• False Starts, Change of direction?

    JNP-3R4 days ago 0 comments

    I've been busy working on the PCB and routing on the computer, but while I'm getting much quicker and better at it, I've ran myself into a corner.

    As far as I got on the PCB before realizing things were a bit to tight
    We're gonna need a bigger board...

    This project is one of odd anachronistic design choices, pulling in design for aesthetics here, cost there, speed of construction in another place. The design so far is a single board computer with onboard front panel, simply because that seemed the most expedient and reminiscent of old kits and educational machines like the KIM-1, MicroProfessor, ELF, Superboard, and so on. I love the vintage look with simple boards and wide curved traces, and I really want the front panel to be organized and clear. That said I also want the board as small as possible to reduce cost. There's no sense paying $100 for bare boards, and I don't want the machine to become too large and unwieldy to use on a small desk.

    This contrast has unfortunately not been working out well. Positioning the front panel on the main board, keeping output LEDs near their related input switches, and keeping the I/O connectors out of the way on the back of the board means a large number of signals have to traverse the whole length of the board, often twice, while also navigating tight areas between the processor section, LED drivers, and clock logic.

    Is it possible to route the board? Yes, definitely, but it means giving up on trace spacing, and conceding to building a tight rat's nest. If aesthetics were the only concern we may be okay with that, but... Well, our nostalgia is for educational and exploratory systems, systems that are designed to be easy to follow along with and understand, and a tight mess severely limits that ability. We'd like to be able to show this thing off and maybe help teach something, you know?

    So where do we go from here? Well, the board could be made larger, but I'm neither interested nor willing to risk finances on having to re-order something like that. That leaves us with a card bus / backplane system. I'd initially rejected this idea due to the cost and complexity of connectors and a chassis, but if we can use smaller boards we can take advantage of special pricing, and there's less wasted in case of a screw up or reorder. Plus, modularization makes further development easier and means we can get more out of what we've spent on the backplane and chassis.

    My next step is going to be doing some research and measurement to see what kind of connectors or backplanes are available cheaply and seeing what it'll take to split the design up. Something with 60+ connectors and a small size would be ideal and allow for a straight through backplane, but we may need a smaller pin count to accommodate fan out on a fine pitch board. Currently PCIe x4 is near the top of the list due to its low price, small size, and availability, but we'd like to not have to "cheat" to route it. Let me know if you have any other suggestions.

    Edit: Wait a second, the biggest trouble I'm having is coming from trying to accommodate the front panel and its wiring on one board while hiding or moving components from line of sight. Simply letting the chassis host the front panel would save a huge amount of space. A short bit of ribbon cable and freedom to place connectors wherever's convenient would help tremendously (though boards may still need stacked / modularized). To the enclosure catalogs!

  • Catch Up and Component Selection

    JNP-3R02/12/2024 at 05:00 0 comments

    We've been working on this project for a little bit now. Mostly iterating through pen and paper designs and descriptions to figure out what we even want to build, up to the first iteration of the schematic from earlier this week. The design is heavily based on Usagi Electric's UETTL, though I have added some features, made substitutions (CMOS mono and astables instead of 555s, 74HC251s instead of 74HC4051s and so on) and most of the clock and control logic is taken from my earlier Uno TTL computer project.

    The front panel design and interface concept is largely sorted, as is preliminary component and footprint selection. I was able to track down some super stylish and cheap paddle switches from Unicorn Electronics (they still have an eBay store!), so I'll be working those in the design.

    Assortment of paddle slide switches. Some are loose one the desk, but 3 are arranged in a row
    Switches for our front panel. Unicorn Electronics SW1004

    Obviously these plus the vintage RAMs with separate ins and outs make the build tricky to replicate, so I may work some modifications into version 2. For now parts are available enough through NOS sellers.