SA4000

SA4000 — HDCA hard-disk drive emulator

A dumb HAT that plugs onto an Olimex RP2350-PICO2-BB48R and emulates a Morrow HDCA-connected M26 (Shugart SA4008) or M10 (Fujitsu M2301B) hard disk drive — four simultaneous drives, all the same type.

The board is deliberately dumb: no MCU, no storage, no logic. It carries only level shifters, RS-422 transceivers, and the two IDC drive connectors. The BB48R (RP2350B, 16 MB flash, 8 MB PSRAM, onboard micro-SD) is the brains: its PIO generates the NRZ data + bit clock and buffers the bit stream against the onboard SD/PSRAM.

Why this is not ST-412

The HDCA has no data separator — its parts list is pure TTL, no VCO/PLL. The drive carries the VFO and a dedicated clock track, so the cable carries NRZ data + a bit clock, not raw MFM. This is the Shugart SA4000 interface family (the Fujitsu M2301/2 CE manual calls it "SA4000 interface compatibility"). NOTES.md is the design authority for the electrical interface, pinouts, timing, and drive geometry.

Hardware

• CNA 50-pin control/status (daisy-chained) + CNB 20-pin data (radial), one CNB per drive.
• Control/status: 5 V TTL active-low (open-collector). Data/clock: RS-422 balanced pairs, 110 Ω termination.
• Level shift: 74LVC245A (5 V → 3.3 V, 12 inputs), 74LVC07A (3.3 V → 5 V open-drain, 6 outputs).
• RS-422: AM26LV31 drivers, AM26LV32 receivers (3.3 V).
• SOIC ICs; through-hole axial resistors and disc capacitors; through-hole IDC headers.

Repository layout

Regenerating

KiCad 10 dropped the Specctra .dsn export/import that FreeRouting needs, so routing goes through two small scripts (ROUTING.md has the full story and the gotchas):

python3 gen_kicad.py       # schematic + sa4000.net
python3 gen_pcb.py         # unrouted board (footprints + placement)

# route
python3 dsn_gen.py         # board -> SA4000.dsn
java -jar ~/.local/share/freerouting/freerouting-2.4.1.jar \
     -de SA4000.dsn -do SA4000.ses -mp 20 -mt 4
python3 ses_import.py      # SA4000.ses -> routed SA4000.kicad_pcb

kicad-cli pcb drc -o drc.rpt SA4000.kicad_pcb

Status

Board generated, routed, and DRC-clean (0 violations, 0 unconnected). The firmware — the PIO clock/data engine, four drive state machines, and SD record/replay — is not yet written.