Dual BIOS. By Lee Davison.

The problem with writing software to update a flash BIOS is that if it doesn't work then you have to pull the flash chip, reprogram it in the programmer, then try again. This gets a little tedious after a while.

Some PC motherboards come with a dual BIOS so that if one gets corrupted, say during an unsuccessfull BIOS upgrade, the machine can boot using the other and fix the first one. So I decided to make a dual BIOS adapter for the EPAC.

The circuit is fairly simple and a suggested board layout, seen from above, is shown here. All the pins of the header and the two sockets are common except for the chip select signal pins. For the sockets this pin is connected via a 4K7 resistor to the +5v on pin 32 and to one side of the BIOS select switch. Pin 22 on the header goes to the common on the BIOS select switch.

The red outlines are those of the sockets, the blue outline is of the header. The two pullup resistors connect between R1 - R1 and R2 - R2. The select switch connects to A, B and C with the common going to B.

The circuit was built on a piece of stripboard not much bigger than two 32 pin devices side by side. The strips were cut where needed then the header was soldered onto the bottom of the board. On top of the board the two 1K pullup resistors were positioned then the two 32 pin sockets were placed on top of them and everything soldered into place. The leads for the select switch were connected then finally the 31 common pins between the two sockets were connected with short insulated wires.

While the circuit is simple mine didn't work first go, but it turned out that it wasn't my fault.

Just plug the adapter into the BIOS socket and put the BIOS chip(s) in either socket, the switch is used to select which chip is accessible and can be switched any time the BIOS isn't in use, e.g. when running the BIOS update software.

There's no reason this board couldn't be used in other systems apart from the EPAC. Any motherboard that has a 32 pin DIL BIOS could be made dual BIOS with this.

Last page update: 16th March, 2006.

e-mail me