TWO-WIRE ANALOGUE
DIMMER MULTIPLEXING
STANDARD D54

This standard is placed in the public domain for the free use of all interested parties, in the interests of encouraging standardisation amongst equipment suppliers. Where compatibility is claimed it is requested that it be described as "complies with Rank Strand Dimmer Multiplex Standard D54".

TWO WIRE ANALOGUE DIMMER MULTIPLEXING STANDARD

1 SCOPE

This standard describes the electrical and mechanical criteria to ensure signal compatibility between control system multiple outputs and dimmer inputs to ensure reliable interworking. A second signal is similarly described to allow the multiplexing of manual faders into the control system.

2 DESCRIPTION

2.1 Outline
The multiplex dimmer levels are transmitted down a screened cable using time-division multiplexing with timing information derived from superimposed sync and end-of-frame pulses for a normal capacity of up to 384 dimmers per cable. An expansion capacity of up to 768 dimmers per cable is described. For larger installations, two or more multiplex cables shall be used. A typical system block diagram is shown in Fig.1.

In all cases the control system is the master and the dimmer and/or fader interfaces act as slaves. Each interface may just drive dimmers, multiplex faders or both as required. Each interface shall be coded to respond only to the dimmers of interest and means shall be provided to assure predictable response in the case of incomplete or absent multiplex drive.

One signal (and conductor) is used to multiplex the dimmer levels (DMX) and an optional second to multiplex faders (FMX).

2.2 Signal Specifications

The transmitter and receiver are specified separately at their output and input terminals respectively to allow for tolerances and signal distortion in typical cables.

The DMX signal is terminated by a distributed A.C. termination configured to provide a noise immunity appropriate to the size of the system.

The FMX signal is fully terminated at its receiver and has a current sink to ensure that when not driven it settles to zero. Each driver for the FMX signal shall be gated onto the cable only for the duration of the fader numbers being multiplexed. Inadvertent connection of two fader multiplexes to the cable, both coded for the same faders, may give indeterminate results but shall not cause damage. A front porch is set to 0V to minimise bus conflict between fader multiplexes at change-over from one interface to the other.

2.2.1 Impedances and Levels

Notes

1) Outputs to be short-circuit proof to 0V to unlimited periods.
2) The termination resistor is paralleled for multiple dimmers received, e.g.24 dimmers use 1k.
3) The termination capacitor is set to give a 1µs time-constant, e.g. for Rt = 1k, Ct = 1,000pF.
4) Full Level may be adjusted at the control system.

INSERT FIG 4

2.3 Cable and Connections

The cable used shall be a single (for DMX only) or twin screened cable generally of good microphone quality. Whilst cables may be chosen for the most economic performance in each particular installation, for optimum performance to achieve a maximum transmission distance of 1km a cable should be used as below:

Where input/output connectors are used, the following type and connections are to be preferred:

XLR 3 PIN (e.g.Cannon AXR-3-XX)

3 EARTHING CONSIDERATIONS

Precautions shall be taken to ensure that no earth loop, leakage or external fault current flows down the cable screen. Particular attention should be taken to avoid multiple earth connections in situations where multiple Neutral-Earth connections are made (e.g. PME mains distribution systems), since these can cause neutral current to flow through the screen.

Connection, if required, between 0V and mains earth shall only be made at the main control system.

Notes

1) FMX Delays start at leading edge of Sync local to the receiver..
2) Provision should be made at the receiver to accept an expanded capacity of 768 dimmers. This will not support FMX signals and Td (min) = 50µs

D54, Issue 1