Verilink PRISM 4101 (34-00230) Product Manual - Manual (Page 45)

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When set to Unidirectional, the NET receive data is looped back to the NET as
NET transmit data and continues to pass through the data port to the DTE as
shown in Figure 4-5. Transmit data from the DTE is terminated.
Figure 4-5 Unidirectional Loop
When set to Bidirectional, the NET receive data is looped back to the NET as NET
transmit data as shown in Figure 4-6. Transmit data from the DTE is looped back
through the data port as receive data to the DTE.
Figure 4-6 Bidirectional Loop
During dial backup, the loop mode field is ignored and all loops are unidirectional.
This allows testing of the DDS interface during dial backup without disrupting the
DTE/DBU data path.
There are four loop types that can be activated from the PRISM 4101. They are
line loop, data loop, V.54 loop, and local loop.
Line Loop. Occurs at the DDS network interface and is activated by the reversal of
the simplex, 20 mA sealing current. This is a unidirectional loop that ignores the
unit transmit data and retransmits the received DDS data. Receive data is
unaffected and circuits DSR and CD are optionally forced Off.
Data Loop. Occurs at the data port (DTE) interface and is activated when the unit
receives alternating loop codes in the network receive data stream. Technically, it is
activated by the receipt of at least four consecutive loop commands and remains
looped as long as each third pattern byte is the loop command. It returns to normal
operation after at least four pattern bytes that are not the loop command. This is a
unidirectional loop that retransmits the PRISM 4101 received data on the unit
transmit data including the remapped loop code. Receive data is unaffected (but
includes the modified loop codes) and circuits DSR and CD are Off.
V.54 Loop. Occurs at the DTE data port interface and is activated upon receipt of
inband V.54 loop codes for at least two seconds followed by all ones in the
network receive data stream. This loop is unidirectional and retransmits the PRISM
4101 received data on the unit transmit data including the remapped loop code.
Receive data is unaffected (but includes the modified loop codes) and circuits DSR
DTE NET
Line Loop
DTE NET Local Loop
✍
DTE NET Line Loop
DTE NET
Data Loop
V.54 Loop