Merge pull request #13 from 0cjs/dev/cjs/200521/rc2014-bus-comparison

Bus.md: Various Updates

Bus.md

@@ -174,12 +174,120 @@ 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
+---------------------
+
+The RC6502 bus is very similar to the [RC2014 bus] used on some Z80
+homebrew computers. The following table compares the two; pins with
+substantially different/incompatible functions are marked with a bullet
+(`●`).
+
+The 6502 uses the Motorola bus prococols; the Intel bus protocols are
+substantially different, precluding sharing of peripherals unless they are
+specifically designed with the extra hardware necessary to support both.
+However, the backplanes are interchagable with some cavats; see below for
+details.
+
+        RC6502  Pin   RC2014      Notes
+    ----------------------------------------------------------------
+           A15   1    A15
+             …   …    …
+            A0  16    A0
+           GND  17    GND
+           Vcc  18    Vcc
+         Φ2out  19 ●  /M1         Low on Z80 instruction fetch/int ack cycles
+        /RESET  20    /RESET
+          Φ0in  21    CLK
+          /IRQ  22    /INT
+     Φ1out,EX0  23 ●  /MREQ
+           R/W̅  24 ●  /WR
+           RDY  25 ●  /RD
+          SYNC  26 ●  /IORQ
+            D0  27    D0
+             …  …      …
+            D7  34    D7
+            TX  35    TX,TX2      May be user-specified function on RC2014
+            RX  36    RX,TX2      May be user-specified function on RC2014
+          /NMI  37 ●  USER1
+         -,EX1  38    USER2       Some RC2014 modules use USER2 and USER3 as
+         -,EX2  39    USER3         IEI and IEO for interrupt daisy chain.
+                40    USER4,IEO   Nonexistent on RC6502
+
+### Backplane Compatibility
+
+The [RC2014 backplanes][RC2014-spec] (as of the 0.4 draft specification)
+can be used with RC6502 boards and vice versa, with some caveats. The power
+(18) and ground (19) pins may be supplied by the backplane and are
+compatible. The following other pins have caveats:
+
+* __/RESET__ (20)
+  * RC2014 backplanes with a power supply (SC105, SC112) have a 4.7 kΩ
+    pull-up resistor on /RESET. RC6502 board reset logic should be able to
+    pull this line low to reset the system. RC2014 backplanes also have a
+    reset switch; this ___must not be closed___ as it will conflict with
+    the standard reset output circuitry of RC6502 boards, potentially
+    damaging the board.
+  * RC6502 backplanes have no reset logic, so a pull-up (4.7 kΩ to Vcc) and
+    an RC2014 board will need to supply a reset signal. The SC108 CPU board
+    supplies power-on reset but has no reset button. The SC101, SC114 and
+    SC130 provide a reset button.
+* __Pin 40__.
+  * RC2014 backplanes provide pin 40 as a bus or daisy chain (with pin
+    80) depending on the model. This pin does not exist on RC6502 boards
+    and thus can be ignored. Be careful not to insert the RC6502 board
+    offset by one pin.
+  * RC6502 backplanes do not provide pin 40, so the USER4 signal cannot be
+    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/
@@ -188,3 +296,15 @@ The following boards may optionally use this line as an output:
 [TIA]: ./RC6502%20TIA%20NTSC/
 [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