From 08996975e0133e4da202ca72259064625df0aa20 Mon Sep 17 00:00:00 2001 From: "Curt J. Sampson" Date: Fri, 22 May 2020 16:52:45 +0900 Subject: [PATCH] Bus: Address decode mods from third party developer appleii6502 --- Bus.md | 52 ++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 52 insertions(+) diff --git a/Bus.md b/Bus.md index 4c19cc9..6a3ddee 100644 --- a/Bus.md +++ b/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