Pathway DMX Pathfinder CR Manual - Manual (Page 11)

DMX Distribution System Page 11
Operation
described, all of these channels include a separate circuit for communications in the
reverse direction. All data channels are fully opto-isolated from the input signal and
each other. Figure 1-2 on the next page shows a simple 1-in, 4-out opto-repeater
application. There is one other important issue to be dealt with:
.15 Line Termination -- When a high frequency electrical signal is transmitted
through a cable, the signal source experiences a certain impedance, referred
to as its characteristic impedance. This impedance is a function of the
design and manufactured characteristics of the cable. If the signal's
impedance changes suddenly as it travels along the cable, for example if the
end of the cable is open, a reaction occurs which causes the signal to
bounce back toward the source. This is known as signal reflection, and it is a
common cause of poor performance in data communications networks.
Reflection can cause communication errors and in extreme cases render the
system inoperative.
This problem is easily solved by connecting a resistance, equal to the
characteristic impedance of the source signal (110 ohms in the case of the
DMXPathfinder), at the open end of the data cable. Most DMX receiving
equipment incorporates this termination feature, often by providing a
termination switch. If several receivers are connected to the same data link,
only the last one in line should be terminated. Multiple terminations will
overload the source line driver and attenuate the signal excessively. The
system’s opto-receivers (input modules) and opto-repeaters (output
modules) all include termination resistors for the source DMX lines and
auxiliary (return) data lines. Figure 1-3 shows the application of termination
devices to a simple DMX distribution network.