Auto-Zone Plus Section 4
Start-Up and Troubleshooting 4-1
1.0 Communications
Overview
Perhaps no other portion of the system seems as difficult to diagnose as the
communications loop, yet it really can be quite simple if you understand how it works. In
fact, much of the troubleshooting can be done by simple observation. We will lead you
through the system functions and then provide you with detailed instructions for checking
each piece of equipment.
1.1 How It Works
Auto-Zone uses a token-passing loop architecture, which means in simple terms that an
“electronic message” called a token is passed from controller to controller in a round-
robin fashion. A controller must wait to receive the token before it can send or request
any data. Only one device on each loop is capable of sending the token, and this device is
considered the loop master. The loop master varies depending on the type of system and
the installed options.
Note: It is very important to know which device is the loop master on the loop
you are troubleshooting.
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