What’s the difference between a 64k ROM in a 28-pin DIP and a 128k ROM in a 32-pin DIP? Besides the obvious answers of “64k” and “four pins,” it turns out that these two chips have a lot in common, enough so that with a few modifications they can be interchangeable — more or less.
In a video, [Anders Nielsen] demonstrates using the SST39SF010, a Flash ROM in a 32-pin DIP, as a substitute for the W27C512, an EEPROM in a 28-pin DIP. By examining the data sheets, he discovered that due to JEDEC standards, the pinouts of the two chips are nearly identical. The only real difference is the position of Vcc and the presence of a 16th address bus line on the Flash ROM, which has greater capacity.
In order to make the 32-pin chip work in a 28-pin socket, [Anders] accepted losing the upper half of the Flash chip’s capacity and began making modifications. This involved creating a jumper from pin 32 to pin 30 on the Flash chip to correctly position Vcc, and adding a couple of pull-up resistors for write-enable and A16. These changes were relatively straightforward. However, [Anders] encountered challenges due to using a Flash ROM with heavily oxidized pins, leading to cold solder joints and intermittent issues during testing. Additionally, not all boards have space for overhanging pins, but this was resolved by adding a socket to create vertical clearance.
This clever workaround makes it easier to use a different chip for the job. To see where [Anders] is using these chips, you can check out his 6502 in an Arduino footprint or the bring-up of an old XT motherboard.
Image Source: Hsyn20 @ shutterstock