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DMX Output board for 6502 based systems
Posted: Sun Apr 28, 2024 5:59 pm
by Sittingbull
Hi Everyone,
I'm a long term 6502 fan in Italy, obviously I had a C64 when I was a child.
It's from 2020 that i'm crafting my own 6502 Singleboard/Multiboard computer (later a brief description)
I wish to share my latest creation, just in case someone is interested, a DMX output board for 6502 based system.
By now it's on breadboard, I'm waiting to receive a PCB from a well known prototype manifacturer.
Here some features:
- 2Kb static ram on board
- It delivers 512 DMX channels without any charge on the CPU
- It has got 3 registers: DMX_ADD_LOW,DMX_ADD_HIGH,DMX_DATA
- Interfaces directly on main BUS
- needs an 1Mhz clock signal to work properly
- 5V power source
Shortly I will post schematics and photos...
just to mention:
SittingBull is my 6502 SBC
classic layout 6502+6522+32Kb SRam+32Kb EPROM (16Kb used+16Kb i/o address space)+6551UART
currently working on:
VGA card 640x480sync, 160x120,3Bpp (8 colours)
SPI interface SD-CARD reader/writer
Re: DMX Output board for 6502 based systems
Posted: Tue Apr 30, 2024 1:29 pm
by Dr Jefyll
"
DMX512, a communications protocol that is most commonly used to control stage lighting and effects"
Shortly I will post schematics and photos...
Welcome, Sittingbull
And BTW just a reminder... on this forum you're allowed to include attachments with your posts.
-- Jeff
Re: DMX Output board for 6502 based systems
Posted: Tue Apr 30, 2024 7:14 pm
by BigDumbDinosaur
"
DMX512, a communications protocol that is most commonly used to control stage lighting and effects"
I have to admit I had no idea what DMX512 was before doing an Internet search on it. My knowledge of stage lighting stops in the era when Variacs controlled things. 
Re: DMX Output board for 6502 based systems
Posted: Wed May 01, 2024 3:37 pm
by Sittingbull
"
DMX512, a communications protocol that is most commonly used to control stage lighting and effects"
I have to admit I had no idea what DMX512 was before doing an Internet search on it. My knowledge of stage lighting stops in the era when Variacs controlled things.
I forgot... DMX512 or simply DMX is a serial protocol used to control stage lights and more.
It's basically an implemetation of RS485 on a balaced 120Ohm line.
It's easy, it's just bitbanging at 250Kbit/s. My first version was made with the 6551 uart but it was so demanding on cpu that it could do only that job.
Re: DMX Output board for 6502 based systems
Posted: Wed May 01, 2024 4:00 pm
by Martin_H
I would love to see the details on your VGA interface. Do your use CPU generated signals or video chip?
Re: DMX Output board for 6502 based systems
Posted: Wed May 01, 2024 6:00 pm
by Sittingbull
Hi Martin,
i'm pleased of your interest.
in brief I use counters to take out timing signals programmed in a EPROM, then counters (llots of 74ls161) to take out 3 bit from a 32Kbyte Sram (video ram) in order to put color signals in sync.
The bus interface is 2x74ls273 and a 74ls245 with some glue logic + 4x74ls157.
The problem is that writing an entire screen causes lots of flickering... sooner or later I will put on the logic to allow writes only during blanking, but I don't want to insert wait states to the processor, I will use a FIFO buffer. To be enginereed...
Re: DMX Output board for 6502 based systems
Posted: Wed May 01, 2024 6:09 pm
by Sittingbull
Here you are...
PCB with chips mounted, soon resistors and capacitors...
Kicad project. please note that kicad project is derived from breadboard construction, and I could not test pcb yet... it can be Buggy...
when everytingh will be tested I will notice here...
Re: DMX Output board for 6502 based systems
Posted: Wed May 01, 2024 6:16 pm
by fachat
Some empty holes are for bypass capacitors? Best practice is to have an own bypass cap per chip. With all chips 74LS types chances are it'll work, but your mileage may vary
André
Re: DMX Output board for 6502 based systems
Posted: Wed May 01, 2024 7:57 pm
by Sean
"
DMX512, a communications protocol that is most commonly used to control stage lighting and effects"
I have to admit I had no idea what DMX512 was before doing an Internet search on it. My knowledge of stage lighting stops in the era when Variacs controlled things.
I forgot... DMX512 or simply DMX is a serial protocol used to control stage lights and more.
It's basically an implemetation of RS485 on a balaced 120Ohm line.
It's easy, it's just bitbanging at 250Kbit/s. My first version was made with the 6551 uart but it was so demanding on cpu that it could do only that job.
