Editing Apple Desktop Bus (section)

=== Communication ===
The Apple Desktop Bus system is based around the devices having the ability to decode a single number (the ''address'') and being able to hold several small items of data (their ''registers''). All traffic on the bus is driven by the host computer, which sends out commands to read or write data: devices are not allowed to use the bus unless the computer first requests it.

These requests take the form of single-[[byte]] strings. The upper four bits contain the address, the ID of one of the devices on the chain. The four bits allow for up to 16 devices on a single bus. The next two bits specify one of four commands, and the final two bits indicate one of four registers. The commands are:

* <code>talk</code> - tells the selected device to send the contents of a register to the computer
* <code>listen</code> - tells the device to set the register to the following value
* <code>flush</code> - clear the contents of a selected register
* <code>reset</code> - tell all devices on the bus to reset

For instance, if the mouse is known to be at address $D, the computer will periodically send out a 1-byte message on the bus that looks something like:

<code>1101 11 00</code>

This says that device $D (1101) should talk (11) and return the contents of register zero (00). To a mouse this means "tell me the latest position changes". Registers can contain between two and eight bytes. Register zero is generally the primary communications channel. Registers one and two are undefined, and are generally intended to allow 3rd party developers to store configuration information. Register three always contains device identification information.