6264s are 8K byte static ROM chips. OK, let me explain a bit more. A hard disk simply records a bitstream on a track of the platter. It has no conscept of sectors, checksums, things like that. But the host ssytem deals with bytes/words. It expects to be able to read a particular secotor from the current track, it expects to be told if there's a checksum error. I call the first the 'raw' data, the second the 'formatted' data. At some point there must be a conversion from one to the other. With some drive units (e.g. IDE, SCSI), the conversion is done in circuitry that's normally considered to be part of the drive, the interface is a formatted one. Withe others, for example the ST412 ('MFM' in PC-speak) drives, the interface is a raw one, what you get on the interface connector is a cleaned up version of the bitstream from the head. The conversion to formatted data is done on the controller card. Now, the OMTI chips you've mentioned would seem to be the chips to convert between a raw and a formatted interface. For that reason, I suspect the 50 pin bus is a formatted interface (to the host computer), the 26 pin connector is a raw inteface carrying the cleaned up version of the bitstream from the head. Unfortunately, while these OMTI chips were commonly used on SCSI-ST412 interfaces, there's nothing in the _chips_ that implies a SCSI interface. That was handled by the microcontroller program (you have a Z8 on your board I believe). So there's no reason to assume that the formatted interface (the 50 pin connector) is SCSI. And while there's probably an ST412-like data stream on the 26 pin connector, there's no reason to assume the control lines (e.g. to position the heads, select a particular head, etc) are identical to ST412 signals. Again, that side of things is handled by the microcontroller. -tony