μs) Break (μs) MAB (μs) Packet (ms) Interbyte (μs) Interpacket Idle (μs)
Maximum 8 176 12 22 8 176
Fast 8 176 12 24 8 2160
Medium 12 240 60 29 12 4200
Slow 13 288 60 33 12 8240

2 Port ETCNet2 Node, Software Version 4.1.3.9.0.6

  Post Start (μs) Break (μs) MAB (μs) Packet (ms) Interbyte (μs) Interpacket Idle (μs)
Maximum 8 88-93 48 22 8 8
Fast 12 156 48 25 12 90
Medium 16 184 74 27 16 162
Slow 24 226 100 31 24 254

Four Port ETCNet2 Node, Software Version 4.1

  Post Start (μs) Break (μs) MAB (μs) Packet (ms) Interbyte (μs) Interpacket Idle (μs)
Maximum 8 94 8 23 8 1140
Fast 8 142 13 26 8 3040
Medium 8 188 18 27 8 5160
Slow 8 282 33-35 32 8 8800
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