ZAP Z80 Alternative Memories - Page 1
Forum section: electronics / projects

author: Sparky49
author_rank: Regular Contributor
timestamp: September 15, 2018, 06:52:17 pm
content: Hi,I've been really interested in building my own small computer for some time, and felt that I should stop thinking about it and do it. I happen to have a Z80 I picked up at a radio fair, so decided to build around that. I quickly grew to enjoy reading "Build Your Own Z80 Computer" by Ciarcia, and enjoy the clear, concise explanations given. The ZAP project appealed to me, as it clearly sets out a complete computer with OS, but also explains each and every step along the way. There's no "program the ROM with this complete OS which is so complex it is beyond the scope of this project", which I like.However, I see that some of the components, (especially around memory) such as the EPROM (intel 2708) and 2114 are now unavailable from reliable vendors like Farnell and Digikey. I've tried some basic searches for alternatives, with memory at least that of the originals (perhaps also moving to EEPROM), with similar parallel address and data lines, but with no luck. Is anyone aware of some suitable alternatives to these memory ICs which could be used without major alterations to the overall design? Or am I 'stuck' with using modern alternatives and needing to alter the design to suit?For the given memory layout, see the pdf's page 127 here:https://www.modmypi.com/download/BuildYourOwnZ80.pdfMany thanks for your help, I understand this might be perhaps a very vague question to ask, but any help or insights would be most appreciated.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: September 15, 2018, 07:00:01 pm
content: I started my Z80 dev board 2 years ago and come back to it once in a while again. Memories I ordered me some CY62256 static rams and AT28C256 eeproms, what worked out flawlessly so far.All a simple setup with the rom starting at 0x0 and the ram at 0x8000

author: spudboy488
author_rank: Regular Contributor
timestamp: September 17, 2018, 11:36:51 am
content: A lot of the older parts can be replaced with modern equivalents. You can use larger PROMs and just tie the extra address lines to ground. The same applies for RAM. Address buffers are now byte wide (vs the 8T97s in the text). You have a lot of options.I put together a simple system a while back loosely based on Ciarcia's book. Some notes can be found here.http://modernretroz80.blogspot.com/

author: Sparky49
author_rank: Regular Contributor
timestamp: September 17, 2018, 07:04:27 pm
content: Thanks both, very useful information.Also, extra thank you to spudboy - I will be reading your website with great interest!

author: SiliconWizard
author_rank: Super Contributor
timestamp: September 17, 2018, 09:48:22 pm
content: For RAM, you can use SRAM chips such as the CY62148ELL-55SXI (512Kx8, 55ns). Cheap, much lower power than the dynamic RAM chips that were often used at the time, and you can design some banking scheme to get access to the full memory.

author: rstofer
author_rank: Super Contributor
timestamp: September 18, 2018, 07:36:19 pm
content: Maybe you can get some ideas from this site:https://rc2014.co.uk/I built up two of their kits and the second one includes the OS "CP/M 2.0" on a compact flash drive.

author: wilfred
author_rank: Super Contributor
timestamp: September 19, 2018, 12:32:38 am
content: Are there examples where people have used DRAM on their Z80 projects? Or non Z80 projects where they didn't use the refresh register or a special purpose refresh controller.I know the world has moved on from DRAM for retro microprocessor projects but I am still interested.

author: drussell
author_rank: Super Contributor
timestamp: September 19, 2018, 04:08:54 am
content: Quote from: spudboy488 on September 17, 2018, 11:36:51 amOr put a couple dip switches or jumpers on those extra address lines and be able to switch between different versions of your firmware for testing, different features, diagnostics, monitors and debuggers, etc.

author: netdudeuk
author_rank: Frequent Contributor
timestamp: September 19, 2018, 10:35:47 am
content: I use these in my Z80 and 6502 projectshttps://uk.rs-online.com/web/p/sram-memory-chips/7444542/https://uk.rs-online.com/web/p/eeprom-memory-chips/1276570/

author: CJay
author_rank: Super Contributor
timestamp: September 19, 2018, 11:44:53 am
content: I'd stick with a 6116, 6264 or 62256 device, they're dirt cheap and should be an obvious alternative when you look at the datasheets.If you're UK based I'll chuck a couple of 6264 devices in the post just for giving me the link to the Ciarcia book.

author: Sparky49
author_rank: Regular Contributor
timestamp: September 19, 2018, 12:07:56 pm
content: Thanks all for the very useful replies! drussell, sounds like a very smart idea using the switches, though I know that will cause me some headaches on my first project, so I'll keep that in mind for my ZAP mk. 2.Thanks for the direct links, Netdude.Cjay, cheers, I might just take you up on your very generous offer!