USITT Inc.
Suite 5A,
10 West 19th st.
New York,
NY 10011-4206
USA
Tel : (212) 924 - 9088
Fax : (212) 924 - 9343
Electronic Industries Association (EIA) 2001 Eye Street,N.W. Washington D.C.20006 USA.
PLASA Ltd.
7 Highlight House,
St Leonards Road,
Eastbourne
East Sussex BN21 3UH
UK.
*----*-----*------------*---------*
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* *
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*---*---*
|
|
*---*
Where * is a DMX plug/socket pair. I have seen this done! It worked
(sort of, sometimes), but would fail in unpredictable ways when
extra devices were plugged in. DMX cannot be split by simply
soldering 3 cables together. Dasy-chain everything, but see the
section on isolation.
2>-----------+------------+
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- -
| | | |
| | | |
| | | |
- -
| +
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| LED
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3>-----------+------------+
The BiColour led has 2 LED's (one Red and one Green) back to back
in the same package. Build this into the back of a 5 pin XLR
with the LED showing out of the back.
To use :
If the led fails to light in any one of the 2 above states then there is a short between one of the lines and pin 1.
Its limited but surprisingly useful!
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).
Most DMX compatible 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. An opto isolator is highly recommended 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 usual 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. However 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.
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 they can usually be solved by making sure that the desk, Monitor and Demux are at the same ground potential. You sometimes get REALLY weird behaviour with this.
PMX (Pulsar) : This protocol is EIA 232. I do not have any more details.
CMX (Colortran) : DMX was based on this (Amongst others). CMX equipment can usually be easily modified for DMX.
SMX (Rank Strand) : Designed to provide fault reporting and other nice features, unfortunately it is so complex that it is seldom seen in practice.
Midi Show Control (MSC) : Not really a lighting control protocol, but so common on large shows that I have decided to put it in. It provides show control functions, like synchronising sound and lighting with pyro. This is used BIG TIME in the theme park industry.
Contributed to by : Loren Wilton.
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