Specifications

Chapter 5 Troubleshooting and Diagnostics FOM-40 Installation and Operation Manual
5-4 Internal BERT
Local Digital Loopback (DIG)
The local digital loopback (DIG) test allows the operator at the remote end to check
the performance of the local and remote FOM-40 units, and their connecting lines.
The DIG test loops the received data back to the remote FOM-40 (see Figure 5-3).
The local digital loopback test is equivalent to activating the remote loopback from
the remote FOM-40.
Local
DTE
Remote
DTE
Link
Interface
Link
Interface
Transmit
DIG
Pressed
Receive
Receive Transmit
Data
Data
Clock
Clock
Local FOM-40 Remote FOM-40
Figure 5-3. Local Digital Loopback

To activate the local digital loopback:
• Press the DIG pushbutton on the local FOM-40 front panel.
The TEST LED lights up.
5.2 Internal BERT
FOM-40 has a built-in BERT circuit consisting of a pattern generator and a pattern
tester. This circuit acts in conjunction with the diagnostic loops and a remote BERT
to verify normal system operation and isolate faulty equipment in the event of
system failure. The pattern transmitted is a RAD proprietary, pseudo-random
pattern.
The FOM-40 built-in BERT circuit can be used for the following tests:
• Modem Self-Test: the pattern transmitted by the pattern generator is looped
back to the pattern tester for comparison (see Figure 5-4).
• Two-BER Test: the pattern transmitted by the BERT is received by another
FOM-40 unit (see Figure 5-5).