Post by thread:Transmitting DMX-512

Hello, FYI, I am new to this list. I know how most lists feel about newbies, so if you'd like to send me a warm welcoming message, feel free. :) I've been working with PICs for around 6 months now, and just recently got a programmer. I wrote a program for a 16C57 to transmit DMX-512, but it doesn't seem to be transmitting the protocol correctly, or it's not at the correct speed (250K bps). I'm not sure exactly what the problem is, and I'm mailing the list in hope that maybe someone has some experience doing this. If anyone is willing to help, or for more information on exactly what I'm doing/using, please e-mail me privately! Thanks very much! :) Cheers, -- ~Keith spam_OUTtsk3000TakeThisOuTProdigy.Net http://pages.prodigy.net/tsk3000/

Keith, Welcome to the list! In transmitting DMX512 with a PIC (I do it with a 16c74a), it's kinda tricky to get the break timing right due to the double buffering of the transmit side of the uart. What I finally did is to use a timer interrupt instead of the uart interrupt. This way the uart transmits immediately on loading. When the timer interrupt comes back again, the uart has finished sending the byte. It is then safe to send the break. By the way, how are you sending the break? The 16c74a doesn't have a break function, so I did it in hardware. I put a resistor between the serial transmit data pin and the driver chip. The end of the resistor away from the chip is tied to another I/O pin that is normally tristated. When I want to send a break, I turn that pin on with a programmed zero. Then tristate the pin when you want to end the break. Finally, I used a state machine to figure out where we are (transmitting break, sending mark after break, sending start code, sending data bytes). Also, to see what's going on, I suggest toggling an I/O pin at various points in the code to trigger a scope. The above discussed "break pin" can be used for this, letting you see the break, mark after break, start code, and first few code bytes. If you have delay triggering, you can lootk at the whole string! Also, the Lil DMXter is a very handy piece of equipment. It will verify the protocol and give you various parameter values. Good luck! It CAN be done! See the StarPort at http://www.dovesystems.com . Harold

> -----Original Message----- > From: pic microcontroller discussion list > [.....PICLISTKILLspam@spam@MITVMA.MIT.EDU]On Behalf Of Harold M Hallikainen > Sent: Saturday, January 03, 1998 14:02 > To: PICLISTKILLspamMITVMA.MIT.EDU > Subject: Re: Transmitting DMX-512 > > > Keith, > > Welcome to the list! > In transmitting DMX512 with a PIC (I do it with a 16c74a), it's > kinda tricky to get the break timing right due to the double buffering of > the transmit side of the uart. What I finally did is to use a timer > interrupt instead of the uart interrupt. This way the uart transmits I was wondering if you could send me the info yoy have on DMX512, protoc ol specs, timing diagrams, schematics, hardware details, anything really. Thanks for any info. TTYL

Check out ABtech's Lighting System's technical expose on "The Anatomy of DMX12" I think you'll find the information there to be very helpful. http://www.abtec.demon.co.uk/index2.htm > I was wondering if you could send me the info yoy have on >DMX512, protocol >specs, timing diagrams, schematics, hardware details, anything really. >Thanks for any info. TTYL

On Sat, 3 Jan 1998 14:53:44 -0500 Herbert Graf <.....hgrafKILLspam.....GEOCITIES.COM> writes: > I was wondering if you could send me the info yoy have on >DMX512, protocol >specs, timing diagrams, schematics, hardware details, anything really. >Thanks for any info. TTYL > A good source for this info is "Recommended Practice for DMX512" by Adam Bennette. As I recall, info on ordering it is available at the USITT web site. Here's a quick summary... Connector 5 pin XLR 1 - shield 2 - data complement 3 - data true 4 - spare data complement 5 - spare data true Electrical Levels EIA 485 (0 volts and 5 volts), 120 ohm characteristic impedance Data rate 250 Kbps Byte format 1 start bit 8 data bits no parity 2 stop bits Frame format break - 88uS min mark after break - 8 uS min Start code (generally 0x00) - 1 byte Data bytes - 24 (min) to 512 Mark before break - 8uS min (corresponds to 2 stop bits of last byte) Harold