Newsgroups: rec.arts.theatre.stagecraft
Subject: Re: Ideas for special DMX-512 controller
References:  <[email protected]>
Eric N Johnson  writes:

> Tomi Holger Engdahl  wrote:
> > Idea is that normally thing are controlled with buttons, no computer 
> > needed, it can be disconnected, powered off or crashed.. 
> > When needed, the computer can take in and do controlling. 
> > The computer has always information on the state that goes to 
> > the dimmer. 
> 
> Almost any microcontroller can do this.  It's easier if you have a full
> universe worth of RAM (512 byes) and a hardware UART, but it's not
> absolutely required.  

I know that. I have played with few DMX-512 circuits based on PIC 
microcontrollers.. both receving and transmitting. 
I have used project found from Internet (parallel to DMX-512 interface, 
Digi Pack dimmer). I have looked their code, and even tried 
some small modifications to them. 

> Fading is easier if you have a hardware multiplier,
> but it's also not required. The RS-485 interface is easily accomplished
> with a line driver, and a few passive protection components.  

I know. Been there, done that. 

> The PIC 18F series is great for DMX, many commercial products use variants for
> the 8051, and I've even seen a few AVRs.  Since you only need 6 channels,
> I'll bet you could even pull this off with some clever programming at a
> PIC12 series.

I have Velleman K8048 PIC programmer / test board 
(details of it can be found at 
 http://www.epanorama.net/phpBB2/viewtopic.php?t=1259
and http://www.velleman.be/product/view/?lan=1&id=350903 )

It supports the following controllers:

PIC12F629, PIC12F675
PIC16C83, PIC16CR83, PIC16F83, PIC16C84, PIC16CR84,PIC16F84, PIC16F84A
PIC16F870, PIC16F871, PIC16F872, PIC16F873, PIC16F873A, PIC16F874, PIC16F874A, PIC16F876, PIC16F876A, PIC16F877(A)(ICSP only)
PIC16F627, PIC16F627A, PIC16F628, PIC16F628A, PIC16F648A
PICF630, PIC16F676
PIC16F818, PIC16F819

I have mostly myself used PIC controllers from PIC16F series 
(PIC 16F84, 16F627, 16F628, 16F870

> There's enough information online to build a DMX controller, but if you're
> serious, you'll want to get a copy of USITT DMX512/1990 and ESTA E1.11
> DMX512A.  These are the standards documents that define DMX.

I have the public review version of DMX512 standard...
> 
> The one thing watch out for if you're only sending 6 channels is the minimum
> breat-to-break spec.  (IIRC it's around 1100 us, but I don't have the spec
> in front of me).  It's often a good idea to send more than 6 channels, even
> if you're only using 6 of them.

Thank you for your tips. 

I already know that if you just send few channels it is 
not a good idea to repeat the data sending too often. 
Some pause between different data sending is a good idea. 
 
> > Feedback from what is fed to the dimmer to computer direction is 
> > a plus (doable with bidirectional other DMX-512 interface). 
> 
> DMX by itself is uni-directional.  There is an up and comming standard for
> talkback called RDM (E1.20), but it's not widely supported yet.  Most
> existing talkback systems use vendor proprietary protocols.

I was thinking on the way that the computer has DMX-512 interface 
that has also input functionality... it just stays on the DMX-512 
line together with the dimmer pack listening to the data getting there. 

Other option to get feedback if I make my own microcontrollerboard 
is to make it to send the status of different channels to the PC 
though the serial port that it receves the commands from PC. 

The idea is that the controlling can be done with the buttons 
independent od the PC, but when PC is on it all the time knows 
what is the status of the dimmer channels no matter if the control 
commad came from the PC or from control buttons. 


This dimmer is just a part of the planned system. 
There are also plans to include other things that are 
controlled with control buttons + computer (on/off 
type circuits). Everything should work both with 
computer and without it. And computer when it is  
powered on will always know the status of the system. 


-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/