"DMX512 mini-FAQ" - читать интересную книгу автора (Mills D.)

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The DMX standard specifies 5 Pin XLR style connectors, Howether only 3 pins
have standardised uses. The remaining pair (Second data link) are used for
anything from Overtemp. indication to supplying power to control desks! If
using equipment from more then one manufacturer Check the spec. carefully
as to the use that is made of these pins. Several manufacturers use 3 Pin
XLR's for 'DMX' This is not standard. It may be worth having several jumpers
to convert between these (and to isolate the second data link).


3.5 Splitting a DMX feed.
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You cannot split a DMX feed by simply soldering 3 cables together. Doing
this may work some of the time but it cannot be relied upon. Most DMX
compatable equipment has a DMX out socket fitted and this may be used to
loop into other equipment. If you need to split a DMX feed a splitter box
can be used. These can also provide isolation such that a fault on one line
does not cause a malfunction on the other line. A opto isolator is highly
recomended on lines feeding dimmers as a fault could destroy other equipment
on the line. Note that a line from a isolated port is considered to be a new
DMX line.
These can be obtained from any of the usaual suppliers of DMX hardware, or
they can be made cheaply by anyone with a electronics background.
The EIA 485 standard supports a maximum of 32 devices on a line. If more are
needed a repeater is needed. This will allow another 32 devices to be added
for a total of 63 devices (The repeater counts as a device). This can be
continued for more devices. Howether Where large networks are required it is
better to use a splitter to produce multiple lines at the source. This means
that if one line fails the problem will be limited to that section.

It is also worth noting that not all instruments have "proper" RS-485
receivers in them. Some of the wiggle lights (early I-beams?) simply stuck
the LED in an opto-isolator across the DMX line, rather than a real receiver.
This is OK, *as long* as you are only driving ONE thing from the line, and
the line isn't too long. I think these things daisy-chained, with each light
regenerating the signal for the next instrument.


3.6 Other protocols.
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Some short notes on other common protocols.

AMX 192 (USITT) : This analog TDM protocol was used before DMX became
common. Mostly found in the US. it suffers from all the problems inherent
in any analog protocol (Earth loops, Noise). It is carried on 5 Pin XLR.

D54 (Rank Strand) : This is also a analog TDM protocol. It is single ended
and VERY prone to earth loop problems. If you have problems with this one