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Programmer's Reference Manual


Monitor power saving

Introduction and Overview

Government agencies and independent organisations worldwide are involved in setting limits or goals for power consumption in office equipment, in order to slow the growth in overall demand for electric power. Desktop computers are one of these agencies' primary targets, especially their displays, which are a significant portion of their power consumption.

VESA (the Video Electronics Standards Association) has produced a proposed standard called Display Power Management Signalling (DPMS), which provides a common means for a display controller to send a signal to the display that makes it enter various power management states. DPMS is likely to be adopted by most major monitor manufacturers.

Where the monitor supports this mechanism, RISC OS 3.5 and later versions can use it. This has been done by incorporating DPMS into the code used in RISC OS 3 to blank the screen after the computer has been left untouched for a certain amount of time.

Technical Details

The DPMS power saving states are distinguished by the presence or absence of pulses on the horizontal and vertical sync lines.

State Power saving Recover time Horiz. sync Vert. sync Video
On None None Pulses Pulses Pulses
Stand by Minimal Short No Pulses Pulses Blanked
Suspend Substantial Longer Pulses No Pulses Blanked
Off Maximum System Dependent No Pulses No Pulses Blanked

To be compliant with DPMS, displays do not necessarily have to have all four states, but they must implement at least one reduced power consumption state.

From RISC OS 3.5 onwards the screen blanking mechanism has been extended so that it can select any of the power states above:

  • The existing RISC OS 3 screen blanking mechanism (ie blanking the video whilst leaving the sync pulses active) must still be possible. This is to avoid problems with older monitors which do not support DPMS and require the presence of sync pulses.
  • Screen blanking must be able to select all three reduced power states, since the DPMS proposed standard does not specify which of the three states a DPMS monitor must support.
Controlling DPMS power saving states

The DPMS power saving state to enter during screen blanking for a particular monitor is configured by an optional line in its ModeInfo file. The different states are specified using the values 0 - 3:

Value Meaning
0 DPMS disabled - screen blank just blanks video.
1 Screen blank enters 'Stand-by' mode.
2 Screen blank enters 'Suspend' mode.
3 Screen blank enters 'Off' mode.

There is no simple user interface to set or alter this value.

The power saving state is actually set by a video control parameter; see Service_ModeExtension. The control index is 11, and the value is in the range 0 to 3, with the same meanings as above. The line in the ModeInfo file makes the ScreenModes module append such an entry to all VIDC lists it passes to the kernel.

This edition Copyright © 3QD Developments Ltd 2015
Last Edit: Tue,03 Nov 2015