Thread: RDM repeater
View Single Post
Old March 4th, 2019   #3
Task Group Member
Join Date: Aug 2008
Posts: 369

To reiterate what Hamish said: an RDM repeater is basically a single port splitter. It will need to meet all of the requirements for a "Transparent In Line Device" that are defined in the standard.

The timing requirements in the standard (delay, break shortening, etc.) are designed to allow up to 4 cascaded in line devices, but you'd need 5 cascaded repeaters in your topology. You might be able to have 5 if your repeater has timing performance that's much better than what's required in the standard.

I built a 1-in:1-out repeater like what you're describing, it was a fun project.

One nit pick: you'd get 30 fixtures per segment under the conditions you describe, not 32. The controller at the start of the segment and the repeater at the end contribute to the limit. Like this


But here's the most important point: There is *no* 32 device limit in RS485. That limit doesn't exist. There's a 32 "unit load" limit, but 1/4, 1/8, even 1/16 unit-load drivers are readily available. With 1/8th unit-load drivers you can hypothetically have 256 devices on one segment.

Make sure you think about fault containment in you design. Wiring them all in an end-to-end daisy chain may make it easier for the installer, but you'll have at least 500 connections along the path. Any single connection going bad can take down a large portion of the chain. Using a single 1:N splitter at the start may give you a more reliable system, and when there is a wiring fault it will only take out a smaller portion of the system.

Last edited by ericthegeek; March 4th, 2019 at 09:00 AM. Reason: GUI Editor added lots of etra line breaks
ericthegeek is offline   Reply With Quote