Maintenance
PSM-500/500L/500LT SCPC Satellite Modem
Page 4-2
PSM-500/500L/500LT - Rev. 0.91
transmit a data signal at one end of the link and sequentially set each of the loop-back options.
Proper reception of the loop-back data verifies all components between the source and the loop.
The simplified diagram below shows the location of the PSM-500’s built in loop-back functions
within the link from one end of a link to the other. The advantage to having these functions built-in
is that they are electronically programmable without having to disconnect existing cabling to
connect equipment that must be available for testing.
Each of these loop-back modes are individually programmable at the modem front panel or
remote control interface. More detail on each of the typical loop back uses is given below.
•
Near End Terrestrial Data Loop-Back
: This will be the closest loop-back to the DTE or
BERT. If data is returned and received properly it indicates that the DTE wiring and
connection to the modem are correct. Note from the position of the built-in BERT in the
diagram above that this test requires an external source of data. The PSM-500 can
individually set the data path terrestrial side loop-back and the data path satellite side loop-
back in
<Int’f: Test - Ter Loopbck>
and
<Int’f: Test - Sat Loopbck>
respectively.
•
Near End IF Loop-Back
: This second loop-back will verify the modem transmit data signal
processing, modulation, demodulation, receive signal processing, and connection to the
receive interface. The PSM-500 sets the IF loop-back in
<Dem: Test - IF Loopbck>
. Note in
this test that the near end satellite loop-back function should not be enabled whether using
the internal or an external BERT.
•
Far End Satellite Data Loop-Back
: This will test most of the satellite link as well as the
functions checked in test 2 above. The signal is sent over the satellite (or test setup) and is
looped back at the satellite side of the data interface on the far end. This tests both modems,
the satellite link and originating end wiring. The PSM-500 sets the satellite side loop-back in
<Int’f: Test - Sat Loopbck>
. Note in this test that the near end satellite and IF loop-back
functions should not be enabled whether using the internal or an external BERT. Setting this
mode slaves the modulator timing to the demod timing and the FIFO buffer remains engaged
if enabled.
In this type of testing an external BERT is typically set to provide a terminal timing output, while
the connected modem is set to use the terminal timing signal as the transmit bit rate clock
source. This modem can alternatively be set to use its internal clock for the transmit clock timing
DTE
Or
BERT
Near End Terrestrial.
Data Loop-Back
Near IF
Loop-Back
Far End Satellite
Data Loop-Back
Satellite
Link
Near Modem
Far Modem
Built-in BERT is
located here.
Near End Satellite.
Data Loop-Back
