VTX5000: Part 4 - Communications

This article is the fourth part of a technical series on the VTX5000, a vintage computing terminal for the ZX Spectrum. It focuses on the Z80 machine code responsible for real-time operations: communicating with the Intel 8251 USART, managing data buffers, and scanning the keyboard with Prestel-specific key remapping. The article provides detailed register bit tables for the 8251's command, configuration, and status registers, and walks through the hardware initialization routine (HW_INIT) with assembly code examples. It explains the asynchronous mode setup, timing, and error handling, making it a valuable resource for retro computing enthusiasts and developers interested in low-level hardware programming.