A very simple board designed to allow two different BIOS ROMs to be installed in the Osborne 1 computer. It can also be used anywhere that a 2732 (4KByte) EPROM needs to be replaced with a 2764 (8KByte) EPROM.

I could've made the board a little bit smaller by using a surface-mount pull-up resistor but I opted for simplicity for everyone by using all though-hole parts.

Background why I designed this board:

The standard BIOS ROM for the Osborne 1 is a 4KByte 2732 which is 24 pins.

Using an 8KByte 2764 EPROM allows one to have two different BIOS installed (i.e. official Osborne and 3rd party OZROM) that can be switched via the A12 address pin.

Alternatively, it allows you to replace an older BIOS using a 2764 EPROM if you can't program or source 2732 EPROMs.

By default the A12 address pin is pulled HIGH by the 10KΩ resistor (so default is upper ROM address range 0x1000 to 0x1FFF) or can be switched to LOW via a strapping or SPST switch connected to the A12 pin header (so lower ROM address range 0x0000 to 0x0FFF). This grounds the A12 address line.

The sample picture shows the 2764 EPROM soldered directly the board. You can use an IC socket instead if you have room (there is room in the Osborne 1) and this was how I originally tested it. Once I confirmed it was working I created a second one soldered directly to the board so it looks more "slimline".