Designing A Macintosh-to-VGA Adapter With An LM1881

October 7, 2023 by 3 Comments
Old-school Macintosh-to-VGA adapter. Just solve for X, set the right DIP switches and you’re golden.

If you’re the happy owner of a vintage Apple system like a 1989 Macintosh IIci you may know the pain of keeping working monitors around. Unless it’s a genuine Apple-approved CRT with the proprietary DA-15-based video connector, you are going to need at least an adapter studded with DIP switches to connect it to other monitors. Yet as [Steve] recently found out, the Macintosh’s rather selective use of video synchronization signals causes quite a headache when you try to hook up a range of VGA-equipped LCD monitors. A possible solution? Extracting the sync signal using a Texas Instruments LM1881 video sync separator chip.

Much of this trouble comes from the way that these old Apple systems output the analog video signal, which goes far beyond the physical differences of the DA-15 versus the standard DE-15 D-subminiature connectors. Whereas the VGA standard defines the RGB signals along with a VSYNC and HSYNC signal, the Apple version can generate HSYNC, VSYC, but also CSYNC (composite sync). Which sync signal is generated depends on what value the system reads on the three sense pins on the DA-15 connector, as a kind of crude monitor ID.

Theoretically this should be easy to adapt to, you might think, but the curveball Apple throws here is that for the monitor ID that outputs both VSYNC and HSYNC you are limited to a fixed resolution of 640 x 870, which is not the desired 640 x 480. The obvious solution is then to target the one monitor configuration with this output resolution, and extract the CSYNC (and sync-on-green) signal which it outputs, so that it can be fudged into a more VGA-like sync signal. Incidentally, it seems that [Steve]’s older Dell 2001FP LCD monitor does support sync-on-green and CSYNC, whereas newer LCD monitors no longer list this as a feature, which is why now more than a passive adapter is needed.

Although still a work-in-progress, so far [Steve] has managed to get an image on a number of these newer LCDs by using the LM1881 to extract CSYNC and obtain a VSYNC signal this way, while using the CSYNC as a sloppy HSYNC alternative. Other ICs also can generate an HSYNC signal from CSYNC, but those cost a bit more than the ~USD$3 LM1881.

Book8088 Slows Down To Join The Demoscene

September 12, 2023 by 9 Comments

As obsolete as the original IBM Model 5150 PC may appear, it’s pretty much the proverbial giant’s shoulders upon which we all stand today. That makes the machine worth celebrating, so much so that we now have machines like the Book8088, a diminutive clamshell-style machine made from period-correct PC chips; sort of a “netbook that never was.”

But the Book8088 only approximates the original specs of the IBM PC, making some clever hardware hacks necessary to run some of the more specialized software that has since been developed to really stretch the limits of the architecture. [GloriousCow]’s first steps were to replace the Book8088’s CPU, an NEC V20, with an actual 8088, and the display controller with a CGA-accurate Motorola MC6845. Neither of these quite did the trick, though, at least not on the demanding 8088MPH demo, which makes assumptions about CPU speed based on the quirky DRAM refresh scheme used in the original IBM PC.

Knowing this, [GloriousCow] embarked on a bodge-fest aimed at convincing the demo that the slightly overclocked Book8088 was really just a 4.77-MHz machine with a CGA adapter. This involved cutting a trace on the DMA controller and reconnecting it to the machine’s PIO timer chip, with the help of a 74LS74 flip-flop, a chip that made an appearance in the 5150 but was omitted from the Book8088. Thankfully, the netbook has plenty of room for these mods, and with the addition of a little bit of assembly code, the netbook was able to convince 8088MPH that it was running on the correct hardware.

We thoroughly enjoyed this trip down the DMA/DRAM rabbit hole. The work isn’t finished yet, though — the throttled netbook still won’t run the Area 5150 demo yet. Given [GloriousCow]’s recent Rust-based cycle-accurate PC emulation, we feel pretty good that this will come to pass soon enough.

Will An 8088 Run DOOM? Now, Yes It Will!

September 2, 2023 by 12 Comments

The question on everyone’s lips when a new piece of hardware comes out is this: Will it run DOOM? Many pieces of modern hardware have been coaxed into playing id Software’s 1993 classic, but there have always been some older machines that just didn’t have the power to do it. One of them has now been conquered though, and it’s a doozy. [Frenkel]’s Doom8088, as its name suggests, is a port of the game for the original PC and AT.

As can be seen in this gameplay video, it’s not always the slickest of gaming experiences. But it works, so the question is, how on earth can a machine that was below the spec of the original, run this game? The answer comes in it being a port of GBADoom for the Game Boy Advance, a platform with less memory than a DOS PC. It still relies on extensive hard disk access for every frame though, which leaves it snail-like.

We set out to install it ourselves on one of the web based PC emulators, but fell over on the size of the required Watcom installation. If any of you have the real thing lying around though, we’d love to hear about how the game performed in the comments.

