Z280 S-100 CPU Card

A 16-bit Zilog Z280 CPU card for the IEEE-696 S-100 backplane. All the glue logic lives in two Atmel ATF1508AS CPLDs, and the entire schematic + PCB are Python-generated (gen_kicad.py, gen_pcb.py).

The board is a bus master by default: it drives address, status, control and data onto the backplane. The CPLDs split the work cleanly:

• Control CPLD (U4, z280-s100-ctl.pld) — "the decider". Arbitration, the master-bridge state machine, local-memory control, and the pre-qualified steering terms (MST_RD/MST_WR/SLV_RD/SLV_WR, XFR16, SPLIT, SLAVE).
• Data CPLD (U5, z280-s100-ad.pld) — "everything that moves bits". The AD0-15 byte mux to the S-100 DO/DI lanes, the burst address counter, and the byte lane.

Address decode is external: a 74F138 (SRAM window) + two 74F521 comparators (flash window, slave window). The A3–A15 address lines are latched out through two 74F573s, and the 16-bit data bus runs through two 74F245 transceivers.

The boot EEPROM pair is word-addressed (flash A_n = byte A_{n+1}), so the two parts span 64 KB. They are not socket-interchangeable with 27C256s (pin 27 is WE# here, not A14).

Another S-100 master can take the bus and read/write this card's local SRAM directly — DMA-style, without the Z280:

1. The temporary master (TMA) asserts HOLD; the control CPLD forwards the request to the Z280 (Z_BUSREQ) and waits for Z_BUSACK.
2. With Z_BUSACK low the board is a slave (SLAVE = !Z_BUSACK # SLAVE_ONLY): the address/status/control 74F245s reverse direction, the data 74F245s float (S100_DODSB), and the 74F573 address latches float their outputs.
3. The TMA reaches this card's RAM through the slave window — a 74F521 comparator (SLAVE_WIN, hardwired to 000000–1FFFFF) that maps the TMA's address space onto the local SRAM.

The SLAVE_ONLY strap (J11) forces this state permanently, turning the card into a plain memory-only card for bring-up and debug.

The Z280's memory burst mode (consecutive accesses without re-presenting the address) is handled in the data CPLD:

• a 2-bit burst address counter (A1/A2) increments during a burst;
• the byte lane (A0) is derived from the low-address bit and the 16→8 SPLIT flag, so byte and word cycles share one data path;
• 16-bit transfers are detected from WORD and the S-100 SIXTN (16-bit-acknowledge) line: SPLIT16 = WORD & !SIXTN.

SIXTN is open-collector (wired-OR) and pulled up via R4, like the ready lines.

Two 24 MHz time-base footprints are laid out; populate exactly one:

• Y1 — parallel-resonant crystal (HC49) across XTALI/XTALO, or
• Y2 — a DIP-can oscillator (DIP-8 or DIP-14) driving XTALI.

The Z280 auto-detects the source: a crystal enables the on-chip oscillator, while an external clock into XTALI bypasses it (leave XTALO open). There is no strap for this. Either way the CPU clock is half the XTAL1 frequency, so 24 MHz → 12 MHz processor clock.

The Z280 samples AD0-7 on the rising edge of reset to load its Bus Timing & Initialization (BTI) register — the wait-state field, clock divider, bootstrap and multiprocessor bits. J10 is a 3×8 header, one row per bit: the centre pin selects +5V (logic 1) or GND (logic 0); a 74HCT244 (U21) drives the value onto AD0-7 for the whole reset window.

Default = 0b10001110 (AD7…AD0): direct clock, no bootstrap, no multiprocessor, 3 wait states, bus clock = CPU clock.

2-pin header. Open (default) = normal bus master. Jumper to +5V = permanent slave (memory-only card). R3 (1 kΩ) holds the line low when open.

A 74F521 comparator hardwired to 000000–1FFFFF (the same 2 MB window as SRAM) selects where a temporary master sees this card's SRAM. Not user- configurable on this rev.

The 28C256 WE# line is normally driven by the control CPLD (in-system EEPROM programming). Strap it to +5V to hard-write-protect the boot ROM.

CPLDs are flashed over the JTAG header (TDI/TMS/TCK/TDO on pins 14/23/62/71).

CERN Open Hardware Licence Version 2 - Permissive (CERN-OHL-P-2.0). Fork, modify and build it freely — just don't claim you designed it: keep the attribution.