Arm Screen (ARM_SCR)

Description

This parameter automates the On-Screen Display (OSD) management by automatically switching to a specific layout when the vehicle arms.

Pilots often prefer a "Pre-flight" OSD screen (showing GPS health and battery cell voltages) while on the ground, but want a "Flight" OSD screen (showing artificial horizon, altitude, and current draw) as soon as they are ready to fly. Setting ARM_SCR eliminates the need to manually toggle screens using a transmitter switch.

The Mathematics

The parameter uses a 1-based index for user convenience, which the internal code converts to a 0-indexed array element:

$$ \text{InternalScreenID} = ARM\_SCR - 1 $$

If $ARM\_SCR = 0$, the feature is disabled and no automatic switching occurs.

The Engineer's View

In AP_OSD::update() (libraries/AP_OSD/AP_OSD.cpp), the OSD system monitors the armed state of the vehicle.

1. It checks for a "Rising Edge" transition where was_armed is false but the current state is armed.
2. It verifies that arm_scr is within the valid range (typically 1 to 4).
3. It checks if the target screen layout is actually enabled in the parameters.
4. If all checks pass, it updates the current_screen variable, forcing the display to refresh with the new layout.

Tuning & Behavior

• 0: Disabled (No automatic switching).
• 1 - 4: Switches to the corresponding OSD screen number on arming.
• Recommendation: Set OSD Screen 1 as your "Summary/Pre-flight" screen and OSD Screen 2 as your "Combat/Flight" screen. Then set ARM_SCR to 2.
• Related: Use OSD_DSARM_SCR to automatically switch back to a summary screen upon landing and disarming.