The other component of DMX512 is the Electrical Specifications. The hardware electrical specs are listed in EIA-485, commonly refered to as the RS-485 specs. The RS-485 standard, specifies only the electrical characteristics of the driver and the receiver to be used at the line interface; it does not specify or recommend any software protocol. Northlight’s encoder is compliant with RS-485. Each Decoder represents less than 1 node, load to the system.

RS485 is a data transmission system using balanced differential signals. That is 2 signal wires and signal ground. 3 wires are required.

Splitters/Repeaters
 Isolation between the console and dimmers is sometimes required to prevent signal degredation and protect devices from damaging voltages on the control cable. Optically isolated splitters help avoid these problems.

 Each DMX512 output can drive up to 32 devices. If there are more devices on the line, a “repeater” or “booster” is required. Some cheap devices  are not fully compliant and actually represent a load equivalent to 2 or more devices.
 Long or improper cables, electrically noisy environment (generators, motors)  and improper use of passive “Y” splitters all contribute to DMX signal degredation. A repeater/booster may help to solve these problems.

Why ask WYE?
 Wye(Y) splitters are NOT recommended for DMX512 systems. Wye splitters are simply a male inline XLR connector, parallel wired to 2 female inline XLR’s. While convienent, Wye splitters cause unwanted signal reflections and possible ground loops, leading to signal degredation.
 The best layout for DMX systems is a Daisy chain configuration, where the signal cable jumps from one device to the next, with no branching. Each chain can have up to 32 devices on it. When using an isolated splitter, each outputs can be a separate DMX daisy chain.

Termination
A common problem with DMX systems is improper termination.
 A simple terminator consists of a 120 Ohm resistor connected across pins 2-3 of the DMX signal. More complex terminators utilize voltage spike protection and bi-color LEDs to indicate signal integrity.
 Terminators are an impedance matching circuit required to damp signals that “reflect “ from the end of an improperly terminated cable, causing signal degredation under certain conditions.
 On devices the have a DMX thru , a male XLR connector with terminating resistor connected across pins 2-3 and installed on the DMX thru connector will suffice. Some devices with isolated outputs will not use a terminator on the DMX out. These usually have an internal terminator that is selected with a switch.

Wire Type
  There is a difference between microphone cable and “Data” cable. Sure you can get away with mic. cable for short runs in many situations. However on longer runs or marginal situations mic. cable will let you down. You may have random errors or the system won’t work at all. It comes down to insurance. If you want to insure the most reliable DMX signal distribution you need the most appropriate wire for the job.
  DMX512 requires wire suitable for RS-485, there is no way to get around that.
 For RS-485 buses , stranded, twisted-pair cable is the most common. You can use a range of wire gauges, but designers most frequently use 22 - 24 AWG. The characteristic impedance of the cable should be 100 to 120 Ohms.
 Some other requirements are, at least 1 twisted pair plus ground and shield. It should have low capacitance  and overall braid and foil shield.

 Data Rate VS Cable Length
You can transmit data over an RS-485 bus up to 4000 ft at slower data rates. The rule of thumb is longer the cable, the slower that data rate, and vice versa. At 250K bits per second the max cable length is about 1000 ft for DMX512 in good conditions.

Connectors
 DMX512 protocol specifies that 5 pin XLR connectors be used. Female on the transmitter and male on the receiver.
 When a 3 pin XLR is used  it is wired the same as the first 3 pins on the 5 pin XLR.
 

PIN WIRE   SIGNAL 1 - shield     ground/return 2 - signal     data compliment ( - ) 3 - signal     data true ( + ) 4 - signal     spare data compliment ( - ) 5 - signal     conductor spare data true ( + )  Conductors 2/3 and 4/5 should be twisted together.