The Z80H by Zilog is a high speed Z80 family, pin compatible
processor. It can be used in place of the Z80,A ,B series with which
it is identical as to function. It can provide at minimum a 10% in-
crease in throughput on most CP/M based motherboards. The chip was
designed to run at 8 to 10 Mhz, with selected and verified chips able
to run at 11 to 14 Mhz (the higher speeds at some degradation to
reliability). The Z80H is a drop in part for all applications that use
the Z80 family In use the Z80H can just replace your processor, but
for the best results you can switch the clock crystal on the board for
one of a higher speed. It is advisable not to go to the highest speed
at once for the support TTL chips may not be rated for the highest
speed at which the Z80H will run. A good idea would to remove the
clock crystal and install a a socket so that you could change the
crystal as you check the reliability at the higher speed. Remember to
check the clock divisor and get crystal's that are correct (If your
board is running at 4 Meg and the clock divisor is four your crystal
is a 16 Mhz a good replacement would be a 24 Mhz crystal which works
out to 6 Meg, a 32 Mhz for 8 Meg, etc. If you have problems with
dropout and lost bits you probably will have to change the TTL chips
on the board for the H versions, such as 74HS14 etc. You may have to
do quite a bit of work and you may still not get a reliable board. One
of the other problems that may occur is that the traces on your board
are too long (large size motherboards) even though electricity travels
at 186,000 mps at the higher clock speeds its a little hairy, and you
also run the risk of higher RF interference. Try one of the Z80H's
without a crystal first, you WILL see a difference without having to
go to the Z280 or HD64180 boards.
