Z80 Mark III (continued)

This is the Z80 Mark III RAM/EPROM board. It contains two 2K byte RAM chips plus three 4K byte EPROM chips for a total of 16K bytes. This memory is hard-wired to be accessed in the address range C000 to FFFF.

At the top are the two 24-pin 2K byte RAM chips. Below them are the three 24-pin 4K EPROM chips. These can be erased my shining high-frequency ultraviolet light for several minutes into the quartz window in the centre of the chip. The third chip’s quartz window has been covered to prevent unintended erasure. It is labeled “System” since it contains the operating system. The other two EPROMs contain FORTH and other programmes.

The components in the top right and the red wire form an “EPROM blaster” circuit. By connecting the red wire to 25 volts and sending the required write instructions, data can be written to the EPROMs. Above each EPROM chip is a transistor that switches this higher voltage.

The other chips are buffer and support chips.