Homebrew Z80 Computer Number Two - 8BitBites
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- เผยแพร่เมื่อ 1 ก.ค. 2024
- #vintagecomputer #z80 #homebrew
In this 8BitBites episode, we take a look at another "homebrew" vintage computer from around 1982 - this one also based on the Zilog Z80 CPU.
This machine is much more capable in many respects than the other homebrew Z80 featured in our previous 8bitbites episode.
A dump of this machine's 'TOS 4-15' monitor EPROM is available here for those curious: drive.google.com/file/d/1vRo9...
0:00 - Intro
0:44 - 10 Cent Tour
1:51 - We're Not Alone
7:27 - Switches, in Stitches
8:01 - Christmas in June
8:17 - Mary Had a Little Lamb (then she ate it)
11:30 - Perfunctory BASIC Program and Test
13:00 - Brad Gets Suckered
14:19 - I Said 'Bugger'. Not what you think I said.
15:25 - Duped by a Z80
15:51 - Let's Try the Lazy Way
16:34 - The Fool Thing
16:45 - Shameless Subscribe Plug
16:47 - Revenge of the Fastvid Terminal
17:07 - Time for a Hangin'!
18:33 - Another Shameless Subscribe Plug
18:38 - One More for the Road (Outro) - วิทยาศาสตร์และเทคโนโลยี
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Hi,
I've started pulling the binary files to bits and the start up process looks fairly simple (if I am reading the code correctly) :)
On RESET the system waits "PULSE" button to be pressed ; it then reads PORT 0 switches and uses the value as an offset into a look up table. The result of the look is the address, within the monitor, that the processor jumps to. Each code segment seems to be no bigger than 256 bytes (so far).
The look up table has code entry points for all but 2 switch combinations (1100 and 1101 do not). I think I've found the two "musical" setting but it would be really useful to know what you see/hear for each combination of the PORT 0 switches.
Thanks for a great video...
These bring back so many memories. I created a Z80 computer on several 44-pin edge connector cards from Radio Shack, and wrote my own monitor for it (and later Forth). I had planned to use it in a homebrew robot but got bogged down by the mechanical design and didn't do much with it.
I saw this very unit on Ebay.. Thought of bidding but and the end price was really good considering the rarity... As we all know though, Vintage computing is _not_ a cheap hobby and diverging into too many machines is quite the money pit ;)
That's a great piece of computer archeology. Thank you very much!
Great video. Would love to watch more.
This is fantastic !!
I just found your channel tonight! Very cool! I'm always fascinated with obscure vintage computers like this.
pretty sure those switches just do what they say: set whatever the computer can read from input port 0x00, and then the rom uses that to jump wherever, because the z80 always resets to 0x0000
tried to disassemble your rom, not sure if that dump got messed with, but from what i can tell, it jumps to 0xe074 on bootup, 0xe3e3 on RST8 and 0xe3e6 on RST10 (all of which are 0xff, which means RST38, which itself is 0xff, so endless loop), and the remainder of the code also doesn't make too much sense to me, so not sure what's going on there. if i'm interpreting that code correctly it should never start up. maybe they connected some address pins in "creative" ways, or the connections to your adapter were just dodgy due to pins having a patina after all that time maybe?
Nice vintage tech 😃 Thx
Keep up the awesome work!
I remember getting that book from the library!
The beauty of the Z80 CPU is the fact that on power up it starts running code from address 0. Easy to program.
Why Jingle Bells though ? Ha ha. (or should that be Ho Ho ?)
Yeah, I saw a few of these back in the day, probably in/on old electronics magazines. Probably a kit.
ZX-81 woodie? I can hardly wait. I always wished for a decent keyboard for it. And props for mentioning the David Ahl book(s). I see that you too are a man of culture! Super Star Trek was my favorite game. Mastermind too.
Yeah the wooden ZX81 video is finished and just undergoing last minute editing. I definitely enjoy using it over the plain Jane version. Yes I've programmed a few of Ahl's recipes over the years. Not so much lately as I don't get as much time as I'd like.
Nice mystery machine. Lots of early machines went down the embedded terminal route, even the Apple 1. There is probably a pile of control/escape sequences you can send to the terminal to do cursor addressing etc. It looks like that terminal has a few issues, maybe thats why it's set back to 300 bps.
As for talking to it with your laptop, sounds like a parity etc problem.
It is now High Summer, as a matter of speaking, roasted bird are falling from the hot tin roof and listing to Jingle Bells is kind of kool.
We need this made commercially!
That's not "Mary Had a Little Lamb" at 8:21, it's "Twinkle, Twinkle Little Star" :-) Great video!
Yeah. Honestly it's been that long since I've heard it!
It could also be the abc's
The designer probably added the tune for fun. The beep command on my HP42S calculator plays the Westminster Chimes.
In the 70's Radio Shack sold a standalone chip that had about twenty little tunes like that. You only needed a few discretes and an amplifier.
Thank you for Sharing. Greetings from Karachi, Pakistan.
