Editing Psion Series 3a (section)

=== Power rails ===

SIBO machines run four power states — '''standby, backup, idle and operating''' — and the supply generates a set of named rails. Knowing what each does turns a dead machine into a traceable fault.<ref name="sibo" />

{| class="wikitable styled-table" style="width:100%; text-align:left;"
|+'''SIBO supply rails'''<ref name="sibo" />
! Rail !! Purpose !! Notes
|-
| '''Vcc1''' || ASIC9 and '''all the memory''' — present in every state || Nominally 3.3 V on ASIC9 machines. Current-limited to a few hundred microamps in standby. Powered from the backup cell in the backup state.
|-
| '''Vcc2''' || The processor and anything that can shut down || Fixed at 5 V even in a 3.3 V system. Off in standby, on automatically in idle or operating.
|-
| '''Vcc3''' || The LCD module and its display drivers || Software switchable, 5 V or 3.3 V
|-
| '''Vcc5''' || The 5 V pin on the external SSD slots || Software switchable
|-
| '''Vee1''' || '''LCD bias — the contrast rail''' || Software switchable negative rail from a flyback converter. '''Contrast is set by a DAC in software''', not by a potentiometer.
|-
| '''Vh''' || '''+16 V programming voltage for Flash SSDs''' || Software switchable flyback converter. Needed to write and format Flash packs — not to read them.
|}