DMX (lighting protocol)

[tannewt]

This is used by DJs to control lighting.

[siddacious]

If this ever gets done a great project would be an Artemis "red alert" siren:

http://artemiswiki.pbworks.com/w/page/68387358/DMX%20Interface

[siddacious]

I found these which might be useful reference
https://github.com/jaydcarlson/microcontroller-test-code/tree/master/SAMD10/dmx
https://github.com/claudeheintz/LXSAMD21DMX

[s-light]

it is also used in theater, music-shows and other stage / event things .
also in architecture (storefront and similar ambient-lighting)

some more references i found on my search that could be helpfull:

• Generating DMX512 - simple demo pointing to clacktronics/pyb_dmx
• claudeheintz/LXESP32DMX
• chrisstaite/TeensyDmx (active development)

[ariherzig]

I'm also very interested in there being a way to send and receive DMX-512 data with CircuitPython.

[tannewt]

Can this be done with UART at the lowest level? Do we just need a Python library for it?

What is the least expensive testing setup?

[wallarug]

Hey @tannewt ,

A Python library would be the easiest way to do it. But it might need doing at the hardware level of the SAMD21/51. I read in the datasheet that these chips should support RS485 at the hardware level.

I know only one python library that does this well at the moment. https://github.com/mathertel/DMXSerial

At the heart of DMX:

• serial RS485
• 250 kbit/s
• 513 byte transactions (frames)

Element 14 DMX Explained

[wallarug]

After extensive reading from the adafruit forums from 2018 ... please ignore my comments about direct hardware support.

The SAMD51 has some support for RS485 but external hardware is still required. The SAMD21 has no built in support.

The only way is to use a transceiver module (MAX3485 is most popular) and normal UART protocols with a packet decoding layer (presumably this is what everyone in this issue is looking for).

I'd imagine anyone who wants this would do something similar to the below:

class DMX512:

    def __init__(self, uart):
        self._device = uart


    def read(self):
        # recieve 513 packets from DMX protocol
        data = self._device.read(513)

        # do something with the data
        #  maybe remove first packet

        recv = data[1:]

        return recv

    def write(self, values):
        # send 512 bytes via DMX protocol
        # check data first
        if len(values) < 512:
            raise Exception("data too short")

        # maybe convert values to bytes

        # send data to devices
        self._device.write(values)

I have been looking into this for about 7 years and been doing stage lighting for 10+ years - that's why I'm interested 😄 .

[s-light]

@tannewt least expensive could be some 'amazon dmx led spot' (you never know if they are conform to the specifications...) -
the better setup: a scope or digital-logger/analyzer thing... ;-)
or just some already 'proven to work' arduino library..
all the implementation can be tested in the 3.3V / 5V TTL levels...
as @wallarug has written - to get to the actuall DMX512 HW-Layer ('just' RS485) some differential driver /receiver is needed..
here would be the challenge to check and find one that is 3.3V compatible..
out of my head i don't know one..
but there are lots of options form different manufactures - example TI -
and than its just a question of 'do you want to be specification conformant' -
if true you have to think about isolation...
if not - just ignore isolation - in most small setups it works ;-)

theoretically you could do it with just an UART implementation..
but this could get hacky - as you need to fastly switch baudrates to generate the stop bit timming.. (if i remember correctly)
lower level - maybe the 'USART' support in the chip can handle this thing already in a smoother way...
DMX is in the basic version only Master → Receiver → Receiver→ Receiver.
so uni-directional.

if someone wants to implement RDM (remote device managment)
it gets more complicated and timing-critical - as now the master has to listen between sending normal frames for the responses from the devices.
so its half-duplex - for this you normally need to toggle a pin to tell the driver if it is in send or receive mode..

@wallarug your example is only partly correct:
DMX is not specified to transmit all 512 channels. you are allowed to send less!
and sometimes that is usefull! in my classic arduino projects i used this to reduce performance /memory problems ;-) (only sending the ~12 channels the device needed...)