This circuit provides the means to add up to 256 additional I/O ports to and Elf computer. Two of the original N ports are used to access the new ports.
Port 5 is used to select an extended port. Port 6 is then used to read/write the extended port.
|MRD||MRD signal from the 1802 processor|
|N0-N1||These are the N lines from the 1802|
|D0-D7||These are the data bus lines from the 1802|
|INP||This signal will be low when the cpu performs any INP instruction|
|OUT||This signal will be low when the cpu performs any OUT instruction|
|PORT1-PORT7||These are active low signals that are decoded from the N lines|
|IO0-IO7||These are the extended I/O address lines|