Editing Psion Series 5mx (section)

=== Display and digitiser ===

The panel is a '''640 × 240 passive-matrix LCD, normally-white''', with a separate '''resistive digitiser''' laid over it: a glass lower plate and a hardened plastic top plate, both printed with resistive ink, held apart by very small moulded bumps, with four connections sensing resistance across the X and Y axes. Each connects to the mainboard by a '''22-way flat flexible cable'''.<ref name="s5sm" />

The LCD needs its supply rails '''enabled and disabled in a defined sequence''' so that no residual DC is ever left across the panel — DC causes the liquid crystal to drift and eventually damages its polarisation permanently.<ref name="s5sm" /> That is why the machine re-initialises the display on every power-up, and why a half-powered machine can show odd artefacts.

The screen coordinate origin (0,0) is the '''top left of the LCD, not of the digitiser''', so the silkscreened shortcut icons down the left-hand edge have negative X coordinates.<ref name="wuffs" />