If you open a recent Atari ST notice that on 68000 CPU is mounted a small PCB. Here is a modification whose goal is to filter the "Overshoot" of BGACK line.
Blitters Some are so sensitive to "overshoot" they causing "Reset" useless. This phenomenon arises, among others, when the printed SLM804 ~ through ScreenDump devil and programs. The following list blitters that require or not require modification
Code: Select all
Vendor Date of production Needed Modification
SGS before 8823 yes
SGS after 8822 no
National before 8908* yes
National after 8907 no
GE / RCA any date yes
IMP any date no
* I had a email from Juliusz saying a 8830 blitter also works ok without the patch. So could be possible Atari made a small mistake on data codes of "problem" blitters.
I'm assuming the 4 number code is YY/WW So 8823 becomes 1988 week 23. Going by a Blitter I have here, it is stamped 9106, so assume 1991 week 6 production and later blitter do not suffer from noise problem. So likely only early MEGA ST's off the production line need the patch.
So again noise on the bus is to blame for chip malfunctions. I guess a alternative method would be to just place some 100R resistors in series with lines which have "overshoot" and that would likely solve the problems without the need to "delay" the signal. It would need testing to prove it.
So do I need this modification? Well likely no, unless you have one of the older ICs which are known to have issues.
It is easy to remove the modification, just simply on solder the board. One thing to bear in mind is there is a track on the bottom of the motherboard PCB which is caught underneath the CPU. If you remove this little board, then you'll need to repair the PCB track.
Source of this research is below.
http://www.exxosforum.co.uk/atari/last/MEGAST/index.htm