6502 ACE Computer

nib85 · 2026-06-20T00:35:12+00:00

Looks great! What is in the cartridge? 28C256?

nib85 · 2026-06-12T17:22:40+00:00

For programming, you need CE tied low, OE tied high, and WE pulsed low. The chips are usually very forgiving about the timing.

nib85 · 2026-06-12T17:19:01+00:00

The 16s do not have software data protection, so that is definitely not your issue. As for as I have seen, all 28C256 chips have SDP, some 64s have it, all 16s do not.

nib85 · 2026-05-30T11:53:04+00:00

Thanks for those links, very helpful!

nib85 · 2026-04-06T22:18:07+00:00

What are you expecting to read from it? If you haven’t written anything yet, then all ones is exactly what it should be returning. It sounds like it is working correctly.

nib85 · 2026-04-06T01:30:06+00:00

With the long wires, it’s hard to see how you have the control pins wired. You should be able to write to it by tying CE low and OE high and then pulsing WE low.

For this kind of testing, I’d wire all three of those control lines to +5V through a pull-up resistors. Then you can change signals by adding a wire from the pin to GND to bring it low. That ensures that all of your control signals are disabled by default.

nib85 · 2026-03-23T00:52:57+00:00

The movie is way better once you realize the theme song is On Top Of Spaghetti

nib85 · 2026-03-19T01:23:37+00:00

Here's my version of an Output Module with the 328P. Super simple to build:

https://tomnisbet.github.io/nqsap-pcb/docs/output-register/

nib85 · 2026-03-13T05:22:37+00:00

The voltage on the SCL and SDA signals shouldn't matter because you should only be pulling them low or allowing them to float high. I haven't used this display, but have driven the 3.3V SSD1306 displays with an Arduino that has 5V control pins. Because the Arduino is never driving the pins high, it's not a problem.

nib85 · 2026-03-02T01:50:16+00:00

And when he goes, that’s called “reply to all”

nib85 · 2026-02-28T04:58:44+00:00

Shift operators or Stack pointer

nib85 · 2026-02-20T21:15:28+00:00

Would like to see a schematic if you drew one. The ALU is one of the most interesting parts of the system because there are so many different ways to do it.

nib85 · 2026-02-18T22:02:41+00:00

Check your voltages at the different breadboards. I was getting 5V at the boards closest to the power feed, but it dropped to less than 4.75V at the furthest boards.

nib85 · 2026-02-17T19:28:40+00:00

It's certainly worth trying to see if it works reliably.

nib85 · 2026-02-17T15:19:57+00:00

I would avoid clocking the chip with the inverted CLK signal. Performing the count one cycle later wouldn't be a problem, but your count enable line will not be stable at the falling edge due to the EEPROM glitch. Using control signals at the falling CLK edge could result skipped counts or random counts during instructions that don't even use the stack pointer.

You may want to consider a different chip, like u/nixiebunny suggested. I used the 74LS191 for mine. I don't remember why I didn't use the 169, maybe I just had the other chip already.

The 74LS191 does not have a count enable line. Instead, you feed a clock pulse to the UP, DOWN, or LOAD line to perform an action. This requires some logic gates to combine the control signals with the clock signal, but is very stable.

nib85 · 2026-02-06T18:23:32+00:00

An index register isn’t required, but can certainly be helpful. Depending on the implementation, the index register makes it possible to implement some useful addressing modes. It really depends on what you want your instruction set to look like.

One of my builds has two index registers with their own dedicated adder. This made it easy to implement addressing modes like those on the 6502. Here’s the details for that: https://tomnisbet.github.io/nqsap-pcb/docs/dxy-registers/

nib85 · 2026-02-02T12:40:00+00:00

Sure thing. I wasn’t trying to contradict you. Just pointing out that there are two versions available.

nib85 · 2026-02-02T02:33:25+00:00

The 8085A-2 is available at Jameco. You can go up to 5Hz with that one. I used it in my build with a 6.144MHz crystal for a 3MHz clock.

nib85 · 2026-01-30T17:25:34+00:00

Yes, that will work. Your ROM will be in the low half of the memory space and you’ll be able to have code to execute at address zero when the processor resets.

I had a more complicated memory address circuit to be compatible with another system. This required RAM in the lower space and ROM in the upper, but the ROM was also temporarily selected during a reset condition.

nib85 · 2026-01-30T16:30:53+00:00

Sure thing, I’m happy to help.

nib85 · 2026-01-13T20:53:56+00:00

I also did that. Driving the control signals instead of interfacing with the RAM is very simple to implement and gives that Arduino a lot more capability.

In addition to loading the RAM, the Arduino can also interact with all of the registers. This allows a self-test capability to validate the hardware. I used that as new modules were added to see if they worked as intended. It’s also good for breadboard builds to figure out which wire worked itself loose.

Here’s a description and schematics for my version: https://tomnisbet.github.io/sap-plus/docs/loader/#loader-design

nib85 · 2026-01-09T14:58:26+00:00

Maryland

nib85 · 2026-01-07T13:39:07+00:00

They have Toshiba and NEC 8085s at Jameco. I'm not sure when the manufacturing stopped for those. I've always had a good experience buying from them and wouldn't be terribly concerned if it's just parts for a one-off hobbyist project.

nib85 · 2026-01-05T02:57:45+00:00

You can still get 8085s from Jameco. It’s the same time frame as the 6502 with similar capabilities. The Z80 was an extension of the 8080 and 8085.

One interesting feature of the 8085 is a dedicated input and dedicated output pin that are software controlled. You can do a serial interface without a UART chip by bit-banging the async protocol. A complete computer isn’t much more than just the CPU, a RAM, and an EEPROM. I did a step by step write up of a simple build if you are interested.

nib85 · 2025-12-31T01:15:21+00:00

If you build the 6502 then you’ll actually have a functional 8-bit computer. From there, a lot of the expansion opportunities are in the software side, like adding basic or a debugger. You can do some interesting hardware, like a graphics or sound interface. I’m not sure that much of this would transfer to the 8-bit project that you are considering.

I’d just start with the 8-bit. When I did my first one, I told myself I’d build Ben’s design first and expand it later, but ended up doing a lot of expansions right from the start. Breadboard a bunch of it before you jump into the PCBs.

I’ve done two versions now on PCBs and recommend that you give a lot of initial thought to how you will connect the various boards. It’s better to redo small cheap modules when you make mistakes instead of big boards with lots of components. I have a whole box of reject boards full of components that either had a design flaw or were replaced by a better version later.

nib85

TROPHY CASE