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Merge pull request #13 from 0cjs/dev/cjs/200521/rc2014-bus-comparison
Bus.md: Various Updates
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Bus.md
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Bus.md
@ -174,12 +174,120 @@ The following boards may optionally use this line as an output:
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joystick/paddle fire button signal.
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Third-party Modifications
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-------------------------
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This section describes changes to the bus interface used by
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third-party boards.
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#### appleii6502
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[appleii6502][a26] (apparently an anonymous user hosting his blog on
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[Ninja Tools][a26-nt]) has been producing and selling RC6502 boards
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and kits in Japan. The technical focus seems to be around re-using the
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signals generated by the [SBC]'s [74LS138 decoder][a26-decode] by
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routing them on to the bus in the following way:
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Decode Range RC6502 Bus Apple 1 Expansion Slot
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$A000 35 TX T (pin L)
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$B000 36 RX S (pin 11)
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$C000 39 R (pin 21)
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The $C000 to expansion slot pin R mapping corresponds to the [Apple 1
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address decoding][a1decode] jumper settings needed by the [Apple Cassette
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Interface (ACI)][aci].
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His [Apple1 SBC Slot Expander][a26-se] adds two 44-pin expansion slots with
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the same pinout as the original Apple 1. RC6502 bus signals TX, RX, 38 and
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39 are brought to pads adacent to pads for the Apple 1 slot signals S, T
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and R to which the builder can solder jumpers. Pin 39 is labeled `$̅C̅0̅0̅0̅`;
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it's intended that the [SBC] be modified to bring its address decoding for
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that signal to that pin. These jumperings are [shown in images
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here][a26-cass-7].
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The [Cassette Interface][a26-cass] is a clone of the original [ACI] that
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plugs into the slot expander. It has a 2K ROM enabled by RC6502 bus pin 39,
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which must be set up as a $C000-$CFFF decode (see above).
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Another [post on slot expander decoding][a26-ab] explains how to disconnect
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the Arduino's TX/RX (by cutting the pins off the Arduino board) and instead
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wire $A000 and $B000 decoding to those bus pins. Presumably this is for a
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future expansion card project.
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RC2014 Bus Comparison
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---------------------
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The RC6502 bus is very similar to the [RC2014 bus] used on some Z80
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homebrew computers. The following table compares the two; pins with
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substantially different/incompatible functions are marked with a bullet
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(`●`).
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The 6502 uses the Motorola bus prococols; the Intel bus protocols are
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substantially different, precluding sharing of peripherals unless they are
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specifically designed with the extra hardware necessary to support both.
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However, the backplanes are interchagable with some cavats; see below for
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details.
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RC6502 Pin RC2014 Notes
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----------------------------------------------------------------
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A15 1 A15
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… … …
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A0 16 A0
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GND 17 GND
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Vcc 18 Vcc
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Φ2out 19 ● /M1 Low on Z80 instruction fetch/int ack cycles
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/RESET 20 /RESET
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Φ0in 21 CLK
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/IRQ 22 /INT
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Φ1out,EX0 23 ● /MREQ
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R/W̅ 24 ● /WR
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RDY 25 ● /RD
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SYNC 26 ● /IORQ
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D0 27 D0
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… … …
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D7 34 D7
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TX 35 TX,TX2 May be user-specified function on RC2014
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RX 36 RX,TX2 May be user-specified function on RC2014
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/NMI 37 ● USER1
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-,EX1 38 USER2 Some RC2014 modules use USER2 and USER3 as
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-,EX2 39 USER3 IEI and IEO for interrupt daisy chain.
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40 USER4,IEO Nonexistent on RC6502
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### Backplane Compatibility
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The [RC2014 backplanes][RC2014-spec] (as of the 0.4 draft specification)
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can be used with RC6502 boards and vice versa, with some caveats. The power
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(18) and ground (19) pins may be supplied by the backplane and are
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compatible. The following other pins have caveats:
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* __/RESET__ (20)
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* RC2014 backplanes with a power supply (SC105, SC112) have a 4.7 kΩ
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pull-up resistor on /RESET. RC6502 board reset logic should be able to
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pull this line low to reset the system. RC2014 backplanes also have a
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reset switch; this ___must not be closed___ as it will conflict with
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the standard reset output circuitry of RC6502 boards, potentially
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damaging the board.
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* RC6502 backplanes have no reset logic, so a pull-up (4.7 kΩ to Vcc) and
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an RC2014 board will need to supply a reset signal. The SC108 CPU board
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supplies power-on reset but has no reset button. The SC101, SC114 and
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SC130 provide a reset button.
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* __Pin 40__.
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* RC2014 backplanes provide pin 40 as a bus or daisy chain (with pin
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80) depending on the model. This pin does not exist on RC6502 boards
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and thus can be ignored. Be careful not to insert the RC6502 board
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offset by one pin.
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* RC6502 backplanes do not provide pin 40, so the USER4 signal cannot be
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used.
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<!-------------------------------------------------------------------->
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[Backplane]: ./RC6502%20Backplane/
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[CPU]: ./RC6502%20CPU/
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[Reset]: ./RC6502%20Reset%20Circuit/
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[SBC]: RC6502%20Apple%201%20SBC/
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[a1decode]: https://github.com/0cjs/sedoc/tree/master/8bit/apple1#address-decoding
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[Debug]: ./RC6502%20Debug/
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[RIOT]: ./RC6502%20RIOT/
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@ -188,3 +296,15 @@ The following boards may optionally use this line as an output:
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[TIA]: ./RC6502%20TIA%20NTSC/
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[Terminal]: ./RC6502%20Terminal/
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[VDU]: ./RC6502%20VDU/
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[ACI]: https://www.sbprojects.net/projects/apple1/aci.php
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[a26-ab]: https://appleii6502.no-mania.com/apple1%20sbc%20slot%20expander/apple1%20sbc%20slot%20expander%E3%82%A2%E3%83%89
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[a26-cass-7]: https://appleii6502.no-mania.com/apple1%20cassette%20interface/apple1%20cassette%20interface%E7%B5%84
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[a26-cass]: https://appleii6502.no-mania.com/apple1%20cassette%20interface/apple1%20cassette%20interface
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[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
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[a26-nt]: https://www.ninja.co.jp/
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[a26-se]: https://appleii6502.no-mania.com/apple1%20sbc%20slot%20expander/apple1%20sbc%20slot%20expander%E5%9B%9E%E8%B7%AF%E5%9B%B3
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[a26]: https://appleii6502.no-mania.com/
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[RC2014 bus]: https://smallcomputercentral.wordpress.com/documentation/specification-rc2014-bus/
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[RC2014-spec]: https://smallcomputercentral.files.wordpress.com/2018/09/modular-backplane-specification-v0-4-2018-09-19.pdf
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