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Bus: Address decode mods from third party developer appleii6502

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Bus.md
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@ -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