DMX512 Physical layer Electrical

- Aug 29, 2017 -

Physical layer

Electrical

DMX512 data is transmitted over a differential pair using EIA-485 voltage levels. DMX512 electrical specifications are identical to those of the EIA-485-A standard, except where stated otherwise in E1.11

DMX512 is a bus network no more than 1,200 metres (3,900 ft) long, with not more than 32 unit loads on a single bus. If more than 32 unit loads need to communicate, the network can be expanded across parallel buses using DMX splitters. Network wiring consists of a shielded twisted pair, with a characteristic impedance of 120 Ohms, with a termination resistor at the end of the cable furthest from the controller to absorb signal reflections. DMX512 has two twisted pair data paths, although specification currently only defines the use of one of the twisted pairs. The second pair is undefined, but required by the electrical specification.

For short cable runs of less than about 45 metres (148 ft) with only a few devices, it is sometimes possible to operate without termination. At short distances, cables with higher capacitance and different characteristic impedance such as microphone cable can be used. As the cable length and/or number of devices increases however, following the specification for termination and correct cable impedance becomes more important.

The E1.11 (DMX512 2004) electrical specification addresses the connection of DMX512 signal common to Earth ground. Specifically, the standard recommends that transmitter ports (DMX512 controller OUT port) have a low impedance connection between signal common and ground; such ports are referred to as grounded. It is further recommended that receivers have a high impedance connection between signal common and ground; such ports are referred to as isolated.

The standard also allows for isolated transmitter ports. The standard allows for non-isolated receivers. The standard recommends that systems ground the signal common at only one point, in order to avoid the formation of disruptive ground loops.

Grounded receivers that have a hard connection between signal common and ground are permitted but their use is strongly discouraged. Several possible grounding configurations which are commonly used with EIA485 are specifically disallowed by E.


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