2–18
DECbridge 500/600 Problem Solving
2.4.6
Performing Loopback Testing
If diagnostic results indicate a port interface problem, you can perform a loopback
test on that interface to help determine if the fault is in the bridge itself, in the inter-
face cable, or in the device connected to the bridge. Special loopback connectors
supplied with the bridge must be installed on each port. Each connector routes the
transmitter output of that port back to the receiver input.
NOTE
When performing port loopback testing, always place
loopback connectors on all the interface ports.
During loopback testing, if the FI port fails self-test, the FI_BAD indicator will turn
on. If any of the NI or NI(3) ports fail self-test, the NI_BAD indicator will turn on.
(Note that on a DECbridge 500 series unit, both the AUI and ThinWire interfaces are
tested but only the selected interface has any effect on the NI_BAD indicator.). If an
optional optical bypass switch is installed, the self-test diagnostics also check the
relay driver on the FI module.
NOTE
If you have an optional optical bypass relay installed,
this test will switch the relay. This could adversely affect
the bypassed network.
2.4.6.1
Preparing the Unit for Loopback Testing
Prior to installing the loopback connectors:
1. Disconnect the bridge power cord from the ac outlet.
2. Set the Enable External Loopback switch (switch 7) to the ON (down)
position.
