Grant Searle's (not-so-) Simple-Z80  :-) - Page 1
Forum section: electronics / general

author: Phil_G
author_rank: Regular Contributor
timestamp: November 01, 2021, 07:43:57 pm
content: Grant Searle's "Simple Z80" in its 32k format is a great computer, reminding me very much of my old Nascom-1 back in 1978. Its true 'Retro' and quick, easy and cheap to make using not-too-hard-to-find parts, and Jeff Tranter has kindly made his PCB available for it via easyeda.As designed, its a BASIC-only machine which is brilliant for playing Star Trek and Sargon Chess, but I always like to have raw access with a machine-code monitor - such as the T2 in my old Nascom and NAS-SYS in later models.The 8k BASIC has had a few redundant routines removed, such as cassette handling, keyboard and screen display, so its actually slightly smaller than its original 8k. In fact within the ROM, the interpreter finishes at address 1DB7 hex and 1DB8 to 1FFF are unused - leaving 584 ROM bytes free!Within the confines of the 8k EEPROM there would be room for a modest monitor if I kept it simple. On the other hand it had to have all the essential commands to be of any genuine benefit - in fact its all worked out very nicely although it is an 'only just' fit!The update is in three parts, first within the interrupt I/O driver the "Warm or Cold?" signon message has been extended to "Warm, Cold or Monitor?" and in the BASIC code, the command 'monitor' has been enabled. For the monitor itself I tried a different cross-assembler this time, AZ80, and I quite like it as it has no syntax peculiarities unlike some :-)With the addition of an 'era typical' monitor, the 32k “Simple Z80” becomes a really handy little hobby computer with much more of a “Nascommy” feel, and it presents the opportunity to try a few machine-code routines, hand-coded or cross-assembled elsewhere. I've had great fun with mine, thank you Grant!A new rom image suitable for the 27C64 EPROM or as in my case an AT28B64 EEPROM is posted onhttp://philg.ukin the 'retro' section, along with a brief PDF document which details everything you see here.Here's a slightly outdated video (its since been tidied a little):CheersPhil_G

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 02, 2021, 05:44:11 pm
content: Did you write the BASIC interpreter, or was borrowed from an existing one?

author: Phil_G
author_rank: Regular Contributor
timestamp: November 03, 2021, 12:33:57 am
content: Hi, no its the Microsoft BASIC adapted for the Nascom in 1978, which Grant has tweaked to suit his board:http://searle.x10host.com/z80/SimpleZ80_32K.htmlI just did the I/O changes and wrote the monitorDetails & a brief document are in the retro section ofhttp://philg.uk(These files were updated Nov 5th '21)CheersPhil

author: Phil_G
author_rank: Regular Contributor
timestamp: November 08, 2021, 08:16:07 pm
content: Should anyone be following this, please pull a fresh copy as I've added a page to the document regarding calling machine code routines and passing parameters between M/C & BASIC. Also tidied up the document a bit and put some more links & credits in.Since the video was taken, I've swapped the Arithmetic command around to make it more logical. On Nas-Sys it always seemed the wrong way round to me:Axxxx yyyy on Nas-Sys gave you the sum xxxx + yyyy, the difference yyyy - xxxx (wrong way around to my mind!)and the relative jump offset. Mine does xxxx+yyyy, xxxx-yyyy and offset. Makes more sense to me :-)CheersPhil

author: CJay
author_rank: Super Contributor
timestamp: November 10, 2021, 07:51:41 pm
content: My board arrived and I managed to find most of the necessary parts in the 'junk' pile, the only part I'm missing is a nice fast Z80 but this ST part does seem to be doing something

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 10, 2021, 09:33:10 pm
content: Completely different architecture, so doesn't apply here, but the Z280 should be fun to test... Not easy to find but there are some on eBay:https://www.ebay.ca/itm/383662018204

author: CJay
author_rank: Super Contributor
timestamp: November 10, 2021, 09:43:41 pm
content: Oh that looks a fun chip, I've definitely got Z180 and I've just ordered some 65C816 and a couple of 68K chips to play with, might have to grab some Z280 as well

author: CJay
author_rank: Super Contributor
timestamp: November 10, 2021, 10:45:15 pm
content: We have life!The ST Z80 seems quite happy at 7+ MHz, so that's every IC and socket sourced from the stash.

author: Phil_G
author_rank: Regular Contributor
timestamp: November 11, 2021, 12:47:48 pm
content: Excellent Cjay, recycling at its bestI've found that overclocking the older Z80s is very dependent on the cleanliness of the clock signal - if you scope the HCT04 output its often quite 'ringy' and contrary to what you'd expect '14 schmitt inverters are often worseso I always use HCU04 'unbuffered' gates for clocks, having less gain they go linear more readily - this Rakon app-note explains it well:https://www.iqdfrequencyproducts.com/media/pg/1861/1452506068/ic-crystal-oscillator-circuits-rakon-application-note.pdfIf all else fails RS have plenty of 10mhz Z80s in stock at the moment, about 6 quid though!CheersPhil

author: CJay
author_rank: Super Contributor
timestamp: November 11, 2021, 09:57:54 pm
content: Yeah, I have been harvesting chips for decades so the junk box is well stocked, I'm rather pleased I've had to spend so little.Clock is quite nice with a 74HC04 (at the expansion port), think I need to write a RAM test as the memory chip is quite old and has been stored without much care.Impressed the Z80 runs at 7MHz.

