Enhanced Options Slot Reference
2025-02-16
Overview
The Enhanced Options Slot is a proprietary 50-pin expansion connector on the Compaq Portable 486c’s system I/O board. Separate from the two full-sized EISA slots. Designed for an internal modem or second serial port. Cards slide in from the rear and engage the connector.
Essentially a simplified ISA-like bus. 16-bit data, 10 address lines, IRQ, DMA, +5V only. Well-suited for simple peripherals that need I/O port access, one interrupt, and optionally DMA without the full ISA/EISA bus.
Full 50-pin pinout and ESP32 wiring reference →
Signal Descriptions
Data Bus
- Data Bit 0-15 (Pins 2-9, 29-36): 16-bit data bus, supporting ISA-style 16-bit transfers.
Address Bus
- XA08 (Pin 18): Address line 8
- Add 1-9 (Pins 19-20, 37-43): Address lines for I/O port decoding (10 address lines total including XA08, providing up to 1024 I/O port addresses).
Control Signals
- IOW (Pin 11): I/O Write, active when the CPU is writing to an I/O port.
- IOR (Pin 12): I/O Read, active when the CPU is reading from an I/O port.
- IRQ (Pin 13): Interrupt Request, directly from the option board to the system interrupt controller.
- Bus Ready (Pin 14): Indicates the device is ready / used for wait-state insertion.
- RST (Pin 17): System Reset.
- Select (Pin 10): Chip select / board select signal.
- IO16 (Pin 47): Indicates the device supports 16-bit I/O transfers.
- SLOT-IOEN (Pin 49): Slot I/O Enable, enables the slot for I/O access.
DMA Signals
- DMA (Pin 28): DMA signal.
- SLOT-DMA- (Pin 44): Slot DMA request (active low).
- SLOT-DRQ (Pin 48): Slot DMA Request.
- SLOT-DAK (Pin 50): Slot DMA Acknowledge.
- T-C (Pin 45): Terminal Count, signals end of a DMA transfer.
Power and Ground
- +5V (Pins 15, 16): +5V DC power supply.
- Ground (Pins 1, 26, 27): Ground/return.
Miscellaneous
- Mclock (Pin 21): Master clock signal.
- Slot-on (Pin 24): Indicates the slot is active/populated.
- SPKDRV (Pin 25): Speaker drive, allows the option board to drive the system speaker (e.g., modem audio).
- Reserve 1 (Pin 22): Reserved for future use.
- Reserve 2 (Pin 23): Reserved for future use.
- SLOT-IR08 (Pin 46): Secondary interrupt (IRQ8 line, from the second PIC).
Configuration
Options installed in the Enhanced Options Slot are configured using the COMPAQ EISA Configuration Utility:
- Method 1 (Manual): If no .CFG file exists for the board, manually set switches/jumpers based on available system resources (IRQ, port addresses) shown in the utility.
- Method 2 (Generic Config File): Use a generic ISA adapter definition in the EISA Configuration Utility, then manually edit port address and IRQ resources and lock the board.
A modem configured at COM1 requires IRQ4 and port addresses 3F8-3FF. COM2 requires IRQ3 and port addresses 2F8-2FF. The EISA Configuration Utility can reallocate resources if conflicts arise.
References
Source: Compaq Portable 486c Reference Guide, Part Number 128984-001, November 1991.