From d9f2d1bc8b8afbbbf8c383e6ecd95efc19b4b060 Mon Sep 17 00:00:00 2001 From: "Curt J. Sampson" Date: Thu, 21 May 2020 18:43:32 +0900 Subject: [PATCH] Bus: Add comparision with RC2014 bus The backplanes (though not the cards) are actually fairly compatible, so we include this information to give guidance on using a backplane from one system with the other. This may also provide helpful insight for RC6502 board designers. --- Bus.md | 68 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 68 insertions(+) diff --git a/Bus.md b/Bus.md index 0bb3c0e..4c19cc9 100644 --- a/Bus.md +++ b/Bus.md @@ -174,6 +174,71 @@ The following boards may optionally use this line as an output: joystick/paddle fire button signal. +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/ @@ -188,3 +253,6 @@ The following boards may optionally use this line as an output: [TIA]: ./RC6502%20TIA%20NTSC/ [Terminal]: ./RC6502%20Terminal/ [VDU]: ./RC6502%20VDU/ + +[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