Pathway DMX Pathfinder CR Manual - Manual (Page 7)

DMX Distribution System Page 7
Operation
RS485, 100 ohms for RS422A). The main difference is that RS485 is
capable of half-duplex transmission on the same wire pair, although this
mode of operation is not compatible with the DMX Distribution System.
.4 Communications Channel -- This is the path (sometimes called the data
link or universe) over which the electronic signals travel from the transmitting
device (i.e. the control console, also known as the data source) to the
receiving device(s) (e.g. the dimmer rack, also called the data sink or
station). The channel may connect to several receivers, however only one
transmitter may be connected to a given channel at one time. Some data
communications systems are capable of multiplexing several sub-channels
over one communications channel simultaneously, however this is not done
in entertainment lighting applications. Note: in this usage the word "channel"
should not be confused with the control channel function within lighting
consoles.
.5 Multiplexing -- Multiplexing, which can be loosely defined as the
simultaneous transmission of two or more signals over the same
communications channel, is usually accomplished by one of two different
schemes.
Frequency-division
multiplexing (FDM) is a technique of dividing a channel's
maximum bandwidth (or frequency) into a number of narrower frequency
bands or subchannels. Modulation and demodulation (as with modems) is
then used to first translate and subsequently retrieve the information being
transmitted over the different subchannels. This method is effective for
moving data over a communications channel of limited bandwidth, such as a
telephone voice line.
Time-division
multiplexing (TDM) involves the interleaving of several input
signals over the same communications channel. The signals are
sequentially sampled at the transmitter, and then separated and
reconstructed at the receiver. This method of multiplexing is preferred where
the channel's bandwidth is higher, as with a dedicated twisted-pair line. A
variation, asynchronous time-division multiplexing, is the approach used for
transmitting digital data in most lighting control systems. The signals carry
level or control information corresponding to various devices, such as a
dimmers or scrollers, and are represented by sequentially transmitted frames