Eprom switcher

[Peter Rueb]

Hi,
I promised to make some suggestions to improve the eprom switcher. Here
they are.
The main problem is to make shure that the adress data does not become
instabil while there is an access to the eprom. The result may be only a
hickup or even worse. My idea now is to allow changings to the two relevant
adress lines only when there is no access.
The parts are a 7474 (double flipflop) and a bunch of or-gatters. Connect
each two normal adress line to a or-gatter and connect all gatter outputs
together to the Clock line of the 7474.  On the input sides of the 7474 are
the two switches with their 4.7k pulldown resistors. The outputs of the
7474 go to the relevant adress lines of the eprom. The preset and reset
lines are always high.
My thought is, that when the adress status changes from low to high there
will soon be an access. At that moment the status of the switches is added
to the other lines and does what it shall do. I also thought of connecting
Clock with inverter direct to the active low OE of the eprom, because the
7474 has a very short delay of around 15ns, but maybe that is a bit to
late. (Maybe someone wants to test it with a fast osci; i have none) It is
still important to use bounceless switches (scsi id-selector) and program
codes that are only different in the table section. If you use totally
different eprom contents even that wont work.
Just some thoughts, every improvement welcome. Maybe there is another way
to get rid of the or-gatters.
Peter Rueb
s68558 at stud-mail.uni-wuerzburg.de