REUs, EXes and You, Oh My!

by Roger J. Long

Recently a fellow club member helped pin-point why my SuperCPU accelerator acts finicky with some
computers. It wouldn't always work when the computer was turned on. He solved it by changing the
angle the SuperCPU connects to the computer at. He propped up the back end of the computer by
laying a pencil underneath it. The connectors on the SuperCPU made full contact, and it worked
reliably afterwards.

This reminded me to dig up the response I had received from CMD regarding another cartridge port
problem I had.


My question to CMD:

I have the EX 2+1 cartridge port expander, but have been unable to use it due to what is probably
signal degradation. My system is a C128.

I have tried just plugging the REU (a 1764 that's been expanded to 1 megabyte) into the 2+1, but then
the REU stops working reliably with GEOS. So, I haven't even tried anything fancier; such as the
Swiftlink, REU, SuperCPU and Novaterm combo that I've been wanting to try out.

Both the C64s and the C128s I have have a heavy duty power supply. Would a C64 provide more
current to the cartridge port? I'd like to keep using the C128 so that when I get Novaterm 9.6, I'll have
access to the 80-column video mode, but if it takes a C64 to make the EX 2+1 work reliably, I'll hook
that one back up.


Response from CMD Support:

You could have either noise or signal strength problems. The power supply isn't going to change the
results in this case, as it isn't a matter of needing to supply more current to the device on the port. It has
more to do with either the ability of the chips whose signals are on the port being able to handle the
loads placed on them without degrading, or it may be that the extra length or connections involved are
allowing too much noise on the signal lines.

The first thing I'd try, if at all possible, is a different REU. Stock REUs often generate a lot of noise by
themselves on the signal lines, and create a fair amount of drain on the signals as well. Expanded REUs
create even more of a problem; a 1 MB expansion of a 1750 places four times the signal load on the
signal lines. In addition, the method of 'piggy-backing' often produces signal noise levels many times
higher than the original chips would have. Anyway, a different REU could be lower in noise, and an
unexpanded one will put less of a load on the bus. If you find this is the cause of the problem, you might
want to look into one of our 1750XLs, as they have only four memory chips loading the bus, where
your expanded unit has, I, believe, 32 memory chips.


Additional notes:

CMD was indeed correct in the amount of chips in my REU. There are 16 connected directly to the
motherboard, and each one has a second chip piggy-backed and soldered directly onto it. In addition,
there are lots of wires running around that make the connection to that extra 512K in the REU. Plus, I
have a switch installed that lets me select between a stock 512K or the full 1 megabyte, so that it
remains compatible with "Maverick." (I have directions somewhere on how to modify Maverick to use
a full 1 meg REU, but never got around to implementing them.) When I was making the modification, I
went for functionality, not FCC certification.

So, I don't doubt that there is both noise and weakened signal strength going on inside the REU:

However, it works fine hooked directly to a C64, C128 or the SuperCPU. It just can't quite handle the
extra distance of being plugged into an EX 2+1 cartridge port expander.

I do have an additional 1764 REU that has 512K in it. This may perform better when attached to the
2+1.. I have not tested this yet due to the fact that it has less memory than the one I use now, and I
need most of it for when I do the newsletters. It will get tested in the future, however. I plan on getting
"GeoFax" and see how well that works as a scanner.


REU History:

Commodore produced 3 RAM Expansion Units (REU). The 1700 REU had 128K and the 1750 had
512K. Both were marketed for the C128. The 1764 had 256K and was marketed for the C64. The
1764 included a heavy duty power supply since a stock C64 power supply didn't have enough strength
for the REU. A C128 power supply is already heavy duty, so the 1700 and 1750 just came by
themselves.

Interestingly enough, rumor had it that the 1700 and 1750 only worked on the C128, and the 1764
only worked on the C64. But it didn't take long before that rumor was squashed. Upon opening the
1764 and taking off the metal shield, the words "C128 RAM Expansion" can be seen on the circuit
board.

