User`s manual

Remote Loopback (RL)
Upon receiving a RL signal from the local CSU/DSU, the remote CSU/DSU enters RT configuration. The local DTE can
then transmit data to determine if the local CSU/DSU, the DDS network, and the remote CSU/DSU are functional.
Figure 12, RL (Remote Loopback)
Remote Loopback (RL) with Test Pattern (TP)
RL with TP is similar to RL except that an internally generated 511 test pattern is used for data.
Figure 13, RL (Remote Loopback) with Test Pattern (TP)