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

DMX 512 is RS485 serial at 250 Kbaud, as such any installation must
meet the requirements of EIA-485. In particular use of suitable
good quality cable is important (Mic cable is NOT suitable) suitable
cables include :

Belden 9841
Belden 9842
Alpha 5274

Amongst others.
This is not a complete list.

RS485 is NOT the same thing as RS232 It cannot be made from a standard PC
serial port. You can damage a DMX device by attempting to plug RS232 into it!


3.1 Wiring practice.
--------------------

At the speed DMX works correct wiring practice is important. Always arange
wiring such that the data source is at one end of the cable. NEVER do this :


*----*-----*------------*---------*
| |
* *
|
*---*---*
|
|
*---*



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. Dasychain everything, but see the
section on isolation.


3.2 Line termination.
---------------------

All DMX lines must be correctly terminated if reliable operation is
to be obtained. Some equipment has a switchable line terminating resistor
built in. In these cases make sure that only the last item in the chain has
its terminating switch set to on. If the last item (Farthest from data
source) does not have a terminating switch then a external line terminating
network is needed. To make one you will need the following parts :