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So I wan to control a bunch of devices. I though first of using RS485 but after some digging I figured out this might not work as I don't really have a pure daisy chain. The configuration I would like to use is 'star-daisy-chain':

enter image description here

Each 'Box' has an input and output for a main line and 4 small outputs. The length of the cables between boxes is around 10 meters and the length of the cables going out of the boxes is 2 meters at most. Both synchronous and asynchronous connections can work here. The bandwidth can be as low as 19200.

I would like to have 5 'Boxes' and 20 devices (at most)

What would be the best and simplest way to deal with this? It isn't so simple to daisy chain everything even though possible.

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    \$\begingroup\$ What does "Box" do? To what kind of device are "4 small outputs" going? Is "Controller" supposed to talk directly to devices on "small outputs"? Is it master/slave or do devices send data whenever they like? I think you need to be much more specific. \$\endgroup\$ – Rev1.0 Sep 25 '18 at 7:16
  • \$\begingroup\$ The box is just a box. I thought just connecting the differential wires together without any 'active' components. Or not. Box also has the power lines \$\endgroup\$ – Gilad Sep 25 '18 at 7:18
  • \$\begingroup\$ As @Rev1.0 pointed out, what is missing from the picture is what is at the end of those "small outputs", because that makes a difference between the junction box and active hub. For example if you want to connect another "just a box" to one of the small outputs to start another daisy chain then you need an active hub. Also, this topology is usually called "star bus", not "star-daisy-chain" \$\endgroup\$ – Maple Sep 28 '18 at 7:46
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You could use RS-485. The wires not following the main bus are called stubs and if they are short enough they don't cause problems (depending on the drivers, speed, type of cable etc.).

For example this design guide explains stubs, the limitations and formulas to make everything reliable: http://www.ti.com/lit/an/slla272c/slla272c.pdf

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  • \$\begingroup\$ This answer could benefit from a little bit of elaboration. e.g. dependence of stab length on network data rate and slew rate of the drivers. Ideally maximum data rate for OP configuration should be mentioned \$\endgroup\$ – Maple Sep 28 '18 at 7:36
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You can still make them daisy-chained if you design the cabling accordingly. Here is a suggestion:

enter image description here

It implies that for each device, there is an "enter" pair and a "return" pair. But if both pairs are in the same cable, it doesn't make things much more complicated to install. This is what ethernet actually does (with old-school passive hubs).

This will however divide by two the maximum cabling length you can use, but given the required speed and the constraints you gave, this is not a problem for RS-485 (max length is a several kilometers at this speed). It also implies that there is more wires per cable, but if you use something like regular cheap cat5 cable, you'll have enough pairs available anyway.

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  • \$\begingroup\$ The tag on the question implies this is another DMX setup. Interesting that DMX devices often have both IN and THRU connectors. DMX512 spec actually defines IN as male, OUT and THRU as female XLR jacks. Now, DMX512-A spec allows for RJ45 connectors with two data links, suitable for just the setup you describe. Unfortunately the pinout does not define the direction \$\endgroup\$ – Maple Sep 29 '18 at 17:21

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