Needed: 1 IBM 8" alignment disk.

Please see embedded remarks below. Dick -----Original Message----- From: Tony Duell <ard(a)p850ug1.demon.co.uk> To: Discussion re-collecting of classic computers <classiccmp(a)u.washington.edu> Date: Tuesday, November 30, 1999 2:16 PM Subject: Re: Needed: 1 IBM 8" alignment disk. It's not as odd as you may think. The CRC is for the ID field, so it's got to keep trying to read the ID before it can determine whether it should read the following data field. The controller doesn't have a fixed number of retries, since the infinite loop, in the designers' opinion, allowed you to poke around with your instruments looking for the problem, and is probably as good as any other alternative. Remember, there's a CRC field after the ID field, AND one after the data field. They both must pass check before you get a valid sector. This behavior is, however, quite consistent with a misadjusted PLL, or whatever other mechanism they're using to extract the clock e.g. one-shots. If someone has fiddled with the pots on the device, all bets are off as to its ability to lock on the data. Since the data is synchronized differently on every sector, it should be easy enough to determine whether lock occurs just by triggering on the data stream while observing the PLL's oscillator. Each sector should show a discrete and easily visible acquisition window. Of course, that won't be the case with a naked but formatted diskette, since its data and ID fields were written in one fell swoop, with no asynchronism between sectors. However, if you format a new diskette on some OTHER and WORKING compatible machine, writing a file to it will be a sector-by-sector operation which will ostensibly have at least a short transient of phase change reflecting the difference in phase between the relative position of the diskette during formatting and during writing not to mention the minor speed variations. In any case, you should, with your oscilloscope set up properly, be able to watch a consisten acquisition of the data for each sector. If you're not getting that, which is what I suspect, then your error lies in that, the most basic task of a disk interface, without which NOTHING else having to do with data transfer will work properly.