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Potential DMX problems

 

  DMX512 data link reliability
  Improved DMX512 data link reliability

  DMX GENERAL
  About DMX512
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Here are some DMX512 "gotchas" -- general aspects of DMX usage to watch out for. (For specific aspects related to LD2000, see the Troubleshooting DMX page).

  • 3-pin vs. 5-pin connectors: In some places, particularly when working with older lighting fixtures, 3-pin XLR cables are used instead of 5-pin XLR cables. Sometimes you will need to create a 3-pin to 5-pin converter.
            The main thing to remember here is that pins 1, 2 and 3 have the same meaning on both 3-pin and 5-pin connectors. This means that when making such adapter cables, you connect pin 1 of the 3-pin connector to pin 1 of the 5-pin connector, etc. (Note: Pangolin's QM2000 board uses an adapter cable with a female 5-pin XLR connector. Click here for pinouts.)

  • Data link termination: Since DMX512 uses relatively high data transmission rates, the end of the cable needs to be terminated, particularly if it is over 25 feet long. A terminator is nothing more than a XLR connector with a built-in terminator resistor that you connect to the output of the last DMX device on the DMX link. DMX terminators can be purchased, or you can make your own by connecting a 120 ohm resistor between pins 2 and 3 of the DMX connector.

  • Refresh rate too fast: Although the DMX512 standard provides for very fast transmission where each data packet can immediately follow the last data packet with no delay, very few DMX devices can actually work with such a fast data rate. Most devices require some delay between packets or a lower refresh rate to process the data received in the last packet.
            If your DMX device is not receiving DMX or seems intermittent, you should try to increase the packet-to-packet delay if your controller allows you to do this. Alternatively, you can increase the number of channels transmitted to lower the refresh rate of the link.

DMX512 data link reliability

Sometimes newcomers question the reliability of a serial data link like DMX512. There are two conceivable problems that could occur in the field:

  • The receiver can "drop data bits" in a channel or channels. The consequences of dropped data bits depends on the device being controlled. For simple lamps, this would result in the lamp level fluctuating. A similar thing would happen for any kind of beam actuator or color selector. A fluctuating lamp level or color might go completely unnoticed. For something like a strobe light, the results could be that the light is triggered at the wrong time which would be very visible.

  • The receiver can "drop channels". When this happens, the data integrity for all data channels is in jeopardy, not only the data channels dropped. No matter what is being controlled, the results of dropped channels would probably be very visible.

In reality, things like dropped bits occur very rarely; dropped channels almost never occur. When you add the "continuous transmission" aspect of DMX512, data link errors are essentially unnoticed. As long as good quality cable is used, and as long as the end of the data link is terminated, you should never see any of these problems.
        If you do need to troubleshoot the DMX data, you need either a scope, or a special DMX receiver. (Try the MicroTech DMX Tester, or the DMX Line Tester from Doug Fleenor Design.)

Improved data link reliability in upcoming DMX512 revision

The proposed DMX512/2000 specification provides for a System Information Packet (SIP). This is a special packet which uses a start code of 207 decimal (CF hex) to differentiate it from the standard data packet which uses a start code of 0. The System Information Packet includes the following information:

  • Checksum of the last packet

  • Number of channels in a standard packet

  • Other information including sequence numbers, software revisions and test information

Perhaps the most significant use of the SIP will be to improve data integrity. Since the SIP provides information about the number of channels transmitted in a standard packet and the checksum of all data sent in the last packet, this information could be used to verify that the data sent in the last packet has been received correctly. To do this, the system information packet is "interleaved" with normal data packets.

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  This page last updated: Wednesday, Januari 29, 2020

 
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