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DMX voltage spike issue

djgeorgef

By djgeorgef
in General Technical Chat

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Brian Production Manager

Brian

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A few comments...

Leads - the cable shield is connected to pin 1 and pin 1 only. It is never, ever, to be connected to the connector body. Ever.

DMX Receivers - there are 3 possible circuit configurations for a DMX receiver.

  1. Isolated. Pin 1 is connected to the input circuit's signal common. It is not connected to the chassis or any other earthed point.
  2. Non-Isolated. Pin 1 is connected to the input circuit's signal common which is then connected to the chassis and incoming earth via a resistor, typically 100R. The resistor is there to limit the current flowing in the cable shield in the event of a system fault.
  3. Grounded. Pin 1 is connected to the input circuit's signal common, the chassis and the incoming earth.

Isolated is the preferred configuration, the other suck from a systems engineering stance. If using Grounded receivers it is highly recommended to have a fully isolated transmitter.

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djgeorgef Part-timer

djgeorgef

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7 hours ago, Brian said:

A few comments...

Leads - the cable shield is connected to pin 1 and pin 1 only. It is never, ever, to be connected to the connector body. Ever.

DMX Receivers - there are 3 possible circuit configurations for a DMX receiver.

  1. Isolated. Pin 1 is connected to the input circuit's signal common. It is not connected to the chassis or any other earthed point.
  2. Non-Isolated. Pin 1 is connected to the input circuit's signal common which is then connected to the chassis and incoming earth via a resistor, typically 100R. The resistor is there to limit the current flowing in the cable shield in the event of a system fault.
  3. Grounded. Pin 1 is connected to the input circuit's signal common, the chassis and the incoming earth.

Isolated is the preferred configuration, the other suck from a systems engineering stance. If using Grounded receivers it is highly recommended to have a fully isolated transmitter.

Thank you that’s very informative, we appear to have scenario 3 in our dimmer! By a fully isolated transmitter, would a DMX splitter with optical isolation suffice in your opinion. 

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Jivemaster Production Manager

Jivemaster

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I see two choices. 

A/ send an isolated DMX to each fitting

B/ send DMX and mains from the control position (assuming that you have the power available). It's possible to have leads with DMX and mains correctly built into a single sheath. Remember that DMX outputs on a socket (should be XLR5!!) but audio outputs on pins (look at a mic!) do leads sold for powered speakers and their audio feed will need remaking, Terralec do some cables with XLR3 for DMX and IEC for mains, likewise a powercon/XLR3 lead, likely other suppliers will have them too. 

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