Maintenance M7/M7L/M7LT Modular Satellite Modem
4-18 M7/M7L/M7LT - Rev. 0.05
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 M7.
4.7.5. Data Interface
4.7.5.1. 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.
4.7.5.2. 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”.
4.7.5.3. 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.
4.7.5.4. 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 M7, but a
linked modem may have the ability to turn it on or off.
4.7.5.5. 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 M7 BERT now has
the ability to be switched to look at the data line side.
4.7.5.6. 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 M7 is the ability to electrically switch the direction that the BERT “looks” toward the line
side. However, because of the hard wiring of the interface the pinout is fixed as a DCE device. See
Maintenance Section 4.1.2 for information on using the built-in BERT.
4.7.5.7. Why doesn’t my 1:1 redundant switch on certain alarms?
The 1:1 redundancy logic is programmable on two levels. First is the <Unit: Redundcy – SW Rqst>
parameter which selects whether a switch is requested on all alarms, alarm A and/or alarm B. If it is
set to any options but “On All Alarms” then the particular alarms that are summed into the A and B
alarms are themselves programmable, creating the second level. See the discussion in section for
more information.
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