Official ZX Interface (ZXI ) Standard

The ZXI™ (ZX Interface) standard is for devices that are attached to the Spectrum edge connector. ZXI™ compliant devices are inter-operable with each other without conflicts. Certified devices may display the logo on their case, in their manuals, and on-screen. Background The Z80 provides 16-bit I/O decoding, so in theory the Spectrum has 65536 ports available for hardware devices. In practice the Spectrum does not fully decode the I/O and there are no spare I/O ports available that do not clash with at least one existing peripheral. For example, the ULA alone uses 32768 ports (every even port...