Friday, June 7, 2013

MKHBC-8-R1 homebrew 6502 computer - assembling CPU card.

Very slowly however consequently I move forward with the project. I finished (well, almost) my back plane and now I am ready to assemble my first two cards - CPU card (which is really a combination of CPU, address and I/O decoders, ROM and RAM) and the serial port/uart card.
Every engineering project should be well documented. I created circuit diagrams in Express SCH and component layout for CPU card in Express PCB.
I also created a design and specifications document which I update regularly.

(NOTE: You may want to download this document to your computer and view it locally with Open Office instead of the Word viewer provided by SkyDrive - for some reason it can not display pictures embedded in the document when viewed in the web viewer. Probably Open Office/Word format incompatibility issue.)

CPU card circuit diagram.
CPU card - EPROM.

CPU card - base RAM.

Some of the circuit elements are missing on the board, I will update the blog shortly as the work progresses.
It is worth the note that despite the fact that I use a multiple board back plane design, I decided to make the CPU card able to function in a single board mode (sort of). Additionally to the euro 64-pin male connector that goes into the back plane's slot, the CPU card will also have the IDE type 40-pin male connector on the opposite side (or on the top) from euro connector (J4 on the diagram). This 40-pin connector, or rather slot will be identical that I used for the I/O expansion bus slots. The connector's pins will be connected to the same signals on CPU card that go into the I/O expansion bus (will be compatible with I/O expansion bus pin-out and IOSEL signal will go directly to one of the /IO0../IO7 pins on CPU card, perhaps via some jumper selector to allow configuration choices) thus creating ability to connect the I/O card (e.g: serial port card, general purpose I/O card etc.) directly to the CPU board. The euro connector will be used in the SBC mode to attach the CPU card to the control panel board with power supply, control buttons, LCD, generic I/O etc. - we can call it a simplified back plane, with one 64-pin slot only (luckily I have one spare). The details are included in the design and specifications document mentioned earlier. When inserted to back plane, the J4 slot on the CPU card will not be used. This design may require some creative programming so the firmware would not require to be updated if I move between back plane and SBC modes (auto detection of some sort).
I made some layout work already on the CPU card and soldered the IC sockets and J4 connector into the board.

CPU card - PCB layout.
The actual CPU card board in its current building phase.

The back of the CPU card - there will be a lot more soldered points and wires here soon.

CPU card sitting in the back plane's CPU bus slot.
Since the back-plane is almost finished, I include the circuit diagram of CPU and I/O expansion buses below as well.

Back plane - CPU bus.
Back plane - I/O expansion bus.
Thank you for visiting my blog.