This is the place to discuss our glorious C64/128 machines and their very active scenes. Other C= 8-bits welcome, too!
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Seattle, WA, USA

by intric8 posted Mon Jan 07, 2019 8:02 pm

My latest acquisition for my glorious C128DCR is the Commodore 1750 RAM Expansion Unit. This REU device, for C128 owners, is really very intriguing indeed for a number of reasons.
The outer box was water stained and mildewed to the point of being gag inducing. I don't care if it was "original" - it went straight to the trash. But the rest looks freaking virgin.
I mean this thing looks almost brand spanking new!

First the basics, though. The 1750 REU was originally made for the C128/C128D, and it provided a whopping 512KB of extra RAM. At the time, for 8-bit folks, having a machine with that much RAM seemed borderline bonkers. Who would need it?

Well, BBS sysops hosting Bulletin Boards definitely used that extra memory and speed. No doubt. But also folks who used programs like GEOS had a very strong advantage. And this was especially true for C128 users who were using GEOS 128 in 80-column mode. "What are you doing in there, junior?" "I'm being highly productive, mom!"

One of my local friends uses his C128 and REU like this:
The GEOS KERNAL supports Commodore’s RAM Expansion Unit (REU) via the 128 configure file. Specifically, the KERNAL allows up to 512K of RAM to be configured to provide 1541 or 1571 RAM disks, the shadowing of a real 1541 disk drive, fast movement of data, and fast rebooting of GEOS from BASIC.”

He went on:
The drive shadowing is awesome, so basically I can shadow my GEOWRITE disk and then everything that’s loaded from it gets “cached” into RAM. If I close and re-open GEOWRITE, it’s all immediate.

Whatever changes you make in RAM get written back to disk (so the disk always remains the source of truth).

It also has a RAM REBOOT mode where if I flash-restart the 128, it boots most of GEOS back from RAM.

So in other words it's a RAM disk that can survive a warm reboot. A cold reboot would wipe it out. Though he’s persisting the RAM to disk via a shadow function.

Folks, that's pretty danged coolio!

And like the C128, which has an empty ROM socket for a adding an optional function ROM, the 1750 has a spot for one, too. However, it's not quite as cut and dry as the 128, which I'll demonstrate below.
Opening the REU case is surprisingly easy. You merely work at each corner gently but firmly. I was able to use my fingers and no tools. It's just a pressure fit. The REU is surrounded in a heat shield, which you have to also pry open just a bit at each corner in order to get to the board inside. My shielding has some very minor corrosion but nothing major. The photos look worse than reality, as the metal actually has a flaked appearance by nature - not a smooth visible surface.
Underneath the REU Commodore even applied a thin piece of paper, presumably to protect the solder points from ever touching the heat shield.

After opening the REU for the first time (and voiding the warranty!) I was stunned to discover that the function ROM bay was not only un-socketed, but the holes for a socket were filled.

This leaves me with a few questions.

1. Should I de-solder the plugged holes and solder in a socket for a function ROM? I wouldn't ever want to solder in a function ROM as there'd be no way to disable it. If I installed a socket for a function ROM, would I need to remove a section of the heat shield for it to fit?
2. And look at that area above the ROM bay. It looks like it's made for a jumper - for adding an on/off switch, perhaps? Does anyone reading this know if that's for enabling/disabling the function ROM when it is installed? I found the schematics if you're skilled in reading those.

My original plan was to install a GEOS ROM into that empty spot and make this REU a GEOS Cart-on-Steroids. The GEOS ROM speeds up the boot times a bit, which is nice. But adding the GEOS function ROM has other benefits, too.

When GEOS first came out every application boot disk had a serial number. The GEOS Boot ROM solves the “disk serialization problem” so you don't have to worry about your GEOS programs matching your GEOS serial number anymore, which is very cool.

Things to ponder and figure out in the coming weeks... If you have a personal favorite way of doing things, I'd love to know.

For what it's worth, back in the 1990s some folks figured out how to push the Commodore REUs to 1mb and even 2mb of RAM, which is just insane. From what I've read, you have to remove all of the heat shielding as the case will barely fit after modding it like that. But holy smoke... 2mb must have been very cool. (I don't need that much - I'm just saying.)
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Seattle, WA, USA

by intric8 posted Mon Jan 07, 2019 10:32 pm

In my March 1987 issue of COMPUTE's Gazette, I found an ad for the 1750 in the back of the magazine. It cost $169, which is the equivalent of $375 in 2019 US dollars. For half a meg. As a comparison, that same vendor sold an entire C128 machine in 1987 for $239.

That being said, I think the REU was technically released in 1985. Finding the introductory price would be interesting.
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Zippy Zapp

by Zippy Zapp posted Tue Jan 08, 2019 7:34 am

Nice! Can you take a picture of the whole board? Apparently on the 1750 there is an extra resistor that I am not really sure what it does. I have read that it is a timing fix for C128 use. The 1764 made for the C64 does not have this resistor. But I have also heard the 1750 works fine in the 64 too.

I have 2 1764s, one is my original bought in 86/87 and the other is one I found a while back and converted it to 512k. Of course to use it on a 64 you are supposed to use a heavy duty power supply which everyone using a 64 should have vs the original Brick of Death™, or at least a modern replacement or the 1764 PSU.
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Seattle, WA, USA

by intric8 posted Tue Jan 08, 2019 8:36 am

Can you take a picture of the whole board?
If I perform the GEOS ROM mod on the board, I'll take one for sure. That resistor might be found in the linked schematics, too. When I opened it up last night the heat shield was really delicate and a lot thinner than I was expecting. So I'd prefer to leave it shut until I do the surgery if that's cool.
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Zippy Zapp

by Zippy Zapp posted Wed Jan 09, 2019 6:29 am

No, that's cool, I thought you might have had one already. Yeah, I wouldn't open it again unless I had to either as those plastics get brittle too. The resistor is on the schematics I was just curious if they actually put it in or if all of them, even the C128 versions, are left off.

I have heard of people using the 1764 on a 128 with no problems so it was just a curiosity.
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by Mr.Toast posted Sat Jan 12, 2019 7:50 am

Oo sighted the VicModem ad in that! Of all the stuff I kept from my youthful 64 days, that is the one thing I couldn't part with as a memento. Now, of course, I wish I had all my old kit but-who knew I'd be playing with C= 8bit stuff nearly 40 years later!

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