# RS485 Termination Resistor **URL:** \ **Category:** Support\ **Created:** [18 October 2022 01:27 UTC](https://forum.core-electronics.com.au/t/rs485-termination-resistor/15392 "2022-10-18T01:27:22Z")\ **Posts on this page:** 1\ **Showing post:** 6
**Author:** ![Murray125532](https://yyz2.discourse-cdn.com/flex028/user_avatar/forum.core-electronics.com.au/murray125532/32/6734_2.png) [@Murray125532](https://forum.core-electronics.com.au/u/Murray125532)\ **Post date:** [18 October 2022 21:14 UTC](https://forum.core-electronics.com.au/t/rs485-termination-resistor/15392/6 "2022-10-18T21:14:54Z")
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 🤪 ), but also gives a good description of the binary divide-and-conquer method of quick fault finding. [![](https://img.youtube.com/vi/skwUNRp-Rrk/maxresdefault.jpg "DIY keychain DMX tester for lighting technicians.") ](https://www.youtube.com/watch?v=skwUNRp-Rrk) A reasonable reference for using RS-485 (in the guise of DMX512) is here > **[DMX512](https://en.wikipedia.org/wiki/DMX512)** > > DMX512 is a standard for digital communication networks that are commonly used to control lighting and effects. It was originally intended as a standardized method for controlling stage lighting dimmers, which, prior to DMX512, had employed various incompatible proprietary protocols. It quickly became the primary method for linking controllers (such as a lighting console) to dimmers and special effects devices such as fog machines and intelligent lights. > DMX512 has also expanded to uses in no... The formal definition of the DMX-512A standard is here ( slow download) > **[ANSI-ESTA\_E1-11\_2008R2018.pdf](https://tsp.esta.org/tsp/documents/docs/ANSI-ESTA_E1-11_2008R2018.pdf)** > > 833.53 KB cheers Murray --- _[View the full topic](https://forum.core-electronics.com.au/t/rs485-termination-resistor/15392)._