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Bus driver card firmware

s5-bus-driver/s5-bus-driver.ino is the Arduino Nano sketch for the System-5 bus driver card (BUSDRV-B, hardware/bus-driver/kicad/rev-b). The header comment has the command list. It has the same wire protocol as Ken's YACC1 bus-driver.ino: CMD:OPERAND#, replies, then >>. The host tool is software/busdrv/s5busdrv.py.

  • Monitor by default. At power-up every expander pin is an input. MASTER:1# takes the bus, and is refused while TPA/TPB toggle (a CPU is running) or the bus has no power.
  • Memory and I/O in the 1802's order:
  • Memory: high address byte on A0–7 with a TPA pulse, low byte, -VMA, then -MRD (read) or -MRW (write), each with a TPB pulse inside it (the 6845's E).
  • I/O: OUT drives the data, holds -MRD low (an 1802 OUT is a memory read), sets N0–N2 and pulses TPA then TPB; INP sets N0–N2, pulses TPA and samples the data while TPB is high. -VMA stays high, so no memory card answers.
  • INBLK/OUTBLK run up to 64 / 32 of them in one command (a CF sector is 8 INBLKs).
  • Fixed 2026-10-11: before that, memory cycles had no TPB (the CRTC never answered) and OUT/INP had no -MRD or TPA (the IDE card read the drive onto the bus during every OUT). Found with software/busdrv/fwsim.py, which runs the same steps against the cards' netlists; keep it in step with any change to a bus cycle here.
  • Open-collector lines (-RESET, -INT, -EF) are only pulled low or released.
  • While bus +5V is off: the expanders are held in reset, and I²C is stopped with no pull-ups, so nothing back-feeds them.
  • No Adafruit library: it writes the MCP23017 registers directly (I²C at 400 kHz, about 1 ms per byte).

Build and flash

Board: Arduino Nano (ATmega328P). Many clones need the old bootloader: arduino:avr:nano:cpu=atmega328old. Ken's Nano (a CH340 clone, /dev/cu.usbserial-3120) needs it: confirmed 2026-10-19 with a Blink upload (the standard setting fails with "not in sync").

arduino-cli compile --fqbn arduino:avr:nano s5-bus-driver
On a Mac without arduino-cli on the path, the Arduino IDE carries one: "/Applications/Arduino IDE.app/Contents/Resources/app/lib/backend/resources/arduino-cli". 
arduino-cli upload -p /dev/cu.usbserial-XXXX --fqbn arduino:avr:nano s5-bus-driver

Status, 2026-10-11 (banner s5-bus-driver blocks-1 io-2 2026-10-11): - Compiles with arduino-cli 1.5.1 (the arduino:avr 1.8.6 core on Ken's Mac; 1.8.8 on 2026-10-06), for both nano and nano:cpu=atmega328old, with 0 warnings from the sketch. - Size: 14,564 bytes of flash (47%) and 772 bytes of RAM (37%). - Not yet run on hardware. Its bus cycles are checked against the memory Rev D, IDE/CF Rev A and 6845 Rev B netlists by software/busdrv/test_bringup.py. - Bring-up procedure for the cards: software/busdrv/BRINGUP.md.