Homebrew 6502 on a backplane (playing Invaders on a TMS9918)

[visrealm]

Greg816v2 wrote:

Very cool, nice work! I've noticed that for every person who says "don't run the busses off the board!" there is a working project like this

Thanks!

I hadn't heard this advice before starting the project.

This is my first custom PCB build of any kind, so I'd say I'm really at a novice level.

It's working for me though so... *shrug*. Beginner's luck?

I'm currently running at 4MHz, so it's not extreme and the TMS9918 would have issues going faster than this anyway.

Have just released a new video covering the design and build of a new card (keyboard and dual NES controller)

[GARTHWILSON]

Troy, I was interested in what parts you're using; but when I went to your github repo and clicked on the schematic link, nothing happens. It says it's a .pdf, but mousing over the link has a ton of random characters in it and no ".pdf" in the mix. (I know the schematic doesn't necessarily show what logic families anyway, since many CAD packages' standard parts only have 74LS and people put those in there even if they intend to use 74HC or something else; so do you have the more-exact numbers someplace?)

4MHz 65cXX parts and 74LS should work fine in a backplane-type project as long as it doesn't get too big and you do the clock-distribution design right (even if by accident); otherwise the many hours of construction work can lead to heavy disappointment, and the builder won't know why, since all the connections are correct. To maximize the success rate among these people who are new to the physics of digital design, we advise not running the processor's own buses out onto a backplane. Even if it works initially, it can be right on the raggedy edge such that one small change somewhere makes it non-op. The expansion-bus page of the 6502 primer on my site tells about this, at http://wilsonminesco.com/6502primer/ExpBusIntrfc.html . (Unfortunately the site is down at the moment, and our son is looking into why. Hopefully he'll have it back up soon.)

[visrealm]

GARTHWILSON wrote:

Troy, I was interested in what parts you're using; but when I went to your github repo and clicked on the schematic link, nothing happens.

Odd. They're working for me. Here are the direct links to the various schematics / board layouts:

Backplane PCB
Backplane schematic
Proto card PCB
6502 schematic

There are schematics for all the cards there too. Not sure why it wasn't working for you.

GARTHWILSON wrote:

I know the schematic doesn't necessarily show what logic families anyway, since many CAD packages' standard parts only have 74LS and people put those in there even if they intend to use 74HC or something else; so do you have the more-exact numbers someplace?

I do the same. I don't show the logic family in my schematics either. I'm using a mix of 74LS and 74HCT parts.

GARTHWILSON wrote:

Even if it works initially, it can be right on the raggedy edge such that one small change somewhere makes it non-op.

You might be right. I don't even have a scope to get a good picture of things . Getting by with a logic analyser and a bucket of optimism and persistence. Anyway, I'm glad it's working for me. I do plan on building an SBC eventually, but I do like the flexibility of the bus for now. I can make changes very quickly while I'm still figuring this thing out.

GARTHWILSON wrote:

The expansion-bus page of the 6502 primer on my site tells about this, at http://wilsonminesco.com/6502primer/ExpBusIntrfc.html

I'll be sure to check out your site once it's up.

[GARTHWILSON]

Your links above work.

visrealm wrote:

I'll be sure to check out your site once it's up.

It's back up now. The server company was migrating to something new, and the DNS was not working while they were in the process.

Edit, a couple of hours later: Down again. I guess they weren't done.

[GARTHWILSON]

Monday morning: They finally got the DNS working. Hopefully it will keep working now.

[BigDumbDinosaur]

GARTHWILSON wrote:

Monday morning: They finally got the DNS working. Hopefully it will keep working now.

Still working as of 2 PM CST.

[visrealm]

visrealm wrote:

GARTHWILSON wrote:

The expansion-bus page of the 6502 primer on my site tells about this, at http://wilsonminesco.com/6502primer/ExpBusIntrfc.html

I'll be sure to check out your site once it's up.

Was a good read. Thanks.

I'll be adding SPI very shortly as I've ordered an SPI SDCard interface - was planning to bit-bang it with a 6522. I still haven't added a 6522 to my build yet, but have wanted to for the programmable timer and now SPI.

Will definitely look into 65SIB - my main concern at a glance is the +/-12V. My build only has +5V.

Anyway, it's all part of the learning experience. My next build will be an SBC, although I do plan on continuing on with this project for the time being. It's working well for me and I'm learning a lot.

[GARTHWILSON]

visrealm wrote:

Will definitely look into 65SIB - my main concern at a glance is the +/-12V. My build only has +5V.

If you'll never have anything that needs the negative voltage, I suppose you could leave it unconnected.

The +12 is not critical at all. A 7805 regulator will need a minimum of somewhere around +7.5V input, as will RS-232 line drivers like the 16-pin SN75C1406 (same thing as the MC145406) triple line driver & receiver which takes less board space than the popular MAX ones because it doesn't have charge pumps and doesn't require the extra pins and capacitors that charge pumps do. The best op amps, or even the cheap LM324, will also require at least about 7.5V to output 5V. There are op amps with rail-to-rail I/O; but you don't get something for nothing.

Nearly everything in such classes of parts could handle at least 13V, which is why the +12V can range widely.

Another reason it didn't really make much sense to put +5V on 65SIB is because with various devices out on ribbon cables and going through several connectors to get all the way to the end of the daisychain, that 5V could droop quite a bit depending on load. It was all thought out pretty well, I think.

For local tiny modules that will be supported by just the connector itself, you can go with SPI-10. It's not a bus like 65SIB is. It meets a different need. Similarly, for I²C, there's I2C-6 for tiny modules that will be supported by the connector.