The storage module uses an Arduino Nano to access a Micro SD Card module and communicates with the Z80 via Serial Comms provided by a 16C550 chip. I chose this method because it would be very memory consuming to have the Z80 access the SD Card Module directly via SPI. The Serial communication also uses RTS/CTS signals on the 16c550 I.C. On Arduino a software serial port is used on pins D2 and D3. This module needs a ChipSelect signal so you need to connect this to the device module through a Spare or extended pin (Yellow jumper cable) or you can make your own out address decoder to connect it.
