"Modern" Z80 system design and cloning old computers.

[GK]

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/#msg1246318

I 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.

 
 

[C]

Which model of the TRS-80
You have
Model 1
Model 111
Model IV
Model IVp
All with small differences

If you want games then you need to match speed.
The cassette port often used for game sound.

Then you have business system Z80
model 2
model 12
Model 16
Model 6000
Last to add a 68k

[GK]

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.

[C]

You can find the Schematic on line
You could do just the keyboard but cheat and put in 48k ram
If you want to play games then more matters
For display you can put graphics in  character generator rom
Important 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-input

Now do you want disks , printer, rs232?

[C]

That was dumb, just match printer
Most did not have RS232

The problem is disks
The 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  WD1793
Was 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.
 

[C]

GK
  The 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.

[GK]

That was dumb, just match printer
Most did not have RS232

The problem is disks
The 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  WD1793
Was 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.

The 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.

[C]

Know a bit about all models
part 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 TV

Keyboard 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 & RS232
With todays parts, you might want to think about putting the few extras of a model III cassette system in your keyboard.
 

[GK]

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.
 

[C]

Just trying to point out that

Model III existed as a cassette only version with very little hardware change from model I

High speed cassette option was one change
Better cassette receive circuit

Think also added rtc interrupt

[C]

No changes from grant's sio init code in use now?