RS485 Termination Resistor

Hi Stuart (and Rob),
This discussion brings up all my thoughts about the DMX-512 protocol used in Theatre lighting systems.

Especially the practice of termination of the data lines. Many Lighting techs carried a connector with a 120R resistor built in, to plug into the ‘last’ daisy-chain output on the string of fixtures (maximum of 30 devices due to the capabilities of the RS-485 line drivers). And a handy simple tool in the back pocket was a connector with a pair of red and green LEDs linked between the common and the appropriate data line ( with appropriate resistors in it too ) as a simple visual tester. Remembering that RS485 is a differential balanced signal - as the Data+ line goes high, the Data- line goes low, and vice versus. - to aid in noise rejection.

  • Both LEDs flickering - seems ok,
  • Only one flickering - the other data line is open somewhere, work backwards along the chain to find the dud.
  • no activity - hmm, is anything actually connected and switched on? Start at the controller…

here is how to make it… (he is very chatty :crazy_face: ), but also gives a good description of the binary divide-and-conquer method of quick fault finding.

A reasonable reference for using RS-485 (in the guise of DMX512) is here

The formal definition of the DMX-512A standard is here ( slow download)