Chasing a DRAM Microbee

[CheshireNoir]

Hi All,

First up, let me know if this is in the wrong area, and I'll delete and move it to the right place.

I'm starting to put the feelers out to "mix up" my Microbee collection. I currently have a Model II, a PC85b and a Personal Communicator 32k.
The PC32k is with box and is in quite good nick, complete with stickers. (No cables / PSU currently, but I can build them these days)
What I'd like to get is a DRAM system, with an aim to install a Gotek. Ideally I'd obviously prefer a "Premium" system, but realistically, being able to boot off disk, and be able to upgrade to the higher RAM count trumps a lot 
I'm also happy to get something caseless, and possibly with some faults. I've got three of these working so far, so happy to explore further.
(I'm also about to test my new laser cut case design. I won't be competing with Ewan any time soon, but it'll do the job for me.)
In return, I'm happy to either swap the Personal Communicator 32k, or do a swap + $$$. I'll cover postage.

I'm not chasing anything right this very second, but if someone out there was chasing a SRAM based system and had, say, a CIAB with a dead FDD and PSU (For instance), then maybe we can talk.
Feel free to PM me.

Cheers and TIA.

Chesh

[someone]

G'Day Chesh,
Why not make this an opportunity to make your own Expansion board that connects to an early Microbee32 using its 50 way connector?
These days its really easy to do.
Someone hopes you enjoy this post.

As a guide, attached below are the modifications to a MWB1732 coreboard to work with the prototype Microbee Cartridge Expansion slot.
(As it panned out, RAMs of a suitable size for the cartridges did not become available in time nor at a satisfactory price. Also, that the original intention was to increase the CPU speed to 4MHz but this was not done due to the expense of faster ROMS.)
[#NOTE: There ware performance issues with the MASK FONT ROMS produced by Texas Instruments even working at 3.375MHz.  It is preferable to burn your own with faster EPROMS].

1. /PHANTOM SIGNAL ADJUSTMENT
This makes the /PHANTOM signal effective only for coreboard ROMS at memory address %8000-EFFF.
Reconfigure the /PHANTOM signal by disconnecting it from IC20, pin 6 and connecting it to IC22, pin 6.

2. /ROMSEL SIGNAL ADJUSTMENT
This makes the /ROMSEL signal switch between coreboard MWB ROM A to the a signal on the 50 way expansion connector.
Connect the /ROMSEL to pin 10 on the 50 Way Expansion connector (i.e. reallocated from Z80 /HALT).
Cut PCB track from /ROMSEL to IC22 pin 11 then insert series resistor between IC22 pin 11 and ROMSEL.

3.*Only for 2MHz: CRYSTAL ADJUSTMENT
This provides the CRTC with a dot clock capable of displaying 80*25 characters.
Change the onboard 12MHZ crystal with a 13.5MHz crystal.
On the Motherboard, disconnect the CPU Clock from the 74LS92 divide by 6 output and connect to the divide by 4 dot clock output on the 74LS161.

4.*Only for 2MHz Systems: MWB ROM UPDATE
Replace MWB ROMS with 3.375MHz compatible version.
This version of BASIC is reconfigured to handle the increased CRTC Dot Clock and increased CPU clock speed.
The speaker pitches and durations have been adjusted sound correct.

5.*Only for 2MHz Systems: DUAL FONT ROM UPGRADE
This enables the microbee to display a font suitable for 80x24 character resolution.

Description
The Cartridge Expansion port enables a microbee 32K to work with plug in ROM and RAM expansion cartridges enabling it to work with disk drives and CP/M and work with extended play Game Play cartridges.  Rather than waiting an eternity for a tape to load, ROM Cartridge based games are loaded instantly.

Theory of operation
When the cartridge port is disconnected, the microbee behaves as normal.
When the cartridge port is connected with no cartridge inserted, the microbee behaves as normal.

When the cartridge port is connected and a cartridge is inserted to the cartridge slot , the ROMs on the coreboard are disabled being replaced with the memory decoded by the inserted cartridge.
This memory can either RAM, ROM or a combination of both.
The /ROMSEL signal is connected to the memory address decoding output of the desired boot address.

For the DISK SYSTEM RAM expansion cartridge, RAM is provided at %8000-%DFFF and ROM at %E000-EFFF. /ROMSEL is configured for address %E000.

[CheshireNoir]

Thanks Someone,
I think I'd still prefer to have a proper DRAM system to compliment my SRAM systems.

Cheers!

Chesh

[Ernest]

1. /PHANTOM SIGNAL ADJUSTMENT
This makes the /PHANTOM signal effective only for coreboard ROMS at memory address %8000-EFFF.
Reconfigure the /PHANTOM signal by disconnecting it from IC20, pin 6 and connecting it to IC22, pin 6.

I guess that is the reason behind the move of the PHANTOM signal on the MB8328 from the configuration that was used in all the previous SRAM Coreboards.
To a large extent, the cartridge idea would have been the influence behind the Multiple ROM Pak's available on the MB8328.

Ernest

[mjelic]

Thanks Someone,
I think I'd still prefer to have a proper DRAM system to compliment my SRAM systems.

Cheers!

Chesh

Is there a definitive guide as to what models had what in them? With pictures?

I purchased a Premium 128k but it may not be what I want. I want one that has a lot of ROM slots that I can burn the specific programs I want in it. Namely, maybe a game or two, but most importantly, an Assembler.  Of course I want some way to save my assembler programs, so the disk drives that come with this unit can be handy... I guess... But it sounds like I should flip them over to Goteks anyway. And then, do I really need two of them? I really don't want the bulk of the drive case, and frankly I don't want to be booting off a floppy. I just want to switch it on and BAM, it's good to go.

The monitor (damn cute!) and the keyboard is what I remember having in high school. So if there is some *compact* way I can save my programs... Or hell, I'd be happy with replacing some of the RAM with battery backed NV-RAM... Or better, how about some FRAM?  But being able to save files off the system onto a USB would be ideal, so I can share my programs with others.

So... Which model should I be looking for?

[CheshireNoir]

Here's the page I always use:
https://www.microbee-mspp.org.au/wiki/ti...ome#Models
Pretty much anything marked as an "SRAM" board is the one full of ROMs you remember. The limitation? They only have 32K or less of RAM.
The "DRAM" boards have much more RAM but assume you're loading off Disk. They don't have the plethora of ROM sockets you can easily interchange.

Hope this helps!
(I'm still relatively new to all this stuff myself. I got my first Microbee last year and I've been learning a lot since!)

Chesh

[ChickenMan]

Your Prem 128k with Gotek will look as simple as this.  I put the larger LCD screen in the top of my Gotek rather than the smaller normal one that fits where the 000 display is in the front panel of the Gotek.

   

   

[CheshireNoir]

One day something like that will be MINE! :-D

Chesh