Reading up on it, I am guessing DMX512 was what the 80186-based digital lighting control board we had in the high school theater used. There was an XLR-type connector on the lightboard which had a cable that ran to a wall socket in the tech booth. Down in the basement there was a similar cable running to a huge rack of dimmer units, that controlled the mains voltage for around a hundred circuits that ran up to and above the stage and out to the catwalks.
Re: DMX Output board for 6502 based systems
Posted: Wed May 01, 2024 9:41 pm
by BigDumbDinosaur
Reading up on it, I am guessing DMX512 was what the 80186-based digital lighting control board we had in the high school theater used. There was an XLR-type connector on the lightboard which had a cable that ran to a wall socket in the tech booth. Down in the basement there was a similar cable running to a huge rack of dimmer units, that controlled the mains voltage for around a hundred circuits that ran up to and above the stage and out to the catwalks.
When I was in high school (I was a freshman in 1959), the auditorium stage lighting was controlled by manually turning the knobs on multiple Variac autoformers. The smoothness of increasing or decreasing lighting levels was entirely dependent on how smoothly one turned the Variac knobs. Sometimes-comical effects resulted when the wrong knobs got turned at the wrong time, or the correct knobs got turned too fast/slow.
Re: DMX Output board for 6502 based systems
Posted: Thu May 02, 2024 4:10 am
by barnacle
I set the lights for this little event, back around 1980. Poor Jan never quite forgave me
https://www.youtube.com/watch?v=n6GgPdpQ7To (sorry about the laugh track that's been added).
Neil
Re: DMX Output board for 6502 based systems
Posted: Thu May 02, 2024 8:51 pm
by Sittingbull
Some empty holes are for bypass capacitors? Best practice is to have an own bypass cap per chip. With all chips 74LS types chances are it'll work, but your mileage may vary
André
In fact I'm not a fan of bypass capacitors, on the other hand I'm a fan of your works...
Thanks for your comment.
Re: DMX Output board for 6502 based systems
Posted: Thu May 02, 2024 9:00 pm
by Sittingbull
Why dmx?? When I started this project I mean to build a uP controlled light desk...
Now it's a bit overengineered... It overwelmed me...
Re: DMX Output board for 6502 based systems
Posted: Fri May 03, 2024 12:29 am
by BigDumbDinosaur
Some empty holes are for bypass capacitors? Best practice is to have an own bypass cap per chip. With all chips 74LS types chances are it'll work, but your mileage may vary
André
Uh...bypass capacitors in digital work should not be considered an option if circuit stability and predictability are expected. Saying you’re “not a fan of bypass capacitors” is kind of like a competition marksman saying “I'm not a fan of rifled barrels.”
Incidentally, 74LS logic is not a recommended choice for new designs. If you are interfacing to a circuit being driven by a 74LS device, make that connection with a 74HCT (fast with gentle edges), 74ACT (very fast with aggressive edges) or 74AHCT (very fast with gentle edges) equivalent. Other devices can be 74HC, 74AC or 74AHC, since that part of the circuit will be 100 percent CMOS and no TTL compatibility will be required. The resulting circuit will have better noise resistance, be more tolerant of bus loading, and will consume less power.
Re: DMX Output board for 6502 based systems
Posted: Sun May 05, 2024 8:50 am
by Sittingbull
In fact I'm not a fan of bypass capacitors, on the other hand I'm a fan of your works...
Uh...bypass capacitors in digital work should not be considered an option if circuit stability and predictability are expected. Saying you’re “not a fan of bypass capacitors” is kind of like a competition marksman saying “I'm not a fan of rifled barrels.”
It was just a pun...
I made everything on breadboard, with 4 capacitors and it worked as espected, hoping on pcb with 6 cap it replays...
It's the first time for me with pcb...
I will put every capacitor needed on rev. 2
Incidentally, 74LS logic is not a recommended choice for new designs. If you are interfacing to a circuit being driven by a 74LS device, make that connection with a 74HCT (fast with gentle edges), 74ACT (very fast with aggressive edges) or 74AHCT (very fast with gentle edges) equivalent. Other devices can be 74HC, 74AC or 74AHC, since that part of the circuit will be 100 percent CMOS and no TTL compatibility will be required. The resulting circuit will have better noise resistance, be more tolerant of bus loading, and will consume less power.
okay, forgot to mention I'm using - not only but as many i can - used parts, wich I took off from 80's Olivetti computer pcb.
A firiend of mine has his backyard full of these parts, time to time I catch something for free...
I know that for the purists it's awful but I also mix 74LS with 74HC, as long as they work togheter for me is fine...
I really appreciate the comments as for me it's schooling... I'm a self-taught person...