Editing Apple Desktop Bus (section)

===Enumeration and identification===
The addresses and enumeration of the devices are set to default values when reset. For instance, all keyboards are set to $2, and all mice to $3. When the machine is first powered on, the ADB device driver will send out <code>talk</code> commands asking each of these known default addresses, in turn, for the contents of register three. If no response comes from a particular address, the computer marks it dead and doesn't bother polling it later.

If a device does respond, it does so by saying it is moving to a new randomly selected higher address. The computer then responds by sending another command to that new address, asking the device to move to yet another new address. Once this completes, that device is marked live, and the system continues polling it in the future. Once all of the devices are enumerated in this fashion, the bus is ready to be used.

Although it was not common, it is possible for the Apple Desktop Bus to have more than one device of the same sort plugged in — two graphics tablets or software [[copy protection]] dongles, for instance. In this case when it asks for devices on that default address, both will respond and a collision could occur. The devices include a small timing variation that allows them to avoid this problem. After receiving a message from the host, the devices wait a short random time before responding, and then only do so after "snooping" the bus to make sure it was not busy.

With two dongles plugged in, for instance, when the bus is first setting up and queries that address, one of them will be the first to respond due to the random wait timer. The other will notice the bus was busy and not respond. The host will then send out another message to that original address, but since one device has moved to a new address, only the other will then respond. This process continues until nothing responds to the request on the original address, meaning there are no more devices of that type to enumerate.

Data rates on the bus are theoretically as high as 125&nbsp;kbit/s. However, the throughput is at best half that, due to there being only one pin being shared between the computer and devices, and in practice, throughput is even less, as the entire system was driven by how fast the computer polls the bus. The [[classic Mac OS]] is not particularly well suited to this task, and the bus often slows to about 10&nbsp;kbit/s. Early Teleport modems running at 2400&nbsp;bit/s have no problems using Apple Desktop Bus, but later models were forced to move to the more expensive [[RS-422]] ports as speeds moved to 14.4&nbsp;kbit/s and higher.