Pathway DMX Pathfinder CR Manual - Manual (Page 6)

DMX Distribution System Page 6
Operation
In order to gain a full understanding of the how the DMX Distribution System is
intended to be utilized, and, by extension, an appreciation of the advantages of this
system, it is necessary to discuss briefly some concepts of data communications,
how they have been applied to lighting control in general, and to this type of
installation specifically.
.1 Digital vs. Analog -- Data communications typically involves the
transmission of digital rather than analog signals since they are less
susceptible to errors resulting from distortion, especially at high speeds.
Since the mid-1980s, digital data transmission has been the accepted
standard in the entertainment lighting control industry.
.2 Serial vs. Parallel -- Most current data transmission methods involve the
use of serial-by-bit techniques (one bit follows the other in a serial fashion)
rather than parallel-by-bit (or serial-by-character: one character follows the
other), since less wiring and other supporting devices are required. Serial
data may also be easily transmitted over significantly longer distances
without as much concern about noise and crosstalk.
.3 RS232C vs. RS422A/485 -- The most commonly accepted electrical
standards governing the serial transmission of digital data are the Electronic
Industries Association (E.I.A.) RS232C, RS422A and RS485. These
standards have little or nothing to do with the format, or protocol, of the data
being transmitted. Rather they specify the physical parameters of the data
link, such as driver/receiver type, operating voltage, cable type, and in some
cases connector type. RS232C differs greatly from the other standards in
signal voltage (+/-12V as opposed to +5V) and often in the additional wire
connections it requires for control and handshaking functions. Further,
whereas RS232C uses just one wire each for transmitted and received data,
the other standards use twisted wire pairs for each function, over which all
control and information data is carried as differential signals. The main
advantages to that arrangement are very high noise immunity and high
operating speed (up to 10 million bits per second versus 38,400 for RS232C)
over cable runs of up to 1000 meters or more.
RS485, essentially an enhanced version of RS422A, differs in that a
maximum of thirty-two receivers may be connected, versus sixteen for
RS422A. Cable characteristic impedance is slightly different (120 ohms for