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.