I think the DB25 on the back is just a male connector with no pins installed. They do look weird with no pins. :)
yeah, my dream computer is sticking with calculator lcds thank you very much
I'm not a car guy, but I got the Doug DeMuro reference! 😹
I wonder what this computer would have been like if it had used the ZILOG Z800 or the ZILOG Z8000 processors and 1MB of RAM.
this looks like it could be reproduced, atleast in some fason, with new(ish) components. a little wood, metal and finding those key caps again (thats why we book mark everything now) and your part way there. I think I even have a veroboard ready to go.
going to make a wild guess on what that tape noise mode might be for the monitor - since it comes out as noise and is part of the terminal ROM, is it possibly something you could send through the tape out as terminal commands? either live or possibly recorded for later use, ie type out a program at home and run it on the mainframe at your institution?
Yes, I kind of think it is like early 70s machines where you could 'punch' memory contents to paper tape, and then later with the Kansas City standard they changed processes to encode to audio. If I ever figure out how the monitor works I hope to try reading in something and seeing if it works as anticipated.
I immediately recognized that RCA connector; Radio Shack sold these a a low-cost alternative to the ones with a plastic back-shell. They work only if you perfectly solder them without melting the low-temperature center-insulator; additionally they are really meant for single use, where you just plug them in and leave them.
Great video, you're quite the digital sleuth and your tenacity is admirable...
That S/D monitor prompts look like it's asking for a source address range and a destination address
It could be that or Single/Double, as in memory size (a byte or word). Especially since they had a different format. S is 0000-0000 and D is 00000. I was also thinking the "P" is for "Push" or "Put", but it's hard to guess without seeing more (if they exist).
it looks a bit like an old Hal Communications Corp terminal but those were marked as such.
There at the end where you said the text got corrupted after a few lines. That looked interesting. In videos like this either computers display perfectly or not all. I don't often see the glitchy side. When you're fixing up these old computers it would be cool to do a little video showing off that gibberish on screen. Any subject along with it will do. Maybe you can just talk about the common glitches on screen a hobbyist would see back in the day. I don't know. I'm probably the only person interested in it though.
(Two years later...)
I wonder if this goofy thing has different send and receive bit rates for the terminal, or if it was secretly running as synchonous instead of async, and that's why your ThinkPad couldn't talk to it.
Hallo there. The DIN connector is an audio connector for some TVs here in Europe, as in Portugal and Germany such as Löewe that plugs the audio into the TV set . It also could be used to save sounds from TV into a tape recorder (like saving some tv series , an A-Team music, whatever).
I read Popular Electronics and Byte during the eight bit era, and I don't recall seeing any kits like that. I remember the Netronics kits and wanted to buy one, but I was a poor high school student and my parents thought building a computer was a waste of money.
Yeah I've looked through back issues of same but nothing so far. I figure it had to be some kind of offered project somewhere.. the similarities are too great. But where? Maybe from an institution or school?
Here's the bin file disassembled using ghidra and quick analysis (maybe incorrect...). From line 185 starts "program selector"...
pastebin.com/epQRB1xU
Thanks so much for this! Am eagerly looking through it as we speak!
You have probably checked this already, but is there anything on the underside of the case? e.g. a manufacturer's name, serial number, anything that may help identify the origins of this hardware
These videos about ultra obscure systems are a refreshing change from the usual mainstream computers that _everyone_ covers! Thank-you. 😃
Nada. I even pulled the computer itself out with the power supply but no joy. I have been perusing old magazines from the early 80s.. I figure if someone did this in some volume, given that these units have turned up in various places around the country, they must have advertised it somewhere.
What's the geographic distance between the 3 known examples? Given the identical keyboard layout, I'm wondering if it may have been a school project of some sort.
I noticed you typed Morristown NJ. Is that where you live? I live in Hamilton. If not is there a reason you typed it?
No that was just the copyright info the original programmer of that Hangman program put in there. Actually I did live in New Jersey for a time, but I actually live in the Greater Vancouver area now.
Were those easter eggs that you stumbled onto
Control-H for delete?
Well... that's confirmed later in the video. ;-)
I had long ago forgotten terminal control keys. Remembering after the fact was a bit painful. :)
Is the output strictly serial out as in no support for cursor movements with control characters?
Seems like after one says "netronics" a couple million times, they'll start leaving out the first 'e' for brevity. n'tronics :) n'tronics :)
I downloaded the TOS .bin file and made a pass at dissassembling it. It would help to know the memory map for this computer. I assume the ROM starts at 0000h, the address where code execution begins after a reset. The first two instructions are:
ld sp,0
jp E074h
There are two other vectors for RST 08h at 0008h and RST 10h at 0010h both with targets in the E000h block. It isn't uncommon for address decoding to result in ROMs or RAMs to appear in multiple blocks of the address space.
Back to the SP, setting it to 0 would result in the first byte pushed onto the stack will be at address FFFFh meaning there has to be RAM there.
Roger, I've had a very quick look at the ROM and notice something of interest: The "LD SP,0000" command occurs every 100H bytes and the preceding bytes are "FF" - padding between ORG statements? The other think is that, if I'm decoding it correctly, the "CALL" all seem to be in the Exxx address space (E3xxH and E4xxH) so that seems to agree with your statement that RST vectors are in the E000H block.
The "mode" switches do seem to act to change the restart vector - maybe force some of the address lines?
I know this is an older video but you might check your power supply. I am hearing a very annoying high frequency squeal towards the end when you are running the hang man game.
Looks like a poor copy of a " SOL", from the 70's .