The HP LaserJet III Technical Reference Manual is a developer-focused guide that explains how to control the LaserJet III at a low level, primarily via PCL 5 (Printer Command Language) and the printer’s hardware interfaces. It documents the printer’s architecture and memory, parallel/serial interface signals and handshakes, and then dives into the full PCL command set: escape sequences for font and symbol-set selection, page layout and margins, orientation, graphics modes, macros, and downloadable/soft fonts, plus details on status codes and error reporting. Together, it’s meant for system integrators, driver writers, and power users who need to generate or parse raw printer streams and fully exploit LaserJet III features like Resolution Enhancement (REt) and scalable fonts, rather than just use high-level drivers.