40
CSU/DSU MS/DBU
LOCAL LOOP (RT)
Fig. 4-17. Local Loop (RT).
3=DTE ONLY
The DTE ONLY test provides a method for testing both the DTE
interface drivers and receivers of the local CSU/DSU MS/DBU plus its loop
transmitter and receiver. For this test, the loop transmit data is connected to
the loop receive data at a point close to the physical network interface. The
data is then sent back towards the DTE. The transmit circuit to the network is
terminated in a zero condition for this test. A block diagram illustrating the
loopback point and the signal paths for this test is shown in
Fig. A-3
in
Appendix A
.
Test patterns from an external BERT tester are routed through the DTE
interface section of the CSU/DSU MS/DBU and then to the output of the
loop-transmitter section, where the signal is encoded for transmission.
Instead of being coupled onto the physical transmit circuit of the network,
the output of the loop transmitter is coupled back to the loop receiver input
where the signal is then decoded and returned to the BERT tester where the
serial receive data stream is checked for any bit errors.
This test is used to verify proper operation of both the DTE and loop-
interface sections of the local CSU/DSU MS/DBU.
Summary of Contents for CSU/DSU MS/DBU
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