50 pin Mac Internal ROM Expansion Connector
The Macintosh Portable ROM expansion connector is a 50-pin proprietary internal connector fitted to the Macintosh Portable logic board, allowing additional ROM modules to be installed alongside the system ROM. It operates at +5 V and 16 MHz and exposes the unbuffered Motorola 68HC000 address bus (A1–A23) and data bus (D0–D15). The chip-select signal /ROM_CS covers the address range $900000–$9FFFFF, permitting ROM expansion up to 4 MB. The connector is physically distinct from the Macintosh Portable RAM expansion connector, which is also 50-pin but includes a notch to prevent confusion between the two.
| Pin | Signal | Description |
|---|---|---|
| 1 | +5V | Vcc |
| 2-24 | A1-A23 | Unbuffered 68HC000 address signals A1-A23 |
| 25-26 | GND | Logic Ground |
| 27 | /DTACK | /DTACK input to 68HC000 |
| 28 | /AS | 68HC000 address strobe signal |
| 29 | /ROM_CS | Permanent ROM chip select signal. Selects in range $90 0000 through $9F FFFF |
| 30 | 16MHz_Clock | 16 MHz system Clock |
| 31 | /EXT_DTACK | External /DTACK signal that disables main system /DTACK |
| 32 | /DELAY_CS | This signal is generated by the addressing PAL and is used to put the ROM board into the idle mode by inserting multiple wait states |
| 33-48 | D0-15 | 68HC000 unbuffered data signals D0-15 |
| 49-50 | +5V | Vcc |
Notes
The ROM expansion connector and the RAM expansion connector on the Macintosh Portable are both 50-pin internal connectors. The RAM connector carries a physical notch that distinguishes it from the ROM connector; the two must not be interchanged. The ROM connector carries unbuffered signals directly from the 68HC000 processor and operates at the 16 MHz system clock frequency. Logic levels are +5 V throughout.
Signals
- +5V — +5 V power supply (Vcc); present on pins 1, 49, and 50
- A1–A23 — Unbuffered address lines from the 68HC000 processor (pins 2–24); 23 address lines providing up to 8 MB addressable space per bank
- GND — Logic ground (pins 25–26)
- /DTACK — Data transfer acknowledge input to the 68HC000; asserted by the ROM board when a bus cycle completes
- /AS — Address strobe from the 68HC000; indicates that a valid address is present on the bus
- /ROM_CS — Permanent ROM chip select; asserted for accesses in the range $900000–$9FFFFF; generated by the system PAL logic
- 16MHz_Clock — 16 MHz system clock signal supplied to the ROM expansion board
- /EXT_DTACK — External data-transfer acknowledge; when asserted by the ROM board, it suppresses the main system /DTACK, allowing the board to control bus timing
- /DELAY_CS — Generated by the addressing PAL; places the ROM board into idle mode by inserting multiple wait states
- D0–D15 — Unbuffered 16-bit data bus signals from the 68HC000 (pins 33–48)
References
- Guide to the Macintosh Family Hardware, Second Edition (1990) — Apple Computer, Inc. / Addison-Wesley; Table 5-10 (p. 234) and Figure 5-13 document the ROM expansion connector signal assignments