7 www.slservice.pl S-LIGHTING
S-3
3.3 dMX-512
- 2-conductor twisted pair plus a shield,
- maximum capacitance between conductors - 30pF/ft,
- maximum capacitance between conductor & shield - 55 pF/ft,
- maximum resistance of 20 ohms / 1000 ft (304.8m in normal),
- nominal impedance 100 - 140 ohms.
Cable connectors:
Cables must have a male XLR connector on one end and a female
XLR connector on the other end (duh!).
A word on termination: DMX is a resilient communication protocol,
however errors still occasionally occur. Termination reduces signal
errors, and therefore best practises include the use of terminator
in all circumstances. If you are experiencing problems with erratic
device behavior, especially over long cable runs, a terminator may
help improve performance.
CAUTION: Do not allow contact between the common and the
device’s chassis ground. Grounding the common may cause
a ground loop, and your device may perform erratically. Test cables
with an ohm meter to verify correct polarity and to make sure
the pins are not grounded or shorted to the shield or each other.
To build your own DMX Terminator:
Obtain a 120-ohm, 1/4-watt resistor and
wire it between pins 2 & 3 of the last
xture. They are also readily available from
specialty retailers.
A DMX terminator
To link devices together you’ll
need data cables. You should use
data grade cables that can carry
a high quality signal and are
less prone to electromagnetic
interference. For instance,
Belden© 9841 meets the
specications for EIA RS-485 applications. Standard microphone
cables will probably be OK, but note that they cannot transmit
DMX data as reliably over long distances. In any event, the cable
should have the following characteristics:
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