author: Phil_G
author_rank: Regular Contributor
timestamp: November 14, 2021, 12:42:38 pm
content: Quote from: CJay on November 11, 2021, 09:57:54 pmMine too but most of it is LS or even straight pre-LS TTL. Theres even a tray of DTL in there!Actually, "show us your chip stash" would make an interesting thread subject, similar to the "what was your first computer" threadIts the HCT's I'm short of so I've just ordered a nice mixed stash from Farnell before DIPs become unobtainable - RS dont stock much inDIP format at all now, I think its a great shame for the hobbyist so the idea was to stock up with genuine chips before ebay clones become the only option!Thinking of doingan I/O boardto match the Jeff Tranter PCB, I've a few 8255 & Z80PIO or even a simple TTL latch would be ok to play with a DAC or whateverCheersPhil

author: CJay
author_rank: Super Contributor
timestamp: November 14, 2021, 09:56:29 pm
content: Quote from: Phil_G on November 14, 2021, 12:42:38 pmYeah, I was thinking of more IO but I've also got a small pile of 6845 chips...

author: Phil_G
author_rank: Regular Contributor
timestamp: November 15, 2021, 12:53:52 pm
content: Quote from: CJay on November 14, 2021, 09:56:29 pmHi CJay I found the 6845 easy to use software wise but from a hardware perspective I always thought it was only half a video chip, leaving so much more to do externally, quite messyFor 'serial' video in a few projects I've used Geoff Grahams single-chip terminal, its very good, I generally use a cut-down-to-basics version, like the idea towards the bottom of this page:https://geoffg.net/terminal.htmlIt does composite or VGA, very clever & rock-solidIt can do 1M bps and can fill its PAL screen-buffer in under 9ms (over 100 frames/sec) so you cant tell its not a memory-mapped display!CheersPhil

author: CJay
author_rank: Super Contributor
timestamp: November 15, 2021, 02:52:24 pm
content: Quote from: Phil_G on November 15, 2021, 12:53:52 pmI quite fancy building as much as possible 'from the junkbox' though and using something like the 6845 to build a video adapter opens up a few enticing emulation optionsGeoff's stuff is pretty excellent, I really like the Maximite for instance.

author: Phil_G
author_rank: Regular Contributor
timestamp: November 17, 2021, 06:58:57 pm
content: Today I did a rom image with these enhancements for the 56k version of the board - its untested as me, Tim & Jeff all use the 32k version but I'm sure it will be ok as the difference is only a couple of addresses. Its on the Retro page ofphilg.ukI'd be interested to hear if anyone does give it a tryCheersPhil

author: Phil_G
author_rank: Regular Contributor
timestamp: December 15, 2021, 07:49:05 pm
content: I finally got around to building a 56k version of the Grant Searle Simple Z80 just to try my revised ROM as I'd posted it without testing it on the 56k version. It works great - but I do miss the NVRAM I have on the 32k ones... I'll see if I can afford an NVRAM for it :-)Edit: I bought a Dallas DS1245Y 128k x 8 NVRAM, works great as a 56k non-volatile system, in fact I have two manually-switchable 56k pages... but I'm going to change it to four manually-switchable 32k pages, I think four non-volatile pages would be more useful, one page dedicated to Star Trek and three to play with! Details onphilg.ukin the retro bit.RS has plenty of DS1245's in stock, £35 which sounds a lot but compare it with my 32k Nascom-1 RAM-A board and buffer board at £250 back in '79.... £35 is a bargain! :-)I also got some genuine Dallas DS1210's and made a couple of cluge 32k & 128k NVRAMS, cheap & ugly but they work great!

author: Warren_D
author_rank: Newbie
timestamp: April 01, 2022, 06:13:05 am
content: Hi Phil_G!My name on eevblog is Warren_D, and I'm a new member of the blog, and I recently saw your post about Grant Searle's not-so-Simple Z80, and noticed you have written a monitor for it. I wanted to know if I could obtain a copy of the source code for your monitor, So I could incorporate, relocate in memory, and compile it for my z80 system, Grant's 9 chip CP/M board that I built for my personal use only. It looks really useful to me and I thank you in advance for responding.Thanks, Warren_D

author: Phil_G
author_rank: Regular Contributor
timestamp: April 01, 2022, 12:50:07 pm
content: Hi Warren, no problem see link below. Remember that this is a very minimal monitor of just 583 bytes, the key point being that it resides in the small free area at the top of an 8k ROM since 8k BASIC is actually about 7.5k with 1/2k unused.As such it doesnt compete with SCM etc monitors which are many times larger in code size. It does however cover the essentials to enter, examine and run Z80 machine code on Grant's minimal hardware, including a hex loader, block copy, hex arithmetic & relative jump calculator.Also remember that there's a small change to the 'monitor' command within BASIC, and an extension of Grants "int32k.asm" I/O program, as well as the monitor itself, ie there are 3 parts to implementing it, which is why I just posted the complete ROM incorporating all 3 to make it an easy mod. I've posted the source for all 3 parts on the retro page ofhttp://philg.uk.I use AZ80 which is a really nice cross assembler:https://www.retrotechnology.com/restore/az80.htmlCheersPhil