We’ve shown you so many ports of DOOM over the years to have lost count. One of our favourite recent ones uses an extremely unconventional but very retro display.

Building The Cheapest MIDI Home Studio From 1988

July 19, 2023 by 7 Comments

These days, there is a plethora of cheap hardware and software which you can use to make digital music. Back in the 1980s, though, this was still a nascent field, with new gear changing the game for musicians. In his latest video, [cTrix] puts together a guide to building a budget MIDI home studio like it’s 1988.

The original Atari ST is the star of the show, which sold for around £300 back then. Unlike most contemporary computers, it came with MIDI ports built-in, and it quickly became a popular platform for music sequencing and controlling external synth hardware. It was often spotted in posh studios being used by big-time musicians, but [cTrix] wanted to look at how it was used by more humble users.

While the Atari ST could output some passable chiptunes, it was most powerful when paired with off-board gear. Industry magazines would readily talk up “DIY” setups worth thousands, but [cTrix] aimed for a budget in the low hundreds. In 1988, that might get you something like the 16-channel Yamaha PSS-580, which rocked full MIDI support and a ton of sounds and drumkits built in. It ran an OPL soundchip, and could play multiple instruments at once under MIDI control from software like Dr. T’s MIDI Recording Studio.

Of course, if you’ve ever played a keyboard like the PSS-580, you’ll have noted that the sound output can be more than a little chintzy. A cheap way to level up was to get yourself something like the Roland MT-32, which rocked a far higher-quality sound module. Everything from slap bass to solid electric pianos could be blasted out from the Roland, which also included a decent reverb unit, too. [cTrix] also dives into other budget heroes, like the rich and expansive Kawai K1 and the Yamaha EMT-10.

If you want to make music like it’s 1988 and you’re hunting for your first gold record, you’ll learn a lot here. We can’t wait for Part 2, that explores the next level of sophistication in this era. His previous lessons on how to make 90s dance bangers on the Amiga are a treat, too.

Continue reading “Building The Cheapest MIDI Home Studio From 1988” →

HP 3488A Teardown, Dismantled For Parts

March 17, 2023 by 10 Comments

[IMSAI Guy] has an old HP 3488A Switch Control Unit that he wants to dismantle for parts ( see video below the break ). The 3488A is pretty simple as far as HP test equipment goes — a chassis that can hold various types of relay cards and is programmable over GPIB. He notes up front that these are plentiful and inexpensive in the used test equipment market. Continue reading “HP 3488A Teardown, Dismantled For Parts” →

DietPi Releases 8.12 With Support For The Rockchip RK3588 SoC

December 27, 2022 by 31 Comments

This month DietPi released version 8.12 of this SBC-oriented Linux distribution. Most notable is the addition of support for the NanoPi R6S and the Radxa ROCK 5B SBCs. The ROCK 5B features the new flagship Rockchip RK3588 SoC with quad Cortex-A76 and quad Cortex-A55. What makes DietPi interesting as an operating system for not just higher end SBCs but also lower-end SBCs compared to options like Debian, Raspberry Pi OS and Armbian is that it has a strong focus on being the most optimized. This translates in a smaller binary size, lower RAM usage and more optimized performance.

The DietPi setup experience is as straightforward as with the aforementioned options, except that right from the bat you get provided with many more options to tweak. While the out of the box experience and hitting okay on the provided defaults is likely to be already more than satisfactory for most users – with something like the optional graphical interface easy to add – enterprising users can tweak details about the hardware, the filesystem and more.

When we set up DietPi on a Raspberry Pi Zero, it definitely feels like a much more light-weight experience than the current Debian Bullseye-based Raspberry Pi OS. Even though DietPi is also based on Debian, it leaves a lot more RAM and storage space free, which is a definite boon when running on a limited platform like a Raspberry Pi Zero. Whether it’s polite to state in public or not, DietPi definitely rubs in that many standard SBC images are rather pudgy these days.

A Cycle-Accurate Intel 8088 Core For All Your Retro PC Needs

December 3, 2022 by 17 Comments

A problem faced increasingly by retrocomputer enthusiasts everywhere is the supply of chips. Once a piece of silicon goes out of production its demand can be supplied for a time by old stock and second hand parts, but as they become rare so the cost of what can be dubious parts accelerates out of reach. Happily for CPUs at least, there’s a ray of hope in the form of FPGA-based cores which can replace the real thing, and for early PC owners there’s a new one from [Ted Fried]. MCL86 is a cycle accurate Intel 8088 FPGA Core that can be used within an FPGA design or as a standalone in-circuit replacement for a real 8088. It even has a full-speed mode that sacrifices cycle accuracy and can accelerate those 8088 instructions by 400%.

Reading the posts on his blog, it’s clear that this is a capable design, and it’s even been extended with a mode that adds cache RAM to mirror the system memory at the processor’s speed. You can find all the code in a GitHub repository should you be curious enough to investigate for yourself. We’ve pondered in the past where the x86 single board computers are, perhaps it could be projects like this that provide some of them.