6502.org

After many months of lurking on this forum, now seems like a good time to introduce myself. I'm close to finishing the first permanent iteration my first 6502 project, the f1 computer.

Some of you may have seen my posts at anycpu.org (https://anycpu.org/forum/viewtopic.php?f=13&t=569). I talked about my first
homebrew computer project there. I was working on that project while on hiatus from employment and had a lot of time to dedicate to it. I got a full-time job and decided to scale back my aspirations (for now). I'd done just a little 6502 programming in vice and found these forums and Garth's primer, so I chose the 6502 over Z80 for the f1.

I slowly built the machine on solderless breadboards, starting with CPU, ROM, and LCD display. Then I added RAM and a VIA. Then I added a UART and wrote a ROM that loads assembled machine code from my desktop computer into the f1's RAM over serial line. This made it much faster to experiment with programs than writing and swapping ROMs with every code change.

After spending a couple weeks writing code for the machine and testing some sound generation ICs, I realized the hardware configuration was stable enough that I should move it off the breadboards. I've never made a PCB before and spent some time experimenting with board layouts in KiCad. I ultimately decided I wasn't ready to move to PCB, and decided to wire wrap a board instead.

Attached photos show my result. All components are down and most of the connections are wrapped. There is a main board with header for expansion boards. The first expansion board has an AY-3-8910 sound IC and an LCD display. The jumper wires between the boards are an unfortunate mistake. My DIY ribbon cables worked well enough in breadboard holes, but don't securely fit the wire wrap pin header I used. One of my short term priorities is to replace all that with nice ribbon cable connectors, like these: https://www.digikey.com/product-detail/ ... 4G-ND/9709

I want to thank Garth for the primer and this forum for all the valuable posts. You've all been very helpful in making my project successful so far.

I would welcome any suggestions or corrections on my board construction practices. I do know of a couple things I should go back and improve, like adding bypass caps where they're missing and debouncing the reset button. In my next posts in this thread, I'll talk about my future plans for the f1 and raise a few questions that came to mind as I built this machine.

Welcome! Looks like you've already come a long way - well done. I look forward to seeing further developments...

This is the major parts list for the main board of my computer:

CPU: WDC W65C02S
ROM: Atmel AT28C16 2K EEPROM
RAM: Alliance AS6C62256 32K SRAM
Parallel I/O VIA: 2x WDC W65C22S VIA
Main system clock: ECS-2100A-010
RS-232 serial: WDC W65C51N + MAX232EIN driver

Additionally, a CD74HCT08 quad NAND IC is used for address decoding, and an SN74HCT08 quadAND IC is used to handle multiple interrupt sources.

I've written a number of test programs for the machine. Things appear stable at the current 1 MHz clock speed, but maybe I'll find some issues when I start working on software again.

Here are a few of my short term priorities, in no particular order:

• Replace the pin header that currently breaks out VIA I/O, power, and phi2 with nice
  ribbon cable connectors.
• Debounce reset circuit.
• Finish adding bypass caps across all active devices.
• Write a music tracker program to make music with the AY-3. I wrote a rough one for the SN76489,
  and I'm in the process of rewriting a second one with AY support.

Longer term ideas:

• Increase serial port speed past the current 19,200 bps.
• Character cell or bitmap graphics. This could be in the form of graphics commands sent
• over the serial line to be rendered by my desktop computer, or a standalone VGA
  subsystem to attach directly.
• Support for playing low rate digital sampled audio. I would need to implement this in software, find a suitable IC, or design a circuit for it.
• Write an RPG game.
• Make a Forth environment.

I think I'll spend a lot of time with the AY-3 sound chip over the next few months. Does anyone here have some experience programming it?

Welcome, fmahnke

Nice photos -- thanks for sharing your project with us.

With the goals you're contemplating I think you'll have plenty to keep you busy! Regarding low rate digital audio, you might be interested in the 9-level PWM that Garth talks about, since it's VIA-based and your project has VIAs. Hmm, and there's more about VIAs here.

I like the idea of getting some Forth working. If you do that sooner rather than later then it would become a means to an end, assisting you with the other goals such as the graphics commands and of course the RPG!

Cheers,
Jeff

I'd like to ask a few questions about wire wrap construction.

First, I've noticed some people use two breakable headers to socket ICs instead of proper wire wrap IC sockets. This has the advantages of lower cost and the potential to more easily get discrete components on the board under the IC, component side. Wondering if anyone's used this technique for a variety of IC sizes and can comment on the resulting construction quality compared to using true WW sockets?

Secondly, from the AC performance construction section of the 6502 primer:
Could someone explain the reasons behind connecting the ICs to power and ground both between each other and also to the power-and-ground hub, rather than either connecting them all to the hub or connecting the closer ones to the hub, then the farther ones to the closer ones? I did as the primer directed on the f1 wire wrap board and found it used most of my available wraps on the vcc and gnd pins, leaving little space to connect other IC pins. This leads me to my next question.

What is your preferred method to tie IC pins to vcc or ground when the IC power posts are already close to full? I had several pins on the 6502 and 6551 to tie high or low, enough that the power posts for each IC didn't have near enough space to bring in its respective signals. One solution is to make a bigger central power-and-ground hub and run pins to that area as necessary. This makes for a cleaner board layout, but makes wires longer than they need to be. Another solution is to break out an IC's power pins to a mini power distribution header next to it, and then run each signal there. This makes for shorter wires, but uses more board space and makes the board less organized.

Thank you for your suggestions.

This advice comes a little late, as I see you already have the signal wiring in place. But I always begin by doing the power and ground wires, and rather than wrapping them I solder them, using fairly fine gauge wire (similar to WW wire).

There are various advantages. Soldered connections hardly consume any height on the wire-wrap post. Also you're going to need lots and lots of connections to bypass caps, and those can't be wire-wrapped anyway. So just solder as much stuff as possible before the signal wiring (using wire-wrap) begins. (BTW, it's sometimes preferable to use a SMD cap and run wires to it, rather than trying to get the leads of a through-hole cap to reach where you want them to.)

Re AC performance: it's important to realize that whenever current (a signal) is sent from Chip A to Chip B, to complete the circuit the current also needs a return path from B back to A. Superficially that may seem like a non-issue. Ground and Vcc connections extend to every chip on the board, so there's bound to be a return path somewhere. But it makes a difference what route that return path takes. No one path is ideal, so we try to provide many return paths. (After all, there are many signal paths.) For each signal, the return current will automatically favor whichever return path it "likes" best.

Ideally you want there to be a return path that's immediately adjacent to every signal path. The more nearly adjacent the two paths are, the less inductance you'll get. That's because inductance results from creation of a magnetic field, and the fields of two nearby conductors tend to cancel if (as in this case) the two current flows are in opposite directions.

One advantage (not the only advantage) of a ground plane is that any conceivable signal path always has an identical return path nearby (ie, the ground plane).

When there's no plane, the next-best way to make lots of return paths available is to implement a grid. Personally I prefer a simple rectangular grid, and see no advantage to the radial arrangement shown in the Primer. But it's the same idea: make lots of return paths available and thus keep inductance reasonably low.

-- Jeff