(18-04-2022, 02:10 PM)someone Wrote: When using larger capacity FLASH ROMs on a 8238 coreboard:Having done a lot of work around the idea of one ROM containing all the PAK ROM's myself, I will add a couple of things to watch out for if anyone decides to give this a go.
one can remove IC31 74XX138 3-8 decoder
the disconnection the related gates to select the standard Wordbee ROM
remove the standard Wordbee ROM
connect the FLASH ROM's higher order address lines (e.g. A13, A14, A15) directly to IC28 4042's quad clock D latch outputs.
restore the Wordbee ROM functionality to newly installed flash (by burning it into the appropriate Flash ROM bank).
Even if you don't go for a Flash ROM and just go for a Single large ROM to replace the many.
1) Rather than isolating and redirecting the !CE signal, fit a 4K7 resistor between Pin15 and Pin 4 of the previously vacated IC 31 (74XX138) location. This holds the input of the OR gate low preventing it from floating and the !CE to the new ROM will happen whenever !C000h is selected.
2) IC 15 (Wordbee) Pin 26 has a +5V track going to it, needs to be cut prior to connecting the A13 signal from the 4042 Pin 2 output.
3) IC 15 (Wordbee) Pin 1 also has the +5V track going to it, so PIN 1 needs to be isolated from this track before the A15 signal from the 4042's Pin 11 output, however track integrity is still required to maintain +5V distribution. A simple cut either side of the pin and a bypass jumper will suffice.
Not having gone down the track of In-circuit programming of the Microbee ROM myself. A possible thought, was to place the program for doing this in the upper bank of a 27128 NET ROM (Bank 3) IC20, which to my understanding has never had an official program released to occupy this bank location. The decoding is already there on the MB8328 Rev 6 board & should be straight forward to add onto the earlier board with very little alterations.
Ernest