Maintenance4-73160-A2-GB22-10 December 1996Starting a Payload LoopbackThe Payload Loopback command (PLB) loops thereceived signal on the network interface back to thenetwork. The signal is looped back as close to the DSX-1Drop/Insert port (DTE) as possible (after it has passedthrough the framing circuitry of the DSU/CSU).Therefore, framing CRCs and BPVs are corrected. If bitstuffing is enabled, pulse density is enforced.To perform a Payload loopback,1. Press F2 to select Test from the top-level menuscreen.F1DS U E SFStat Test CnfigF2 F32. From the Test screen, press F2 to select Lpbk.F1Test:Rlpbk Lpbk PtrnsF2 F33. From the Loopback screen, press F3 to selectPLB.F1Loopback:Abort LLB PLBF2 F3Test Started appears on Line 2. If a Payloadloopback is already in progress, the AlreadyActive message appears. If an invalid combinationof loopbacks is in progress, the error messageInvalid Test Combo appears (see Table 4-2 forvalid loopback test combinations).Starting a DTE LoopbackThe DTE Loopback command (DLB) loops thereceived signal on the DSX-1 Drop/Insert port (DTE)back to the DTE without change.A DTE loopback can be activated and deactivatedbased on the state of an external contact on the DSX-1Drop/Insert port (DTE). If this option is enabled with theDTE Interface configuration option Extrn DLB, closingthe contact (relay contact sense and relay contact return)activates a DLB. The DLB remains active until thecontact is opened. Refer to the DSX-1 Port DTE Interfacesection in Appendix D, Pin Assignments, for moreinformation.
