What's the difference between ISA 430 MK I and II?

The 430 and 430 MkII are sonically very similar. The pre-amp, compressor, EQ, gate and de-esser circuits use the same designs. The elements in the MkII which will give a different sonic result are the vintage opto compressor or VCA limiter modes (instead of VCA compressor) and the switchable input impedance (between high, med, ISA 110 (same as 430 Mk1), and low). Other things added for the MkII:

Mic and line coarse gain on rotary switch instead of pots
'Air' switch which induces inductor circuit for an open sounding high end
VU meter calibration switchable between +4dBu and +18dBu
Compressor 'Blend' control allowing you to mix between compressed and uncompressed signal
Extra insert point, allowing EQ and dynamics to be split

The ISA 430 MkI uses the same digital card as the ISA 220 and it is still available in the shops. For the 430 MkII a new digital card was designed and it will not work with the MkI.

The MkI digital board outputs AES on a balanced XLR connector and S/PDIF on an RCA phono jack and also on an optical connector.
The MkII card supports 176.4 and 192KHz sampling rate which the original card did not and the jitter figure and noise performance are slightly improved. Its outputs are different too - a 9 pin D-sub connector outputs either AES or S/PIDIF and allows you to route a digital signal to a maximum of four different places. There is also ADAT 8 channel optical out and S/PDIF 2 channel optical out. Both cards have word clock in and out connectors and support Digidesign 256x superclock.