About the interrupt triggering. Devices that are not im2 aware should not drive any value on the data bus during an intack cycle. Instead they should rely on the expansion bus pullups to ensure a 0xff value is read. Doing otherwise would be incompatible with devices that are im2 aware. The zx next will be an im2 aware computer and there are original spectrum peripherals that operated in the im2 mode, eg the amx mouse. In general, any device using standard zilog peripheral ics can be im2 enabled.