TRS-80 Model 1 Expansion Interface question?

On Mon, Mar 20, 2017 at 6:46 PM, Win Heagy via cctech <cctech at classiccmp.org All other things being equal, I'd use the FD1771x-01, with x being either A or B. The A vs. B is just whether the chip is in a ceramic (A) or plastic (B) package. This makes no difference in the TRS-80 EI. The numeric suffix indicates functional or specification differences. The -01 suffix is rated for operation at 1 MHz or 2 MHz, which is fine for the TRS-80 EI, which runs it at 1 MHz (as required for 5.25-inch standard-density floppies). The -02 suffix is rated for operation at 2 MHz only, so in principle it isn't guaranteed to work in the TRS-80 EI, but in practice it should work fine. The -11 suffix is for a part that has an internal substrate bias generator, so pin 1 (Vbb) should be disconnected. This wouldn't meet spec in the TRS-80 EI without disconnecting pin 1, though it's possible that it might work OK without doing so. At 1 MHz, the available stepping rate selections for the -01 and -11 are 12, 20, and 40 ms, while for the -02 they are 12, 16, and 20 ms.