Pathway DMX Pathfinder CR Manual - Manual (Page 8)

DMX Distribution System Page 8
Operation
on the communications channel. Each input signal always occupies the
same frame position in the serial data stream.
.6 Channel Transmission Modes -- A communications channel may operate
in a simplex, half-duplex or full-duplex mode. In simplex mode,
transmissions are in the same direction at all times. Half-duplex describes a
transmission that may be in either direction, but only in one direction at a
given moment of time. When transmissions can occur simultaneously in
both directions, this mode is known as full-duplex. Most remote-control
lighting equipment available to date (1997) operates in the simplex mode,
however certain products do make use of half-duplex or full-duplex
transmissions. The DMX Distribution System is, to a certain extent,
compatible with all of these modes of transmission.
.7 Channel Transmission Speed -- The speed of data transfer in a serial data
communications system is described in a number of ways. The most
common unit is the Baud rate, which refers to the maximum rate at which
signal pulses may be transmitted along a data channel. The effective data
rate, however sometimes called net data throughput, is affected adversely in
asynchronous systems by the necessity of transmitting start and stop bits as
well as certain control codes and even long "breaks", along with the
information bits. The DMX Distribution System is designed to operate to at
least two million pulses (or bits) per second (2 MBaud), although present-day
lighting control protocols run at only 250 KBaud or less.
.8
Communications Protocols -- In order for receiving equipment to be able
to understand and process data transmissions from a data source, certain
"rules of etiquette" must be established and agreed upon by both sets of
equipment. This is a simple definition of a protocol. More specifically, the
function of protocols can be summarized as follows:
.1 establish the starting and ending points of a data transmission
.2 assure message integrity during the passage of data from the
transmitter to the receiver
.3 provide for the orderly interleaving of control and data information with
a mechanism to distinguish between them.
Several different communications protocols, each with their own specific
Baud rates, have been in random or widespread use in entertainment lighting