There are 16 spaces for chips on the circuit board, in two rows of 8. At the time they were made,
memory was quite expensive, and they commonly had one chip work with one bit at a time. So, it took
8 of these chips to make a full byte that the computer could work with. The 1700 used 64K by 1 bit
chips (64Kx1), and filled all 16 spaces to get its 128K. The 1764 used 256Kx1 chips and just filled the
first row. The 1750 filled both rows with 256Kx1 chips to get its 512K.

Plans were developed that allowed for going beyond the designed 512K limit. The first step was to
bring the REU up to 512K. If you had a 1700 REU, you had to unsolder all 16 chips and put in 16
256Kx1 chips.. Next, a small controller circuit was added, and then a new bank of 512K would be
added by soldering the 16 new chips directly on top of the first 16 chips. To get 1.5 megs of memory,
you added another 16 chips, and to go out to 2 megs, you put in a last group of 16 chips.

As CMD mentioned, at any level over 512K, you have a lot of noise being generated in the REU and it
gets worse the more memory you have. There is also more heat generated the more chips you have.
And, getting the metal shield put back on the REU gets more difficult when you have 3 or 4 chips
stacked on top of each other.

When I decided to expand my REU, I had two reasons for stopping at 1 meg. First, the GEOS
Configure program that Jim Collette modified would only use the first megabyte in a REU to make its
RAM1581 RAM drive. This is what I use for making the newsletter, since I can store the same amount
in the REU (790K) as I can on a 1581 disk. Second, I had heard of people having reliability problems
with REUs expanded to 2 megabytes. Now I know why.


Other RAM expansion devices:

The REU really found a home with GEOS users.. Being able to work out of RAM instead of constantly
from the disk drive (often a 1541) sped things up considerably. Most GEOS users had C64s, but didn't
want to spend the extra money that a 1764 cost because of the included power supply.

Berkeley Softworks (now GeoWorks) developed their own RAM cartridge called "GeoRAM". It used
newer memory chips so that fewer chips stored the same amount of memory as a regular REU. It fit
inside a regular cartridge shell, and didn't require a heavy duty power supply. But, it didn't include
Commodore's REU controller chip (the REC), so GEOS had to be modified to duplicate all the work
the REC did. Consequently, it was a GEOS-only cartridge, not usable with anything else. A regular
REU worked with more than GEOS, such as some Infocom games. A few specialty programs have
since been written that will work with the GeoRAM.

Between the GeoRAM and today, there have been various RAM devices, such as the BBGRAM, the
Super 1750 Clone, the RAMDrive and the RAMLink, the latter two being made by CMD. Not all of
them provided full compatibility with the original REUs.. CMD's newest RAM devices do provide this
compatibility because they use the REC chip. Modern memory chips allow the CMD 1750 and the
CMD 1750XL to fit 512K or 2 megabytes, respectively, plus the REC into a package the size of a
game cartridge. A heavy duty power supply is still recommended by CMD, and I second that
recommendation. The original C64 power supplies are notorious for killing the C64 also when the
power supply died. Getting a heavy duty power supply will save your C64 from the grave.


Conclusions:

Getting my 1764 REU (and its accompanying heavy duty power supply) was one of the best purchases
I ever made for my C64. It made running GEOS enjoyable because there were almost no delays when
working from the REU. It made Maverick work better because the file copier and the whole disk
copier would store data in it, cutting down on disk swaps.. And, with the new programs like Novaterm
9.6 making use of it, having a REU "isn't just for GEOS any more".


Items mentioned in this article:

CMD 1750: $99.00 CMD 1750XL: $139.00 (Both also available with GEOS 64 or GEOS 128)
Heavy Duty Power Supply for C64: $59.00 EX 2+1: $39.00 EX 3: $34.00 

1-800-6383-CMD (1-800-638-3263)


Roger J. Long, editor for the SWAMI (Amiga) and TC-Cubed (C64/C128) clubs in Southeast
Washington state.

(This article dates back to late 1997, so prices and availability are not guaranteed).

(Via the Commodore Information Center http://home.att.net/~rmestel/commodore.html)