Is it possible to passively convert RS-232 to DMX512

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Twentytwo22

Joined Dec 11, 2018
1
Hey, I'm new here and I wasn't sure which section to post this in...

Anyways

I have to control a light via dmx and don't have any DMX controllers however I have heard over an extremely short run you can control a DMX (RS-485) device with your RS-232 port. If this is the case... Then it only needs to go one way. With a 3 pin XLR connector. It's a matter of input to the light. No reading anything.

Is this possible?
 

dendad

Joined Feb 20, 2016
4,451
Do you already have RS232?
USB to RS485 adapters are pretty cheap.
Sp are RS232 to RS485. If you get one, make sure it has 3 terminals. N+, N- and com.
Do not use a 2 wire network. It does need the common.

Depending on your devices, you may get away with...

RS232........RS485
com............com
TXD............N-

But that is not ideal.
N+ may need to be biased slightly +ve.

Best get a real converter.
 

Sensacell

Joined Jun 19, 2012
3,432
Understand that the TIMING of data is part of the DMX 512 protocol.
Just changing the levels is not enough, you need to replicate the data packet format.
 

Marley

Joined Apr 4, 2016
502
This can be done. Important thing to note is that there is a difference between RS232 and serial.
RS232 specifies the +12V/-12V signalling. Serial is the data structure (timing and start and stop bits, etc.).

I have a device based on a Raspberry Pi that outputs DMX512 from the Pi's serial port. But this is a 3.3V logic level not RS232. An external RS485 driver chip is used to create the DMX signals.

The baud rate is a non-standard 250k Bd but can be set with a little hacking.
A "break" and a "MAB" has to be sent at the start of every set of frames. This can be done approximately by changing the baud rate on the fly and sending a specially crafted byte.

See the picture
 

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Marley

Joined Apr 4, 2016
502
If you are interested, years ago when I was doing some designing of some DMX512 stuff I looked up all the recommendations and produced a little document showing all the required timing and voltage levels required.

Had to look back through my files but here it is!
 

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