Breadboard replica

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CWJ_Wilko's picture
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Some time ago I attempted to create an Apple 1 replica based around Vince Briel's earliest rendition of Replica 1. Mistakes were made, and I never did get the unit working. I know for sure that I blew the 65C02 by piping in 5v where is wasn't supposed to, so I dropped the project for a while.

I've just started up again with a batch of fresh parts. This time I'll be prototyping on a breadboard, and I'm basing it around the second generation Replica 1 Plus that Mr Briel released some time ago.

It's a shame that these kits aren't being sold anymore, as it was a good budget alternative compared to the very nice Mimeo and Newton replicas that are out there. The only other budget replica (that I am aware of) is the L-Star. But rather than buying a kit, I'm determined to try and put this together on my own, on a breadboard first and, if all goes well, on a prototyping PCB. The breadboard version will leave off the expansion port and edge connector, as well as the ASCII keyboard, for the sake of simplicity. The PCB version should include these though.

Apart from stroking my ego, I want to learn more about the low-level fundamentals of computing. I learnt a lot from my previous attempt, and am hoping to learn even more this time around.

I have put together a parts list on Mouser if anyone is interested, minus a few things like the edge connector, expansion port, and the ROMs. Last time I had someone burn me a ROM; this time I've ordered blank ROMs on eBay, a programmer and an eraser to do it all myself. This has pushed the cost up. Additionally my shopping list included a Prop Plug to program the Propeller chip. I'll be using Mr Briel's latest (and last?) Propeller firmware, and the latest version of the 16 kilobyte ROM image that includes the Monitor, Krusader and BASIC.

This probably won't work, I don't really have faith in my abilities to follow the schematics and not make fatal mistakes. But if I go slow and test everything three times over, I might just get a working Apple 1 replica out of all this.

Below you can see the address and data buses for the ROM (middle) and the RAM (right). I'm still waiting on the ROM programmer from China so I'm not rushing to get anything finished at this point.

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CWJ_Wilko's picture
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Joined: Jun 3 2015
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Re: Breadboard replica

So just a couple of days later, I've nearly completed this. I'm stuck trying to load the firmware onto the Propeller and its 256kbyte EEPROM. Does anyone know whether the Replica 1 Plus allowed this to be reprogrammed while in circuit? The schematic seems to imply this, but I can't seem to get it connected to my PC over serial, it can't be detected. I've been able to connect to the Propeller when its out of the circuit, so I might just program it this way and see if I can't get some video out.

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Joined: May 17 2017
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Re: Breadboard replica

I'm not at all familiar with this particular project. However, I've built a similar project that also replicates the Apple-1 called L-Star. And, it allows you to program the EEPROM using the Parallax Propeller Plug. I've never been successful trying to make my own hardware to program the chip, so I've been stuck using the Prop Plug.

If your circuit doesn't have this already, you'll need to modify the circuit to accept the above.

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Re: Breadboard replica

BTW, I have a bunch of the L-Star blank PCBs. If anyone is interested, PM me. I have seven of them left. I also have some Propeller chips, as well as the 24LC256 serial EEPROM and 5Mhz crystal. I use these when teaching soldering at the local Maker Faire.

CWJ_Wilko's picture
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Re: Breadboard replica

Thanks. I managed to get it programming in circuit today by bypassing the capacitor and transistor that sits in the middle of the RES line. I assume this offers some level of smartness to tell the Propeller when it should be in programming mode (when attached to PC) but it just doesn't work for me, I might replace it with a simple jumper.

Also had to fiddle with the video circuitry for a while until I got it working (I bypassed the 220ohm resistor entirely), but tonight I was able to get the Propeller sending characters to the screen - awesome. Just need to burn a ROM and we should be in business.

CWJ_Wilko's picture
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Re: Breadboard replica

Just a couple of update pics. I forgot to mention earlier that I somehow didn't completely mess this up when I shorted 3.3v to ground, and nearly set the FT232 breakout on fire in doing so. All is well now Smile


macnoyd's picture
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Re: Breadboard replica

Well, that looks like someone's been busy! Wink

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macnoyd

CWJ_Wilko's picture
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Re: Breadboard replica

All for naught so far. I've burnt the ROM and got everything installed, but can't seem to get into the Woz Monitor on RESET, nothing happens. I also haven't installed the keyboard yet, not sure if this would make a difference electrically. Bit bummed out. My working theory is that something is screwy with the address and/or data lines, and the 6502 is jumping to the wrong place in memory on RESET.

Everything feels really fragile as well, which isn't helping. The slightest bump is resetting the Propeller, nothing feels very secure in the breadboard. It's handy for diagnosing these issues though.

CWJ_Wilko's picture
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Re: Breadboard replica

It works! Video is here

apple1videointro

So I may or may not have had the 6502 in the wrong way... mistakes were made, and I feel like an idiot. Popped a new (well, old) 6502 in, the right way round, and everything works great.