Update README for ASCII interface

hardware/interface-ascii/README.md

@@ -16,9 +16,9 @@ even slightly simplify the hardware layer of the firmware.
 ## Features
 - Parallel or serial output
 - Up to 8 configuration settings via an up-to-8 position DIP switch
-- Apple 1 and Apple 2 compatible outputs.  Other configurations can be supported by
+- Apple 1, Apple 2, and SOL-20 compatible outputs. Other configurations can be
-  making a custom cable.
+  supported by making a custom cable.
-- Can decode arbitrary keyboards up 16 rows by 8 columns by making a custom keyboard cable.
+- Can decode arbitrary keyboards up 16 rows by 8 columns.
 - Supports up to 3 keyboard LEDs
 - Supports up to 3 "special" host outputs, such as RESET, SCREEN_CLEAR, BREAK, etc.
 
@@ -27,12 +27,14 @@ even slightly simplify the hardware layer of the firmware.
   lines to drive 16 rows.
 - The columns are read in via an 8-bit shift register, controlled by 3 GPIO lines.
 - One 8-bit port is used for the parallel ASCII output.
-- Two GPIO lines are used to generate special outputs to the host. These could
+- Three GPIO lines are used to generate special outputs to the host. These could
-  be RESET, BREAK, CLEAR, etc.
+  be RESET, BREAK, CLEAR, etc. Two of these may be configured as emulated
-- One GPIO line is used to control a keyboard LED.
+  open-collector drivers.
-- Three of the ISP lines are also used as outputs to controll two more Keyboard
+- Three GPIO lines are used to control keyboard LEDs. 
-  LEDs and one more "special" output line.
+
-- The DIP switch is wired into row 15 (last row) of the matrix.
+- The DIP switch is wired into row 15 (last row) of the matrix. In the future,
+the DIP switch will be moved to row 9 to reduce RAM usage and speed up key scanning.
+
 - The top two parallel I/O bits can also be configured as UART I/O. This could
 be used to provide serial output instead or parallel output, to support a
 bootloader, or even to accept a serial input stream from another computer to
@@ -44,7 +46,8 @@ careful timing to avoid conflicts.
 The minimum functioning circuit includes 
 1. the Microcontroller (U1)
 1. the 74LS166 shift register (U2)
-1. one 74LS138 decoder (U3).
+1. one 74LS138 decoder (U3). This means no DIP switch, so keymap must be
+   selected at firmware build.
 1. The resistor network RN1
 1. The two resistors R3 and R4. If you are using the keyboard in only a parallel
 configuration or only a serial configuration, then you can jumper these
@@ -53,31 +56,32 @@ resistors with a piece of wire.
 
 ## Optional components
 ### Diodes D1-D16
-These diodes are intended to mitigate ghosting when used with a keyboard matrix
+These diodes are intended to prevent conflicts between high and low keyboard
-that does not have any diodes installed. These provide one diode per row. If you
+driver outputs. They allow the row drivers to pull rows low, but not high,
-are attaching a keyboard with no diodes, then you only need to populate the
+emulating open collector outputs.
-diodes corresponding to rows on the keyboard. If the keyboard has 8 rows, then
+
-you may want to install 8 diodes corresponding to those rows.
+If you are attaching a keyboard with no diodes, then you only need to populate
+the diodes corresponding to rows on the keyboard. If the keyboard has 8 rows,
+then you may want to install 8 diodes corresponding to those rows.
 
 Note that the footprints for these diodes include a copper jumper on the TOP
 copper layer. If you install any of these diodes, you should cut the jumpers for
 those diodes. Otherwise the diodes do nothing.
 
+If you are attaching a keyboard with a diode per key, then the diodes on the keys perform the same function, in addition to preventing "ghosting", so the per-row diodes do not need to be installed.
+
 ### DIP switch and associated diodes
 The DIP switch is optional. If you don't want to be able to set options via the
 DIP switches, you can set all your preferences in the firmware, or just accept
 the default behavior, and skip the DIP switch and Diodes D17-D20 and D24-D27.
 
 ### Second 74LS138 multiplexer (U4)
-Only needed if you have more than 8 rows.
+Only needed if you have more than 8 rows (including the DIP switch).
 
 ### The 74LS07 hex buffer (U5) and pullup R6
 The 74LS07 is only needed if you are using LED2 or LED3 on the keyboard, or the OUT2 open
 collector output.
 
-R6 is the optional pullup for the open collector output 2 (O2), and only needs to be
-installed if output 2 is used, and if the host does not provide a pullup.
-
 ### Diodes D21, D22, D23
 Some keyboards may not wire all keys into the matrix. For example, some other
 interface modules may assign special hardware functions to certain keys. Those