Talk:DMX512-A

connectors

The use of modular plugs like 8-position modular connector (8P8C or RJ45) or 6P4C (RJ11)? Is there any allowance for other than 8 position modular connectors? Some product (cheap dimmers) have 6 position connectors (RJ11).

Sender/receiver topologies

To avoid ground loops and improve reception performance, transmitters and/or receivers can be grounded/floating/non-isolated ??? http://www.usitt.org/standards/DMX512_FAQ.html#FAQ_15 There are different topologies, and not all will work together. There are no requirement for what topology to use in the DMX specification from 1990, so it was added to DMX512-A.

Transmitters should use "earth ground" as a reference for the positive/negative voltages that is put on the two data lines. If they don't, it must be clearly marked on the product and in the manual. Receivers should be ...?

Voltages

At http://www.dmx512-online.com/physl.html there is an example wit D1+ at +5V and D1- at -5V, and considering page 12 in this document from TI it is presumably a wrong interpretation.

This web page cites RS485 as having a upper limit of +12/-7 V with respect to ground. This probably refers to common mode loading voltages that receivers must work with. I.e. one of the data wires is allowed to reach +12 or -7 V but the opposite wire must not be at the other extreme, but only differ by 6 V maximum.

Debugging tips

Links to simple testers?

How can reverse polarity be detected?

• The break at the very beginning of a packet must be low
• If all transmitted data bytes have value zero, the DMX line should be low about 80 % of the time if there are no extra idle time between slots/frames or packets.