Emailing: ST506 and-or ST412 hard drives.htm

From: lynchaj at yahoo.com

Hi, Yes, I'll admit this can be a bit confusing and I am bit puzzled by it as well. I had not heard of anything like it either but I have to believe the VG engineer to know what he is talking about. However, I have heard from a former Vector Graphic engineer who is intimately familiar with the VEDMCS (aka, the integrated FD/HD controller) and he tells me that the ST506's used for the VEDMCS are different than the generic "off the shelf" ST506 hard drives. I believe the stock ST506 is modified or configured to provide the signal in some manner. The different signal is called a "constant index" signal. Apparently, it is similar to an /INDEX signal from a floppy drive interface but the signal tells the HD controller everytime sector 0 passes around. The HD controller requires it for setting up the PLL. Here is an excerpt on the subject from the VG engineer: " All hard drives working with a Vector FD/HD must have constant index. There is a phase locked loop that is controlled by U20 which is CMOS 4040 chip and unless you have constant index the PLL never has time to sync up and gives read errors. " I have seen the VEDMCS driver source code and it specifically lists the ST506 and the ST412 as its only two supported drives. I'd like to start witho one of those and see if I can get it work. Maybe I could convince it to accept a ST225 instead of a ST412 but so far I haven't had any luck. I keep getting "drive not ready" errors. I hope this helps explain this situation.

Thanks! Andrew Lynch