Bus: Address decode mods from third party developer appleii6502

Bus.md

@@ -174,6 +174,47 @@ The following boards may optionally use this line as an output:
   joystick/paddle fire button signal.
 
 
+Third-party Modifications
+-------------------------
+
+This section describes changes to the bus interface used by
+third-party boards.
+
+#### appleii6502
+
+[appleii6502][a26] (apparently an anonymous user hosting his blog on
+[Ninja Tools][a26-nt]) has been producing and selling RC6502 boards
+and kits in Japan. The technical focus seems to be around re-using the
+signals generated by the [SBC]'s [74LS138 decoder][a26-decode] by
+routing them on to the bus in the following way:
+
+    Decode Range   RC6502 Bus   Apple 1 Expansion Slot
+        $A000         35 TX          T (pin L)
+        $B000         36 RX          S (pin 11)
+        $C000         39             R (pin 21)
+
+The $C000 to expansion slot pin R mapping corresponds to the [Apple 1
+address decoding][a1decode] jumper settings needed by the [Apple Cassette
+Interface (ACI)][aci].
+
+His [Apple1 SBC Slot Expander][a26-se] adds two 44-pin expansion slots with
+the same pinout as the original Apple 1. RC6502 bus signals TX, RX, 38 and
+39 are brought to pads adacent to pads for the Apple 1 slot signals S, T
+and R to which the builder can solder jumpers. Pin 39 is labeled `$̅C̅0̅0̅0̅`;
+it's intended that the [SBC] be modified to bring its address decoding for
+that signal to that pin. These jumperings are [shown in images
+here][a26-cass-7].
+
+The [Cassette Interface][a26-cass] is a clone of the original [ACI] that
+plugs into the slot expander. It has a 2K ROM enabled by RC6502 bus pin 39,
+which must be set up as a $C000-$CFFF decode (see above).
+
+Another [post on slot expander decoding][a26-ab] explains how to disconnect
+the Arduino's TX/RX (by cutting the pins off the Arduino board) and instead
+wire $A000 and $B000 decoding to those bus pins. Presumably this is for a
+future expansion card project.
+
+
 RC2014 Bus Comparison
 ---------------------
 
@@ -240,11 +281,13 @@ compatible. The following other pins have caveats:
     used.
 
 
+
 <!-------------------------------------------------------------------->
 [Backplane]: ./RC6502%20Backplane/
 [CPU]: ./RC6502%20CPU/
 [Reset]: ./RC6502%20Reset%20Circuit/
 [SBC]: RC6502%20Apple%201%20SBC/
+[a1decode]: https://github.com/0cjs/sedoc/tree/master/8bit/apple1#address-decoding
 
 [Debug]: ./RC6502%20Debug/
 [RIOT]: ./RC6502%20RIOT/
@@ -254,5 +297,14 @@ compatible. The following other pins have caveats:
 [Terminal]: ./RC6502%20Terminal/
 [VDU]: ./RC6502%20VDU/
 
+[ACI]: https://www.sbprojects.net/projects/apple1/aci.php
+[a26-ab]: https://appleii6502.no-mania.com/apple1%20sbc%20slot%20expander/apple1%20sbc%20slot%20expander%E3%82%A2%E3%83%89
+[a26-cass-7]: https://appleii6502.no-mania.com/apple1%20cassette%20interface/apple1%20cassette%20interface%E7%B5%84
+[a26-cass]: https://appleii6502.no-mania.com/apple1%20cassette%20interface/apple1%20cassette%20interface
+[a26-decode]: https://appleii6502.no-mania.com/apple1computer/apple1%20sbc%E3%81%AE%E3%80%8C4k%20blocks%E3%80%8D%E3%82%A2%E3%83%89%E3%83%AC%E3%82%B9
+[a26-nt]: https://www.ninja.co.jp/
+[a26-se]: https://appleii6502.no-mania.com/apple1%20sbc%20slot%20expander/apple1%20sbc%20slot%20expander%E5%9B%9E%E8%B7%AF%E5%9B%B3
+[a26]: https://appleii6502.no-mania.com/
+
 [RC2014 bus]: https://smallcomputercentral.wordpress.com/documentation/specification-rc2014-bus/
 [RC2014-spec]: https://smallcomputercentral.files.wordpress.com/2018/09/modular-backplane-specification-v0-4-2018-09-19.pdf