Top secrets sources NedoPC ngs

zx ports:

$33 -- reset/init (r/w)
$3b -- check presence (r/w)
$b3 -- ROM addr (wo)
$bb -- ROM data (r/w)

ports description:

$33 -- reset/init/ctrl.
 write: $80 -- reset/init board
        $40 -- toggle LED
        $20 -- turn autoincrement ON
        $00 -- turn autoincrement OFF
 read: bit7: 1 when reset/init in progress, 0 otherwise

$3b: check presence.
 write: write value to mangle
 read: read mangled value

$b3: ROM addr.
 write: write address in 3 phases: low, middle, high. after any access to $bb port address phase is set back to low.

$bb: ROM data.
 write: write byte to ROM
 read: read byte from ROM

byte from ROM is read to the buffer, and ZX is shown with previous buffer value, so to read single byte after address setup,
 read $bb twice.

after any access to $bb ROM address gets incremented (if enabled in $33 reg!) so you can do burst transfers.