info-mac@uw-beaver.UUCP (07/21/84)
From: Randy Frank <FRANK@UTAH-20.ARPA> the following comes from a friend at Apple. It is passed along without further comment: Upgrading a Macintosh to 512K Bytes is rather more complicated, at this time, than replacing 64K Dynamic Ram chips with 256K Dynamic Ram chips. In addition to swapping the RAM chips, at your own risk, of course, you have to add a multiplexer in a very strange way. The current Macintosh digital boards have a place for a seven-pin single in-line header. If the proper combination of a multiplexer chip of the right speed and a number of pull-up resistors is placed on a miniature daughter board with a single in-line header, and then 256K Dynamic Rams replace the 64K Dynamic RAMs, you would have a 512K Macintosh! The ROMs are smart enough to explore the machine to see what kind it is, and configure RAM appropriately. You will be much much better off to wait for the Apple-label upgrade kit, which will come with a warrantee, surely. Upgrading a Macintosh to 512K Bytes by adding three banks of 64K Dynamic RAM is extremely unwise electrical engineering practice, unless timing margins are very carefully examined. The RAM address line drivers are designed ONLY to drive the capacitive load of the RAM address bus lines on the Macintosh digital board combined with the capacitive load of the address receivers on all RAM chips combined. Adding the wiring and the RAMs to connect three more banks of 64K DRAM would throw the load estimates, the timing, and the whole design way out of kilter. -------