I finally identified the issued that made some CPM (non banked version) hang the system : the RAM bank switching system had a timing issue. More specifically, when the monitor initializes the D2/D3 ports (with 00 and 04 respectively) the second IO access was actually trigerring also a 04 write into the D2 port.
I solved the issue by replacing U25 74LS04 by a 74F04 but a little glitch on D2 port enable still appears that makes me a bit concern about the reliability :
I was wondering whether the problem would come from sOUT : for U39 I used a 74LS08 instead of a 74S08 thinking speeds would be the same and considering the CPU operates at 4Mhz ? Do you that would fix the problem ?
thanks in advance for your feedback
Philippe