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DMX512 = RS485
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DMX512 = RS485
PostPosted: Fri Aug 11, 2006 4:15 am     Reply with quote

Hi !

This is protocol known in music industry becuse it is used for lighting effects, dimmers, fog maschines etc.

Does someone knows if this protocol can be used with combination of WLAN and rs485 ? Is this protocol the same as rs485 ?.

Thanx !
neil



Joined: 08 Sep 2003
Posts: 128
Location: Oxfordshire, UK

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PostPosted: Fri Aug 11, 2006 4:43 am     Reply with quote

A quick search on Google found this...
http://www.euro-pa.be/dmx.html
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PostPosted: Fri Aug 11, 2006 4:55 am     Reply with quote

I saw this pages ( the dmx-online has even more infos ), but what i dont understand is :

If i use only rs485, can i make such data transmition ?
Rocket



Joined: 29 Aug 2005
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PostPosted: Fri Aug 11, 2006 5:03 am     Reply with quote

Quote:
If i use only rs485, can i make such data transmition ?


NO.

The DMX512 standard uses a serial protocol. as seen at http://www.dmx512-online.com/packt.html
It is not directly compatible with a 'normal' uart.

An implementation would require a protocol converter e.g. a PIC.

Do you need a transmitter, or a receiver?
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J.I.L.
Ttelmah
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PostPosted: Fri Aug 11, 2006 5:22 am     Reply with quote

It is perhaps worth understanding, that 'RS232', and 'RS485' etc., are _not_ 'protocols'. They are _signalling standards_ (defining the voltages used to send the data). You can then 'talk' using these standards, using different 'protocols'. This is one reason why the '#use RS232' command, is incorrectly named (you use the same command to talk over RS485, and the PIC cannot actually generate 'RS232' voltages...).
Now, DMX, is a protocol definition, which can (just) be generated by the PIC UART. You have to use 9bit mode, and program the ninth bit to a '1'. The actual signalling standard used, is RS485.
You will need a fairly high clock speed PIC, if you intend to 'do much', while handling DMX, since the data rate is high (250Kbps).

Best Wishes
Guest
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PostPosted: Fri Aug 11, 2006 8:29 am     Reply with quote

Quote:
An implementation would require a protocol converter e.g. a PIC.
Hm, like Ttelmah wrote, i need some high clock speed pic to make this work, but the question is how much of the dmx do i need and how much can be done with rs485


Quote:
Do you need a transmitter, or a receiver?

Well, the idea was to have one transmiter ( on some control board ) from which i can send data to 5 devices ( lights ). Is this possible on this way ?
Rocket



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DMX512
PostPosted: Fri Aug 11, 2006 8:34 am     Reply with quote

Ttelmah is correct in differentiating between a protocol and the electrical interfaces.
The challenge to generate a data stream which conforms to the DMX512 protocol standard using a PIC and its USART is to generate the ‘BREAK’ and ‘MAB’ signals which indicate the start of the data stream.

Best wishes
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Rocket



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DMX512
PostPosted: Fri Aug 11, 2006 9:13 am     Reply with quote

The PIC needs to do the whole data stream generation, the ‘BREAK’ and ‘MAB’ and the data . Once the ‘BREAK’ and ‘MAB’ signals are generated with bit banging, the rest of the signaling could be done with the USART.

A 4Mhz PIC could generate the 250Kbps needed. Faster would be better (4Mhz increments).

The 0 to 5V output from the USART is sent to a RS485 driver IC e.g. LTC485. Thus, the serial protocol (now DMX512) is placed on a pair of wires. RS485 is thus just an electrical interface standard.

Regards.
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PostPosted: Fri Aug 11, 2006 12:36 pm     Reply with quote

Can u please bescribe me in more detail MAB and BREAK signals and "bit banging " part...
Humberto



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PostPosted: Fri Aug 11, 2006 4:47 pm     Reply with quote

MAB and BREAK are short names of some of the frames that belong to the DMX protocol.
If you want to implement a DMX512 with a PIC, you will need to generate every single
frame according to a predefined standard.
The different stages for a full packet are:
1) Idle
2) Break
3) Mark After Break (MAB)
4) Start Code (SC)
5) Mark Time Between Frames (MTBF)
6) Channel Data (CD)
7) Mark Time Between Packets (MTBP)

The only part of the protocol where you can use the regular CCS RS232 format (using
9bit mode as Ttelmah said) is in the frame called SC (Start Code) where at 250Kb every
single bit width should be of 4us.


Humberto
Ttelmah
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PostPosted: Sat Aug 12, 2006 2:35 am     Reply with quote

I have to disagree slightly.
The whole protocol can be managed using the standard RS232 command, _with the exception of the break_.
The rest is just a matter of pausing transmisions at the right time.
MAB, requires an 8uSec pause.
SC can just be sent using the RS232 command.
MTBF, is just a matter of a pause, and the protocol allows this to be as short as 0uSec. Most modern equipment accepts this.
The channel data sends using the standard RS232 commands.
MTBP, can also be as short at 0uSec, but many systems do require a pause. For both this, and the MAB, I just used a interrupt driven timer, to allow a programmed transmission pause.

Best Wishes
Rocket



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PostPosted: Sat Aug 12, 2006 3:31 am     Reply with quote

Hi Ttelmah

You are 100% correct.. Embarassed

If you have the time, and the inclination, would you please post sample code for the transmitter?

Kind regards
Rocket
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Ttelmah
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PostPosted: Sat Aug 12, 2006 4:07 am     Reply with quote

I can't unfortunately, this was done as a 'job' a while back, and the agreement requires 'non disclosure' on the sources.

Best Wishes
Mark



Joined: 07 Sep 2003
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Location: Atlanta, GA

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PostPosted: Sat Aug 12, 2006 3:25 pm     Reply with quote

Actually the break can be generated with the RS232 as well. The trick is to set the UART speed slower and transmit a 0. Once the TSR is empty, switch back the UART speed and transmit the data. If you really want to make it simple and only use the UART, you can generate the other timing parameters by disabling the transmit and then transmitting a char.

On the receiving side, I look for a framing error to detect the break signal. The received data should be 0 as well. Once I receive the break, I look for the start code of 0 and the next data byte will be the first channel.

You do not need to transmit all 512 channels if you are only concerned with 5 channels. I would only do those and only repeat the stream as often as you need to in order to allow the tranmitter time to do its tasks.

Now it sounds as though you are only looking for a protocol to use for your own purposes. If you do not need to interface to other DMX devices, then you can certainly slow down the transmission rate. The product that I have running DMX uses a 20MHz crystal and works just fine. For the transmitting device, I did you a high priority interrupt (PIC18) since the interrupts occur about every 44us.
Jakin
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PostPosted: Tue Aug 15, 2006 6:50 pm     Reply with quote

Hi, All:
I am asking if anyone has a sample code for RS485 Master and Slave. If you do, could you please post it? Thank you so much ~!

Regards,
Jakin
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