6502 RAM/ROM Expansion construction
If you ordered a kit, you should have gotten all the components on this
picture. If you ordered just the PCB, you'll have to get these or
similar components yourself (see the main page
for a parts list). Note that the GAL is not included in the kit and
therefore is not shown here. The same goes for the 6502 CPU!
Disk drive special notes
The disk drives turned out to have a very special memory map that can
lead to problems with an unmodified 6502 expansion board. If you want
make use of the expansion RAM at $2000-$7fff, a small hardware patch
to be applied. All other functions work without the patch as well:
Expansion RAM at $0000-$07ff and $8000-$9fff, and the Flash ROM at
$8000-$ffff. The patch has to be undone if the board is to be used in a
PET or VIC-20 computer. This is as simple as soldering one wire end to
another terminal of the GAL socket. A small switch could also be added.
The fix involves cutting the connection of the A15 signal between the
expansion board and the 6502 socket adaptor, and applying a modified
signal from the GAL chip. The following steps are needed:
Installing the patch:
1. Cut the two copper traces (on both the top and bottom side of the
board!) between pin 25 of the CPU socket IC1 and pin 25 of the
socket adaptor CN1.
2. Solder a wire from pin 25 of the CPU socket adaptor CN1 to pin 18 of
the GAL socket IC4.
Uninstalling the patch:
1. Disconnect the wire from pin 18 of the GAL socket IC4 and install it
to pin 4 of the GAL socket IC4.
A picture that shows the changes
from the bottom side of the
Patch for Version 1.1 PCBs:
There is now a jumper for selecting the patch mode. You can install a
jumper header here, or a simple wire link if you don't plan to change
the mode often.
For PET and VIC usage, make a link between the left and middle pads,
for 15xx (disk drive) usage make a link between the middle and right
the new Jumper field shown from the top side of the PCB.
Assembling the device includes a few tricky steps, especially
installing the socket adaptor. The instructions below are not the only
way to do it, but it is what I thought is easiest and most straight
Please read through the whole set of instructions first before you
solder anything. It's also a good idea to do a simulation run and
the components without actually soldering them, so you can see if your
end results matches the pictures you see on these pages. If not, you
still undo everything and start over without having to desolder
|I'm now using and supplying
another type of socket adaptors, this makes the soldering of the 40 pin
socket much easier, but requires an extra step to keep the socket
adaptor rows aligned. Simply put the two rows into a socket for this,
solder the four corner pins to keep the rows in place (I do this from
the bottom side so the adaptor rows don't get misaligned), and then
remove the socket again. The remaining pins should be soldered from the
I find it easier to install these adaptor rows as the first step,
before installing IC5. Please also read the explanations under step 2 before soldering anything!
|Cut out the central stabilizer
bar from the 40 and 32pin socket. I use a small wire cutter for this.
the 40 pin socket, remove the remaining plastic carefully with a sharp
knife, because the 74HCT245 fits very tightly inside the socket. Now
insert the 74HCT245 into the board, but do not solder it yet. Also
insert the 40 pin socket, you might have to press a little bit to get
into place. Now turn the board around and solder the 74HCT245 into the
place it has been put now (it might not be centered in its holes). You
can also solder in the 74HCT157 now. Do
not solder the 40pin socket yet!
At this point, you can also solder the SMD-SRAM at IC3a if
you decided to use it instead of the Cache-SRAM.
you already installed the socket adaptors in step 1a, please go on with
Now the socket adaptor will be installed from the lower side of the
board. The pins have a thick and a thin end, the thick (and probably
shorter) end will have to be soldered into the board. If you insert the
adaptor, you will see that it can't be inserted completely because of
the 40pin socket, but the pins can stick out of the top side enough to
be soldered from there (This is the same as the high-profile version on
the 2364 adaptor page, look there for
pictures). So remove the 40pin socket
now and solder the socket adaptor from the top side (see the
yellow markers). It's a good
idea to solder the four corner pins first and check the alignment of
socket and if the 40 pin socket can be inserted far enough. When you
have completed soldering the socket adaptor, insert the 40 pin socket
again and finally solder it into place. Some pins may be hard to reach,
but it's possible without melting the socket. Believe me :-)
|The hardest part is already
done, the rest is easy: the remaining sockets must be installed now.
This is pretty straightforward, just insert and solder them. The
resistor package will also be soldered now, pay attention that Pin 1 is
|As the last soldering step, the
DIP switch and the capacitors will be soldered in. After this, control
the lower side of the PCBs carefully for short circuits that are caused
by using too much solder.
|Now insert the remaining
components into the sockets: The AT29C010A Flash-ROM and the 256kbit
SRAM. You can also insert the GAL and the 6502 CPU now.
the device is ready for use now!
If your device isn't working as it should, then there is some help
here. Of course, you should check for short circuits or forgotten
solder joints first, but there is also a special procedure that you can
use to do a basic test:
- remove all socketed components, i.e. the GAL, the RAM, the FlashROM.
- set all the DIP switches to the OFF position
- now put a wire into the GAL socket, connecting pins 10 and 16. This
will enable the 74HCT245 bus transceiver.
- if you have installed the 1541 patch wire, also connect pins 4 and
18. This will connect the A15 line of the CPU.
- now install the board into the CPU socket and plug the CPU into the
board. Turn on the device, it should work now.
If this test was successful, it means the board has no short circuits,
and the 74HCT245 is working. Now you can remove the test wires and
install the GAL chip instead. If the board stops working, then there is
a problem with this chip, or with the socket connections.
Next, install the Flash chip and try the board again. You should be
able to flash a ROM and then run your device from the Flash chip. If
this doesn't work, be sure to try all four possible banks. Maybe there
is only a problem with the 74HCT157 chip, which is connected to the
banking lines. If you have the possibility, you can program an EPROM
accordingly and use this instead of the Flash chip and see if at least
the reading works. A 27(C)256 or 27(C)512 can be used, probably the
If you have questions about this project, you can mail me: please
visit my contact page.