Engineering of the Gameboy Extender
This is version two of the Gameboy Extender board. You can read about version one by selecting the topic in
the navigator frame on the right.
Several things went wrong when trying to get me the GB extender board version one:
On the left, you see the new circuit drawing for the Gameboy extender card II, codename GBexx. GBexx is easier
to draw as a circuit because 'things happen' on it.
On the far left, is the 32 pin GB edge connector. It is named J1. On the far right is the 34 pin boxed header style connector (for ribbon cable) called J2. The lines betwen the connector pins are the connections. Easy.
The three integrated circuits are 74HC245 bus buffers. Again I chose for three identical IC's since these are
easier to obtain and probably cheaper (than a set of 74HC541's) as well.
The address bus buffers are 'upside down' in the circuit. This was necessary to let have pins 1 (DIR) and 19 (/OE) the same logic level input (both ZERO).
The address bus buffers are easy to understand: just straight through. They are just dumb amplifiers.
The databus buffers require some intelligence: they may only work when we are in Userspace (lower 32 KB of memory, when A15 = 0) and they may only send data to the CPU when asked to do so. Therefore the /RD signal controls data direction.
We can pick the A15 signal directly from the buffered signal. The /RD signal will take some more cunning routing, but not too difficult.
At the top of the drawing are 6 inverters of type 74HC14. These six are placed two in series in order to keep the polarity of the original signal. I would have preferred to use a 74HC243 buffer here, but that was not available in SMD. So I had to find another way and this seems to be the best, for me.
If you click the picture a new window will open with a larger version.
The printed circuit board
I managed to route the printed circuit board. It fits on a plate, approximately 55 mm wide and 80 mm long. This is slightly longer than required but it will also ensure that there is some room to spare for the connections with the system board.
The board has a 32 pin edge connector on the far left and a 34 pin boxed header (it mates with a standard
floppy disk cable) on the right. In between are the interconnects and the busbuffers.
Due to the limited headroom and width of the PCB, the busbuffers are SMD devices. To prevent going double sided, 6 wire bridges (the coloured lines) were necessary. But everything now fits neatly.
Below you see what it looks like. The drawing is not perfect, but you get the picture.
The checkered structures are copper planes connected to +5 Volts and Ground to minimize EMC radiation effects and to minimize losses in the power lines.
The board, a screenshot
On the left you see a real screenshot of what the board looks like. This is a picture taken with my digital camera from the CRT while it ran PADS for DOS. That's why the curves are in the picture.
In the picture you see the three IC's, the 6 wire bridges (vertical blue lines) and the copper area's (green). The pads and connectors are in purple.
If you click the picture, a full screen screenshot of the PADS environment is visible. Try and see if you like it. I still do, after a lot of years.
The board, after it came from Bellmann
Today, three weeks after I ordered my first lot at Bellmann PCB shop the boards arrived. A nice stack of well fabricated PCB's. On the left you see a picture of the board. I guess the photo flash ruined the picture again so I will see if I can fix it in the daylight, tomorrow.
On the left is the GBC connector. On the right is the 34 pin boxed header, we all know of 3.5" floppy disk
drives. In between are the busbuffers.
The board is made according to the new ROHS standard enforced by the EC. There is no lead (Pb) in the board anymore. So the tin is extra shiny. The GBC connector looks like it is gold plated, but it isn't. The flash ruined this as well.
Fitting the board inside a GBC
When designing the board, there were trade-offs, as usual. I chose to use the maximum width for the PCB. This would assure the best possible fit. For the GBC this worked out. For the GBA the boards are a fraction too wide. But with a piece of sandpaper this isn't much of a problem.
Another engineering challenge was the thickness of the board. Normal PCB's are 1.6 mm thick. The GBC carts are 2.2 mm thick. So I need to increase the thickness of the PCB in order to let the contacts mate with the etched connector on the PCB. I did this by attaching a piece of 0.5 mm aluminium to the bottom of the board by means of a piece of doublse sided adhesive tape. The kind used for glueing rugs to the floor. That settled things. The contacts now have a really positive grip with the edge connector.
But the aligning slot of the GBC connector will not fit a PCB that is thicker than 1.7 mm. You can see what I mean in the picture above left. Above right shows how the side of the connector looks like. This narrow slot is something like 1.7 mm wide.
In the above left picture you see a small ridge on the far right side of the GB connector. Normal carts 'embrace' that ridge with cutouts in their cart-case. So I had two options:
A quick test
After I adapted the board thickness and the GB connector, I just needed urgently to see how it worked. So I inserted two batteries in my prepared GBC and inserted the GBexx card. After switching on, there ought to be +5 Volts on the edge connector. So I pulled out the (t)rusty old Fluke 8020A and probed around on the board.
At first, I couldn't find the 5 Volts (although it is at pins 1 and 2) but at the second attempt, I measured 5 Volts! That means:
Page created January 2006,