Document "*EXT.DTK" signal properly.

PDS slot pinouts have now been verified, remove TODO note.

hardware/fpga/bbu/README.md

@@ -100,12 +100,20 @@ only simple, single-pin interfaces.
 
 Peripheral device signals, input or output?
 
-* `*ENTD1K` is very likely an input signal, for PDS use.  Why?  The
+* `*EXT.DTK` is very likely an input signal, for PDS use.  Why?  The
   Macintosh Classic is essentially a stripped-down Macintosh SE that
   uses the same BBU.  In that schematic, pin 11 is indicated as
   connected to a pull-up resistor.  So, clearly this must be an input
   since it cannot be connected.
 
+  Actually, the full purpose is documented right here.  If this pin is
+  pulled low, then the system expansion card is responsible for
+  generating the `*DTACK` signal to indicate to the CPU that the data
+  transfer is complete.  The BBU puts the signal in a high-impedance
+  state.
+
+  20200808/https://web.archive.org/web/20190909060927/http://www.ccadams.org/se/pinouts.html
+
 * `*EAREN` is very likely an output signal (also for PDS use), for the
   reason it is not indicated in the Bomarc Macintosh Classic
   schematics, i.e. it could be disconnected entirely.

old_artifacts/schems/README.md

@@ -22,11 +22,8 @@ included to reproduce the processing steps.
 
 Notes related to the accuracy of the retrace.
 
-* The pinouts of all integrated circuit components and the RAM SIMMs
-  have been verified with second sources.
-
-* The pinouts of the PDS slot have **not** yet been verified with
-  second sources.
+* The pinouts of all integrated circuit components, the RAM SIMMs, and
+  the PDS slot have been verified with second sources.
 
 * The resistor values have been checked to be consistent with the
   reference designators, but **not all reference designators** have

old_artifacts/schems/retrace_se_mlb_p1.svg

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