I certainly have that information for the 5150, it's in the Techref (I have a shelf of said manuals, I may not _like_ PCs, but at least I am going to have the information on them :-)). A 0 bit consists of 1 cycle taking a total of 500us (that's 2kHz I think). A 1 is 1 cycle taking 1000us (1kHz). The data record consists of : Leader (256 bytes of all 1's) Sync bit (a single 0 bit) Sync byte (Hex 16) Data block (256 bytes) CRC (2 bytes) Data block (256 bytes) CRC (2 bytes) etc... The CRC polynomial is the usual one X^16+X^12+X^5+1 I don't feel like typing in the relevant bits of BIOS source code, though :-) -tony