Difference between revisions of "Talk:DMX512-A"

Revision as of 12:33, 31 March 2009

Timings

The clock rate is 250 kHz so each symbol bit on the wire is 4 microseconds long (period time).

Frame rate is ?? Packet rate is typically around 44 Hz when all frames are sent.

RDM protocol additions

The name of the standard is: ANSI/ESTA 1.20, Entertainment Technology - Remote Device Management over USITT DMX512 ??? Mark (beginning of frames) Mark After Break (beginning of frames)

Mark time between packet Mark time between frames

Use of category 5 UTP or STP

New cable types is used, and may be officially accepted. http://www.usitt.org/standards/DMX512_FAQ.html#FAQ_08

The use of modular plugs like 8P8C (RJ45) or 6P4C (RJ11)

The cabling for DMX512-A should be described in the document called "BSR E1.27-1 -- Portable Control Cables for Use with USITT DMX512/1990 and E1.11 [DMX512-A]"

Sender/receiver topologiess

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 ...?

More suggestions and info

Here is also a nice overview of the different parts of a DMX packet with timings etc.: http://www.erwinrol.com/index.php?stagecraft/dmx.php

Voltages

The power dissipation in the 120 Ohm terminating resistors depends on the differential voltage between the two data wires. If the transmitter only makes a 5 V differential voltage, the power dissipation is P= U*U/R= 5*5/120 = 208 mW.

According to http://focus.ti.com/lit/an/slla070c/slla070c.pdf the maximum absolute differential voltage allowed by the EIA485 standard is 6 V. This give the maximum power dissipation is P= U*U/R= 6*6/120 = 300 mW. So it is best to use 1/2 W resistors.

This web page cites RS485 as having a upper limit of +12/-7 V with respect to ground: http://www.dmx512-online.com/physl.html

Transciever chips made for 5 V: Linear Technology LTC485: http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1007,C1017,P2064 National semiconductor LMS485: http://www.national.com/mpf/LM/LMS485.html National semiconductor DS75176B (used in Martin PAL 1200, Lite-Puter DX-625 and a cheap Eurolite DMX console) http://www.national.com/mpf/DS/DS75176B.html

Debugging tips

Links to simple testers?

How can reverse polarity be detected?