Updating Bios on FM Towns CX20

[Jehuty]

On my Question what A+B does i got the answer that it bring a Little bootmenu.
And Reset+T brings the Fast Mode with that littel Yellow man.

Not on my both CX20 FM Towns.

I did a Little Research with UNZ and found out that there are several SYS Rom Versions, with and without the bootmenu / yellow runner.

After Looking  on the Mainboard there is a sharp IC called LH53807Z. While in my Amiga Computers sometimes the Kickstart is in Sharp ICs i tried to read it and yes in that IC is the SYS, FNT and Dos Rom.
FNT and DOS Rom are the same as the ones for UNZ.
The SYS Rom on my FM Towns is V04 - on UNZ i got a V03 (without) and V10 (with bootmenu).

So next i tried to Exchange the sys rom but it doesn´t work.
The FM Towns brings a error message in japanese (what else?) that says (on Google translator) something like "can not start because the free-run timer function is not working" or something similar.
To check if my Rom burnning is correct i tried Burning a Rom without changes an that worked like the original.

So it seems that the sys Rom is missing some Hardware (Timer?) in my FM Towns.

Anyone an Idea what this could be ?
Or if there is a working sysrom newer than my V04 for CX20 ? Could i try the UX Rom ?
Is there a shop who still sells updates ?

[famiac]

nice work. Do you have the datasheet for the Sharp chip? all i found is LH538000. How are you reprogramming it? I can provide my roms. I guess there's also a tool 'rombeger' i can use to dump my roms.

[Jehuty]

No i don´t find anything about the Sharp IC. So i measured where VSS and VCC is on the chip and figured out that this is like the 27C400/800/160.
So i gave it a try and read as 27c160 all 4 Banks. I got 4 files each 512kb but 2 where the same. So the IC could be a 27C800 with 2 banks. I read them and splittet the first 512kb file into two 256kb.
and see the first was identical with FNT Rom, the 2nd was nearly identical with sys rom and the 2nd 512kb file from bank 2 is identical with dos rom.
I compared the sysrom and in that there is a Vxx info so my FM Towns got V04 and that bootmenu is in at v06 and v10
Meanwhile i got the info that Freerun timer is sonething that exsist since UG Models. So i think i doesn´t have a chance to get a sysrom running with the bootmenu and that little yellow man until my FM Towns modell has no Freerun timer.

Last question for me is - where are F20 and DIC rom stored ?

[famiac]

thanks for that info. the pinout from the SHARP LH538000 mask rom matches the 27c800 pinout exactly.

i am wondering if we can modify the contents of the dosrom by rebuilding the reserved region and using it as a virtual FAT hard drive. maybe there's an easier solution

[Cyothevile]

Your post has me interested in possibly upgrading my BIOS on UX20 to the BIOS of the UG20. Theoretically could achieve fast mode on this PC and cut wait states. It will be in the back of my mind. Fortunately I took some photos of UX20 during disassembly but not very detailed and zoomed in and I would have to dismantle the PC and potentially run a LOT of extender cables.
Floppy emulator PCB and alternative tower PSU is my priority though.