ZX Spectrum+ Expansion Port Pinout Configuration Guide

This article provides a comprehensive technical overview of the expansion port pinout configuration for the Sinclair ZX Spectrum+. It details the 40-pin edge connector assignments, including power, data, address, and control signals required for hardware interfacing. Readers will find specific pin numbers and signal names necessary for developing peripherals or troubleshooting connections on this classic home computer.

Overview of the Expansion Interface

The Sinclair ZX Spectrum+ retained the same 40-way edge connector found on the original 48K model. Located on the bottom edge of the motherboard, this expansion port exposes the Z80 CPU bus signals, allowing users to connect external hardware such as memory expansions, disk interfaces, and joystick adapters. The connector operates at 5V TTL logic levels. Care must be taken when designing hardware for this port, as incorrect wiring or voltage spikes can permanently damage the ULA or CPU.

Pinout Assignment List

The expansion port consists of 40 contacts, numbered 1 through 40. When viewing the bottom of the computer with the keyboard facing away, Pin 1 is located on the left side near the power socket. The pins are divided into two logical sides corresponding to the physical edge connector tracks.

Side A Pins (1-20)

The first half of the connector primarily carries address bus signals and power connections.

• Pin 1: +5V DC Power
• Pin 2: /M1 (Machine Cycle One)
• Pin 3: Address Bus A0
• Pin 4: Address Bus A1
• Pin 5: Address Bus A2
• Pin 6: 0V Ground
• Pin 7: Address Bus A3
• Pin 8: Address Bus A4
• Pin 9: Address Bus A5
• Pin 10: Address Bus A6
• Pin 11: Address Bus A7
• Pin 12: Address Bus A8
• Pin 13: Address Bus A9
• Pin 14: Address Bus A10
• Pin 15: Address Bus A11
• Pin 16: Address Bus A12
• Pin 17: Address Bus A13
• Pin 18: Address Bus A14
• Pin 19: Address Bus A15
• Pin 20: 0V Ground

Side B Pins (21-40)

The second half of the connector carries data bus signals, control logic, and video synchronization outputs.

• Pin 21: /IORQ (Input/Output Request)
• Pin 22: Data Bus D0
• Pin 23: Data Bus D1
• Pin 24: 0V Ground
• Pin 25: Data Bus D2
• Pin 26: Data Bus D3
• Pin 27: Data Bus D4
• Pin 28: Data Bus D5
• Pin 29: Data Bus D6
• Pin 30: Data Bus D7
• Pin 31: /WR (Write)
• Pin 32: /RD (Read)
• Pin 33: 0V Ground
• Pin 34: /MREQ (Memory Request)
• Pin 35: /RFSH (Refresh)
• Pin 36: /INT (Interrupt)
• Pin 37: /NMI (Non-Maskable Interrupt)
• Pin 38: Video Signal (ULA Output)
• Pin 39: HSYNC (Horizontal Sync)
• Pin 40: VSYNC (Vertical Sync)

Technical Considerations for Developers

When utilizing the ZX Spectrum+ expansion port, it is critical to note that the +5V supply available on Pin 1 is limited by the computer’s internal power supply capacity. High-power peripherals should utilize external power sources rather than drawing solely from the edge connector. Additionally, the video signals on pins 38, 39, and 40 are TTL level outputs intended for modulation or specific monitoring equipment, not direct connection to modern HDMI or VGA displays without conversion hardware.

Signal integrity is another major factor. The address and data lines are shared across the bus, meaning any expansion device must properly tri-state its outputs when not selected to prevent bus conflicts. Most interface designs utilize the /IORQ and /MREQ lines to decode addresses and ensure the peripheral only drives the data bus during relevant cycles. Adhering to these pinout specifications ensures compatibility with the vast library of legacy hardware designed for the Sinclair ZX Spectrum platform.