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I read this topic DMX on Arduino with RS485 and I want to use this in my new project.

Good to know is that my project concerns daisy chaining several DMX devices, so I need DMX IN as well as DMX OUT on every device.

These are my questions about what I read on the mentioned topic:

  • the first schematic has 3 resistors, R1, R2 and R3, with funny values: 562 Ω and 133 Ω. What is meant by that (the second schematic has resistors with common values)?
  • the second schematic has resistor R5 with value 470 Ω. Is that correct?

I checked the datasheet and I think the values 330 Ω and 470 Ω are mixed up. This is the recommended interface circuit as mentioned in the datasheet:

enter image description here

As you can see, the resistor on pin 2 has 470 Ω, the one on pin 6 has 390 Ω.

Or ... am I missing something? Please, correct me if I'm wrong.

  • in my project I will try to implement this DMX circuit. I understand that this schematic is meant for a DMX IN connection.

I want to take this a step further and daisy chain several devices: I need a DMX OUT. What do I need for that? I expect I need another MAX485/481 chip. I'm unsure on how to do that. The same way as the first MAX485 by connecting RO, RE, DE and DI in parallel with the first one?

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  • \$\begingroup\$ Just to be sure, you want to be the "lamp" that can only receive data? Not the lighting contol desk that can only transmit data? Standard DMX chain, no need to alter the data between input and output? Also you seem to have too many questions in one post, some of the questions are due to misunderstanding so when some questions are answered, other questions are irrelevant. \$\endgroup\$
    – Justme
    Commented Jan 13, 2022 at 12:52
  • \$\begingroup\$ @Justme - My device is on the receiver side, indeed. But it is nog alone, some more devices are in the chain that also need receiving DMX signals. So, I need a DMX IN (=dmx coming from the sender) and DMX OUT (=to the rest of the chain). Pretty much what other DMX device like lighting ie. do. And no, the signal is unmodified (otherwise would be strange). Sorry for being confusing. Better split up in different posts, I guess? \$\endgroup\$
    – Ivan Burvenich
    Commented Jan 13, 2022 at 16:22

1 Answer 1

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By specification, the DMX in and out connectors on a DMX device are just wired together - all devices are on a common bus. If you have a DMX input, you don't need anything else for the DMX output.

Your first question about a device with "funny values: 562 Ω and 133 Ω" are for a device that is at one end of the bus - so it's the DMX controller driving the bus. Since it has the bias resistors to keep the bus in idle state even when controller is not transmitting, it means the DMX controller supports receiving RDM messages.

The second question about a 470R resistor driving an optoisolator - Compared to what the optoisolator needs, and what the RS-485 receiver can output, the 470 ohm resistor does not sound a good value.

The third circuit you show contains a completely another type of driver so it requires completely different resistance values.

And no, DMX output is definitely not added by putting another MAX485 and connecting the RO, RE, DE and DI in parallel with the first one, it makes no sense in any context to connect two RS-485 transceivers in parallel like that.

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  • \$\begingroup\$ I apologize for, perhaps, 'silly' questions, but hey, I'm novice ... So, thank you for taking time to make things more clear. I can go on now with my setup and start basic tests. I will follow the recommendations of the datasheets and certainly implement opto-isolation which seems a wise thing to do. Greetings from Belgium. \$\endgroup\$
    – Ivan Burvenich
    Commented Jan 13, 2022 at 21:04
  • \$\begingroup\$ It might be worth to download and read the freely available standard for DMX specifications. Just remember that opto-isolation does not isolate, unless you also use an isolated power supply, and that RS-485 transceivers with built-in isolation do exist, so there is no need for separate isolated supplies and opto-isolators. Happy DMXing. \$\endgroup\$
    – Justme
    Commented Jan 13, 2022 at 21:14

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