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Adding A Narrow ROM To the 1802 Olduino

June 12, 2017

The 1802 Olduino is based on the 1802 Membership Card by Lee Hart.  The board is quite full so in order to allow both ROM and RAM, Lee put a footprint for a .3″ wide memory chip under the .6″ wide footprint I use for non-volatile memory. I like the idea of adding a ROM so I can eventually get rid of the AVR on the olduino board and have the 1802 bootloader in ROM.  I have in mind to do a bit-bang serial xmodem loader but as a trial I’m going to burn the “fakeloader” into an EPROM so its gone off to Josh Bensadon who has a programmer that can handle the chip.

Because my NVRAM is .6″ wide, I have to put the ROM in the narrow socket.  Herb Johnson on the COSMAC mailing list was nice enough to let me have a couple of Cypress CY27C256s.  These are UV erasable EPROM so the pin layout is slightly different than the RAM that Lee was thinking of when he laid it out.  PIN 1 needs to go to 5V instead of A14, and pin 27 needs to go to A14 instead of /Write-Enable.

Boggling over the datasheets, the schematic, the actual board, and a copy of the board layout gerber files from Lee I’ve concluded that I can clip the traces leading to those pins under the board without hurting anything else and then jumper pin 1 of the inner chip across to its pin 28 and pin 27 over to pin 1 of the outside chip.  I’ve cut the traces and verified that the board still works but i’m waiting til I get the chip back from josh to do the jumpers. In the images below the segment of the board with the red Xs for the cuts is reversed from the gerber and the actual board because i found it easier to work with the top and bottom copper images if I was looking at both from the top.

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