Z80H

       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. 
