Specifications

Maintenance PSM-500/500L/500LT SCPC Satellite Modem
Page 4-20 PSM-500/500L/500LT - Rev. 0.91
impossible. However the PSM-500 actually contains both its own and a copy of the PSM-4900
protocol, allowing the PSM-500 to accept and respond to PSM-4900 packets within the limitations
of the PSM-4900 capabilities. This allows mixed systems of 4900 and 500 series modems
without initially changing control software. The compatibility does not currently extend to
operation over the link on the MCC channel.
Is there a “compatibility” mode for the remote control binary packet protocol that looks
like that in the PSM-2100?
No. The new commands dictated a new structure to implement the significantly greater number of
commands available in the PSM-500. Many items such as the interface structure, data rates and
available options are so different that creating a compatible command set was impossible. The
packet structure itself is virtually identical though and in most cases the new command set can be
accommodated by a “driver” tailored to the PSM-500.
D. Data Interface
How do I make a cable to connect to my V.35 (or EIA-530) device?
See the “Installation” Chapter 2 and Appendix C on “Cabling Specifications” which shows how to
make cables to interface between the modem’s DB37 connector and other types of common
connectors used.
Where do I get a “Y” cable to implement 1:1 redundancy?
These may be purchased from Datum Systems or it is possible to build your own. The
connections are discussed in Chapter 2 “Installation” and shown in Appendix C, “Cabling
Specifications”.
Why do I keep getting “sync losses” on my link? Or why does a BERT test show “sync
losses”?
This is usually a sign that some section of the link has a clock or data inversion. See Chapter 4 of
the manual for “Loop” testing to try to determine where the problem is and correct either the
wiring or change the modem data or clock sense.
Why does the modem occasionally fail to operate with my DTE equipment, and to correct
it I have to invert the data or clock?
The only cause in an otherwise functioning modem for this symptom is that the differential
encoder/decoder is turned off. Modems use the differential encoding to determine the proper
relationship between the clock and data. If the encoder is turned off the modem has a possibility
of locking to a signal with the wrong phase. For all normal operation of the modem the Modulator
differential encoder and the Demodulator differential decoder must be “Enabled”.
The Turbo Product Codes (TPC) option does not use the differential encoder, and when it is
enabled the differential encoder and/or decoder is turned off and the option is removed from the
parameter matrix. Other modes also automatically control the differential encoder and decoder in
the PSM-500, but a linked modem may have the ability to turn it on or off.
How do I use the built-in Bit Error Rate Test (BERT) set?
See Maintenance Section 4.1.2 for information on using the built-in BERT. The PSM-500 BERT
now has the ability to be switched to look at the data line side.
Can I use the built-in Bit Error Rate Test (BERT) set to test the line or DTE side
equipment?
Yes with reservations. The BERT is designed to normally transmit and receive to the modem
side. New in the PSM-500 is the ability to electrically switch the direction that the BERT “looks”