TRS-80 Model 1 Expansion Interface question?

Eric Smith spacewar at
Mon Mar 20 23:17:07 CDT 2017

On Mon, Mar 20, 2017 at 6:46 PM, Win Heagy via cctech <cctech at
> wrote:

> The expansion interface hardware manual indicates
> it is an FD1771B-01, but the service manual indicates a couple
> possibilities....FD1771 A/B -01 -11.  Any considerations to look for here?

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
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.

More information about the cctalk mailing list