Electronics 2000

As far as I can tell ... a DMX is a device with a USB input and some software and a computer... one way or another it can switch lots of switches very fast.
What about flinging the computer and software and controling it with a home made curcuit.
If there are only 2 output wires ... how does it switch 128 or so switches? does it have a dmux at the switch end?

Tar&stuff.

DMX is a standard protocol which is used to transmit data along a twisted pair (RS485).

The data basically has a start, then data bits. A DMX transmitter can be a PC with a DMX dongle or it could be a lighting desk. After the start bit the transmitted data consists of 512 channels of data, each channel being a value between 0 and 256. I think this is the pulse width that says what the data is but not 100% sure.

A receiver such as a moving head, LED par can etc has a setting in it, usually by means of a dip switch (using binary), rotary switches or a menu screen which is used to set the start address which is what the fixture will respond to.
Fixtures may do different things to a number of channels, for example an LED par can may use 5 channels. If you set the start address to 24 it will react to channels 24 through to 28. It may be channel 25 is red, 26 green, 27 blue and the value of the channel setting the relative intensity. Channels 24 could tell the par can what to do i.e. chase between colours and channel 28 speed depending on the value of each channel. (I have some LED pars like this).

You can daisychain devices together using the same line, up to 32 drops I think. It is possible to have more using powered splitters.
You can have more than one fixture responding to the same address if you wish, LED par cans are OK like this but I wouldn't do it with a moving head at it looks rubbish.

DMX should only be used for things that do not pose danger, for example you would never use it to fire pyrotechnics as is isn't fail safe.

The data is sent over two wires and ground. It is a 9 bit serial signal at 250 k baud. The receiver waits for an 88uS low and then waits for data. The data consists of a string of 512 sets of 9 bit data (each set being a lighting channel). Each set of 512 channels are known as a universe. DMX can very easily be transmitted and received with a PIC micro controller using a very small amount of code. The line driver and receiver device is usually a SN75176.

I have built many DMX units including RGB, LED lamps, strobes, dimmers and effects. For instance, you can build an RGB, LED lamp using a PIC16F737, SN75176, six transistors, a few resistors, caps, diodes and a crystal. You use an 8 way DIP switch to select the start channel by selecting it on the switch using binary 1, 2, 4, 8, 16, 32, 64, 128 (256 channels).

Hope this helps.

Daniel.

Well that sort of answers my question but I am still in the dark.
What I would like to do is to decode usb information and manipulate it for what ever I want.
When I plug into a usb there are 2 power (no probs ... understood) and 2 data lines 1 in and 1 out...

How does it recognise a number of devices and turn them on and off?
How can I control in/out data from the key board?
How do I read the code?

Tar&stuff.

Hi again.

USB communication is quite complicated. You would have to use a PIC micro that has USB capabilities and write the code to convert USB serial data into DMX signals. There are many PC based stage lighting programs that output to USB, but they require a dongle to convert to DMX. However, I have used a standard serial (RS232) port and a PIC to convert to DMX. I used Visual Basic to write to program to send the relevant serial data.

Daniel.

Yea... eeeerrrrrrrmmmmm... ... ...

I am not sure if I need DMX and am confusedish, I understandish, I am not realy looking for lighting circuits but want to understand USB data.
Suppose you plug in a number of USB devices, one comunicates with the computer then the next, then the next etc.
How does it do that, why do they not all switch on at the same time?
I can understand multiplexing but how does it organise its self?
Any books on the subject etc?

Tar&stuff.

Hi again.

Take a look at this link, it may help.

http://electronicdesign.com/boards/how- ... sb-devices

Daniel.

Thanks, I have blown my computer up so am mostly off line just now...
Will get back soon.
The link looks good.

Tar&stuff.