Alcorn Mcbride V+ series, User Manual

Page: 216 / 276

13-4 DMX Machine Hardware Reference
DMX Output Port
The DMX Output port connections are as follows:
Pin Connection
1 Common (Shield)
2 DMX TX-
3 DMX TX+
4 N/C
5 N/C
Table 4 – DMX Out connections
The DMX line is a differential communications signal which conforms to
RS-485 standards. This means that it utilizes both positive and negative lines to
transmit a signal, thereby reducing is susceptibility to external noise. Even
though the DMX signals are well protected from external noise, they must be
protected from INTERNAL noise as well. When the DMX controller sends a
DMX signal down the line, that signal has a tendency to reflect back toward the
DMX unit once it reaches the end of the line. This “reflected” signal is harmful
because it can degrade the quality of real signals.
To prevent signal reflection, a 120 Ohm terminating resistor should be placed
across the positive and negative receive terminals (RXD+ and RXD-) of the
LAST receiver on the line. Placing the resistor across the pins will effectively
terminate the signal at the last unit while still allowing the last dimmer to receive
the signal. Many dimmer packs provide a built in resistor and switch for such
termination. Otherwise you will need a discrete resistor.
Figure 1 – A 120-Ohm Terminating Resistor is placed across the RXD+ and RXD- lines
of the Last Receiver.
Note Some DMX Dimmers provide a switch or jumper to activate internal
termination. If your Dimmers include this feature, do not place the terminating
resistor across the receive terminals.