5.2 LOOP (V.54 & TELCO) DIAGNOSTICSThe NetLink-T1™ offers three V.54 loop diagnostics and is com-patible with two Telco loop diagnostics. Use these diagnostics to testthe CSU/DSU and any communication links. These tests can be acti-vated via the software control port (See Section 3.2.3 SystemDiagnostics), via signals on the serial port interface or the front panelswitch.5.2.1 Operating Local Loopback (LL)The Local Loopback (LL) test checks the operation of the localNetLink-T1™, and is performed separately on each unit. Any datasent to the local NetLink-T1™ in this test mode will be echoed(returned) back to the user device (i.e., characters typed on the key-board of a terminal will appear on the terminal screen).To perform a LL test, follow these steps:1. Activate LL. This may be done in one of two ways:a. Enter Local Loop from the SystemDiagnostics/Statistics menu and toggle the until “LL” appears next to the Local Loop option.b. Toggle the front panel switch to the “Local” position.2. Verify that the data terminal equipment is operating properlyand can be used for a test.3. Perform a V.52 BER (bit error rate) test as described inSection 5.3. If the BER test equipment indicates no faults,but the data terminal indicates a fault, follow the manufactur-er’s checkout procedures for the data terminal. Also, checkthe interface cable between the terminal and the NetLink-T1.385.2.2 Operating Remote Digital Loopback (RL)The Remote Digital Loopback (RL) test checks the performance ofboth the local and remote NetLink-T1™, as well as the communicationlink between them. Any characters sent to the remote NetLink-T1™ inthis test mode will be returned back to the originating device (i.e, char-acters typed on the keyboard of the local terminal will appear on thelocal terminal screen after having been passed to the remote NetLink-T1™ and looped back).There are two Remote Loops that can be initiated from theNetLink-T1 unit: (1) V.54 Loop, and; (2) CSU Loop. The user canselect the type of loop that can be initiated from the SystemDiagnostics/Statistics screen or with Switch S1-1. When a loopbackis initiated this is the type of loop that the unit uses to loop up theremote unit. NOTE: The NetLink-T1 will respond to both loops regard-less of the state of the RDL Type.To perform an RDL test, follow these steps:1. Activate RDL. This may be done in two ways:a. Enter Remote Loop from the SystemDiagnostics/Statistics menu and toggle the until “RL” appears next to the Remote Loop option.;b. Set the front panel switch to ‘Remote’.37Figure 8. Local LoopbackEthernetDeviceEthernetDeviceReceiveRecoverClockingInternalClockingCable SpanT1/FT1 CSU/DSUModel 2720/IT1/FT1 CSU/DSUModel 2720/IClock/DataClock/DataClock/DataDataDataModel 2720/X Model 2720/ILLB InitiatedFigure 9. Local LoopbackEthernetDeviceEthernetDeviceReceiveRecoverClockingInternalClockingCable SpanT1/FT1 CSU/DSUModel 2720/IT1/FT1 CSU/DSUModel 2720/IClock/DataClock/DataClock/DataDataDataModel 2720/IRL InitiatedModel 2720/Ibbaax
