Tarbell is making me insane

On 13 Nov 2007 at 22:57, Grant Stockly wrote: Wait one darned second. There's a lot of confusion here--many vendors refer to the *data rate*, not the actual clock. Since the 1771 is an FM-only chip, that translates to 2 cells per bit time, or a 250 KHz clock rate. The frequency of the clock at pin 24 of the 1771 should be 1MHz. If it's 2MHz, it's running at the 8" rate (500KHz = data rate of 250KHz). I don't know which Tarbell card you've got, but if mine is anything like yours, Don takes a crystal at 2x the required clock and divides it down by 2 with a flip-flop (the 1771 requires a 50% duty cycle clock and is not very forgiving). On my controller, Don uses a couple of LS161s and some gates to form a data separator. I'm not thrilled with it--the bit shift on the inner tracks of your 3.5" drive are probably on the edge of causing it to lose sync. You might want to run some pattern-generating diagnostic tests to see if anything really pushes it over the edge. You can get around this by limiting your use of the inner cylinders (just tell CP/M that you've got, oh, 70 tracks instead of 80) or you can junk the onboard separator and replace it with a somewhat more modern WD9216 8-pin DIP. Let me know if any this doesn't make sense. Cheers, Chuck