Each of the three LAT communications fabrics ( command, trigger, event) has a redundant set of wires. This redundancy is referred to as the "A and B path".
The interfaces for switching between the redundant paths for each fabric are:
In addition to configuring the test stand for the A/B path the TEM/AEM also must be configured. For event and trigger path selection set the corresponding bits in the CONFIGURATION register of the TEM/AEM. See the specific documentation for the TEM/AEM for more details.
To configure the TEM/AEM for the Command/Response A/B path, send the TEM/AEM the "Look At Me" (LAM) command. For instance configure the test stand to use command path B and then send the LAM command – upon receiving the LAM command on its B path the TEM/AEM which switch over to path B. You can see which path is being used by checking the corresponding bit in the CONFIGURATION register of the TEM/AEM.
The interface for the LAM command is described here.