The engine screen

Right-click the Billboard Engine to open its screen.

Part	What it does
Left slot	The Selector Wand with both corners marked on the wall this engine touches. Other wands are refused. Breaking the engine drops it.
Right slot	The pixel block. Click it with any full block, or drag one there from JEI or REI. The slot only keeps a faded copy as a setting: your block stays in your inventory and is never used up. Click with an empty hand to clear it.
Text	Up to 64 characters. See Letters and sizes for the characters.
Speed - / +	1 to 10 block columns per second. Default 5. Takes effect at once.
Size - / +	Letter size 1 to 9. A size change redraws a shown text at once.
Go	Starts the text from the right edge of the wall.
Stop	Freezes the text where it is. Go starts it again from the right.
Restore	Puts the original wall back.
Status line	What the engine is doing, or why it can't start. The same message also goes to your chat, so long messages can be read in full.

Messages

Message	Meaning
Put a selector wand with two marked corners in the left slot.	No wand, or one without both corners.
Pick a pixel block	The right slot is empty.
Engine must touch the wall.	The engine does not touch the marked wall with a full face.
The engine must stand behind the wall, not beside it	It stands in the same plane as the wall.
The wall is ... It must be flat	The corners are not in one plane.
The wall is ... Maximum is ...	Too big; see settings.
Can't use this wall: ...	A block entity (chest, sign, ...) is in the wall.
That wall overlaps another billboard.	Another engine is using part of it.
Part of the wall is not loaded. Come closer.	Some of its chunks are not loaded.
The engine is controlled by ... Only Stop and Restore work.	A program has claimed the engine. Text, speed, size and Go are greyed out.

Breaking the engine

Breaking the engine restores the wall and drops the engine and the wand. The pixel block is only a copy, so nothing else drops.