First Modules

Posted by Joe George at

At my second job out of school, I worked for Xylogics designing disk controllers. Now days a disk controller is a single chip in its biggest form (raid controller, etc.) and generally a small piece of a SOC or I/O controller. When I started at Xylogics it was to work on a single board disk controller. At the time the companies revenue came from a disk controller for DEC PDP-11 computers that required 4 boards in their own backplane. The first product I designed was a controller for QBus designed on a single board that was 1/3 the size of their existing controller boards. The reason it could be designed so much smaller was it used a bit slice microprocessor. The picture below is of a single board Xylogic disk controller that I didn't work on but will give you and idea of what I am talking about. (Thanks to Robert Harker who licensed it under the Creative Commons Attribution-ShareAlike 2.5 License.).

At that time a circuit board layout was laid out. You would start with a large 4:1 piece of acetate (film) for each layer of the board. These would be placed on top of each other on a light table and the top edge would be taped down. The pads and traces of the board would then be laid down using black tape and stickers with pads drawn on them. So I drew the schematics for the board on velum with a pencil and made serious use of an electric eraser. Dave then took my schematics and laid the board out with tape and stickers. If Dave made a mistake or I made a change he would have to pull up the tape and stickers and replace them. It was a challenging art and took a lot of time. When we were done we would send the layout to a photo shop and they would photographically reduce the layout to a 1:1 mask so we could send it to a board vendor. Dave was a very clever layout guy so he used photographic methods to layout boards more quickly. He would have custom stickers made so he could do a copy and paste function.

After learning how Dave did things I had an idea to design disk controllers using modules. We designed a bit slice processor that could be used for all the disk controllers. For each of the different computer buses we designed a module. We also designed modules for different disk and tape drive families. Dave then laid each of the pieces out and photographically put films together for each controller using the three appropriate modules. He then taped all the connections between the modules. We were able to design and fabricate printed circuit boards for a number of controllers in record time. Okay, getting the firmware done for each of the controllers didn't go so quickly but it worked great for the hardware.

Back in those days it was a lot more difficult to use modules. It is a lot easier today with all the wonderful tools we have.

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