"Modern" Z80 system design and cloning old computers. - Page 1
Forum section: electronics / projects

author: GK
author_rank: Super Contributor
timestamp: July 02, 2017, 03:26:50 am
content: Well my 1st attempt at an old-school microprocessor system was to dive right in and clone a Commodore PET-2001 using current-production/equivalent 6502 and VIA/PIA peripherals:https://www.eevblog.com/forum/projects/otp-eprom-programmer/msg1246318/#msg1246318I am now finalising the technical details of that design (just a little more perfboard prototyping to do) and the next step will be to draw it all up in Protel and layout a (mostly SMD) PCB for it before building/presenting/publishing it as a completed project with professionally manufactured PCBs.While plodding away at that I'm making a start at working out the preliminary requirements for cloning a different computer - the Z80-based "Tandy-Radioshack" TRS-80 ("Trash 80").The current plan is to use a current production Z84 in LQFP from Zilog. Once upon a time I learnt the basics of MPUs on a Z80 system but I have never designed with one. Is there anything important that I need to know about the compatibility of these Z80 variants with the original that ran the TRS-80 operating system? There were a number of important issues that I had to get down pat in order to properly and successfully implement the PET clone with the modern 65XX-family variants from WDC.Like the PET clone I want to do this entirely in current production logic and again like the PET clone the circuitry can be simplified hugely because much of the original circuitry was just decoding and buffering logic for a gazillion small capacity RAM/ROM chips. The only exotica inside the TRS-80 was a (now long obsolete) "character generator" IC by Motorola which was little more than a pre-programmed ROM containing the character set. The video generation circuitry of the TRS-80 won't be much more difficult to functionally replicate in current production CMOS logic than was the video generation circuity of the PET.Thanks.

author: C
author_rank: Super Contributor
timestamp: July 02, 2017, 03:55:26 am
content: Which model of the TRS-80You haveModel 1Model 111Model IVModel IVpAll with small differencesIf you want games then you need to match speed.The cassette port often used for game sound.Then you have business system Z80model 2model 12Model 16Model 6000Last to add a 68k

author: GK
author_rank: Super Contributor
timestamp: July 02, 2017, 03:59:59 am
content: The first one (Model 1), with fully expanded RAM and the 12kb "level 2" ROMs (actually user-switchable between "level1 and level2; I have the original binaries) of course.I will not endeavor to replicate the bouncy response to button presses on the keyboard, however.

author: C
author_rank: Super Contributor
timestamp: July 02, 2017, 04:13:26 am
content: You can find the Schematic on lineYou could do just the keyboard but cheat and put in 48k ramIf you want to play games then more mattersFor display you can put graphics in character generator romImportant for games to have both 32 & 64 char display modes.Games do a fast switch between 32 & 64 modes to shake what shows on screen.Games with sound connected cassette output to speaker for sound. would need to match ratio of output levels.Very few games did anything with cassette input.Need to match keyboard in memory as it is used as multi-inputNow do you want disks , printer, rs232?

author: C
author_rank: Super Contributor
timestamp: July 02, 2017, 04:29:54 am
content: That was dumb, just match printerMost did not have RS232The problem is disksThe model I used a WD1771 I think number was.Used a different sector ID code as a second way to find directory sector. May not have said the correct, Sector mark on disk. You will find it can create a deleated sector mork. From memoryThe WD1771 could create 4 different marks. The one picked to mark directory can not easily be read by the DD WD1793Was an option for DD, what they did was put in both chips and used invalid track or sector value to swap chips.Protected disks used any thing these chips could do to try to prevent copies.The reason they stopped making the model I was due to RFI some programs were good on AM band radio.

author: C
author_rank: Super Contributor
timestamp: July 02, 2017, 09:05:02 pm
content: GKThe video of the model I is simple.Have one thing you might want to think about that might make it even more so.The Z80 reads and writes to video memory. Back then it was a cost issue.Nothing says a Z80 can not write to two memory chips.Think of using a 64k static ram to replace the 48k.A video memory read could then come from the 64k chip while a video memory write could go to both. The Z80 would not know the difference.Might save some logic, might not.You might like to know that the changes from Model I to Model III were not large, but some did cause problems for Model I programs.The last of the Model IV & Model IVp did add the equal of a boot rom that could test hardware accessed by holding down some keys at start.

author: GK
author_rank: Super Contributor
timestamp: July 03, 2017, 03:15:55 am
content: Quote from: C on July 02, 2017, 04:29:54 amThe basic stand-alone keyboard unit was only provided a cassette interface. RS232 and disk drive interfaces, etc, were provided by an optional external box (that you typically sat your monitor on top of) that plugged into the expansion port.I am only really interested in functionally replicating the basic keyboard unit for now.

author: C
author_rank: Super Contributor
timestamp: July 03, 2017, 05:06:57 pm
content: Know a bit about all modelspart time sold and repaired them. My expansion interface serial is in 400's.The model 16b was not bad as a multi-user computer.Note that Model I video out in 64 char mode was a bit much for US TVKeyboard only is not as much fun as having disks.One thing common with cassette use was putting a machine code program at start of tape that could then load remaining tape at higher speed.Model III added faster cassette speed.You might want to think about level i, level II & model III roms.a Model III had add on boards for Disk & RS232With todays parts, you might want to think about putting the few extras of a model III cassette system in your keyboard.

author: GK
author_rank: Super Contributor
timestamp: July 06, 2017, 01:48:18 pm
content: I'm not particularly interested in accumulating old hardware such a floppy disk drives and media; will be quite content with the cassette interface only. Any digital audio recorder/playback device (such as my PC) can serve as program storage.As a potential future project I'd rather make an embedded disk emulator with flash mass storage to plug into the expansion port than attempt to interface an old crufty disk drive.I have a Z84 in 40-pin DIP for prototyping on the way.

author: C
author_rank: Super Contributor
timestamp: July 06, 2017, 02:03:40 pm
content: Just trying to point out thatModel III existed as a cassette only version with very little hardware change from model IHigh speed cassette option was one changeBetter cassette receive circuitThink also added rtc interrupt

author: C
author_rank: Super Contributor
timestamp: July 06, 2017, 02:06:39 pm
content: No changes from grant's sio init code in use now?