VGA isn’t much used anymore, but it’s not hard to get a hold of monitors with that input. How about the older standards like EGA, CGA, or MDA? Well, it’s good luck on eBay or at the recycling yard to get a period-appropriate monitor, but the bulky, fragile CRTs seem to have been less likely to survive than computers that drove them. That’s what [Scrap Computer]’s MCE Blaster is for: it sits betwixt the retrocomputer’s TTL output and the VGA input of a (more) modern monitor, be it CRT or LCD.
Taking the TTL input and spitting out a properly-formatted VGA signal isn’t trivial, but with a Raspberry Pi Pico (or Pico2) sitting in the middle, there’s plenty of horsepower to act as a translator. There’s a certain irony in having a video dongle with more horsepower than your whole machine, but that’s the price of progress, perhaps. This project is cheaper and simpler to implement than others that connect to upscalers.
MCEBlaster is now on rev 0.3, which aside from adding compatibility with the Pi Pico, also allows a TLL offset. The older versions had a problem where it was possible to perfectly synchronize the sampling rate of the Pi Pico with the TLL signal from the computer’s graphics adapter… but during the crossover period between pulses, which wasn’t exactly ideal. The offset allows for that. There are also trim pots to manually adjust the R, G, and B channels. This has the fun result of allowing you to turn down the R and B channels so you can pretend your CRT has a green phosphor, or dial down the G and zero the B to get a homey amber glow. It also lets you correct the brightness, if things are looking a little dim or overblown.
The whole thing is open source on GitHub, with an active development community. We’ve embedded the rev.0.3 release video below so you can see how it works and how well it works.
Of course, CGA and EGA weren’t the only options back in the day, and we covered some of the competition before. Thanks to the open-source nature of this project, MCE Blaster may support some of them someday, too.
I need to get around to building the opposite device, so I can drive my Samsung amber MDA monitor from a VGA/HDMI signal.
BTW, turning down green would result in magenta, not amber. You need to turn down green and turn off blue entirely for that.
You only need two things to drive your MDA monitor from a VGA output:
A correct video modeline to get the right hsync+vsync rates. (Potentially tricky especially with modern video cards that don’t reliably support pixel clocks below 25MHz)
High bandwidth voltage translation to translate from 0.7Vpp analog voltages to two digital TTL outputs.
I’ve driven an ECL monitor recently from VGA; the hard part is that you have to keep the BJT in the “active” domain instead of cutoff or saturation (those are both slow to exit).
Specifically narrowly in the case of the IBM 5151, the primary VIDEO signal goes directly to the base of a NPN transistor – there’s a very real chance that you could just directly connect analog video from the VGA port to the VIDEO input and have it work.
(Not so with INTENSITY / BRIGHT / DUAL, which both has 150Ω loading and goes to the input of a TTL part.
Wouldn’t be surprised if just biasing it up with a variable resistor divider would work — abusing the TTL input’s threshold voltage being somewhere in the 0.8-2V range, and hoping it’s fairly sharp. But not knowing exactly where the threshold is, a trimmer is needed.
Video amps aren’t too bad to design, given a modest background on amplifiers. Design gets real tricky as you approach fT, especially if good isolation and impedance flatness is required, but it’s pretty understandable at fT/10 or lower (mid frequency model applies: basically, hybrid-pi model with capacitances added). 2N3904 claims 300MHz or so (and the 1pF Ccb, 800MHz MMBTH10 is still available), so 20MHz is quite accessible.
Unrelated – you by any chance da same guy who was into induction heating circuits yearrrrrs ago? If so, thanks for inspiring me to go down that rabbit hole 🤣 I finally ended up with a parallel LC system with a Celem cap, driven directly with a current-fed inverter. Worked great from 120V, but sadly everything totally fried when I tried it on 230V, and I never had the enthusiasm to resurrect it. There’s some photos here – https://www.flickr.com/photos/48329631@N08/albums/72157624215995767/
Now we’re spoiled with cheap ZVS induction heaters and bolt heaters from AliExpress!
You’re right about the colours.
Now you’ve got me wondering why magenta-phosphor CRTs weren’t a thing. Blue was restricted to O-scopes, but that’s probably an eyestrain thing. For displays I’ve seen white/grey, green, and amber, but I’ve never anything even like a “red” in the wild.
VGA is very much alive in industrial settings. This is entirely because VGA analog signaling can work over longer distances. Any digital monitor signal is going to be trouble beyond 6 meters.
Not to mention HDMI and Display Port connectors are entirely too fragile for the job.
Not true if you use RDP. It can work over a distance of 8500 km with very little delay. Maybe not for fast-paced games like TF2 but good enough for retro stuff like Pokemon or Golden Sun on a GBA emulator.