Comtech EF Data SDM-309B - Manual (Page 122)

SDM-309B Satellite Modem M&C and Interfaces
MN/U-SDM309B Rev. # 4-4–45
4.2.3 External Channel Unit Interface
Refer to Appendix C for a complete description of the External Channel Unit Interface
and system integration. This is an optional mode of operation, making the SDM-309B
modem compatible with a variety of stand alone channel unit equipment for IBS service.
Please refer to Appendix C for full details.
4.3 Interface Clocking Options
Clocking of the data from the terrestrial circuits to the satellite and visa versa will
depend upon the application. The most common options and recommended
configurations are described in the following Section.
4.3.1 Master/Slave
This application is used where the far end earth station does not have local access to a
high stability reference clock, or where it is not required to synchronize with a local
clock.
The disadvantage of the master/slave application is that the signal received at the slave
station is subject to doppler shift.
Note: The length of the buffer at the master end will need to be twice the length that is
normally required since it will be compensating for the doppler shift on the outward and
return paths.
4.3.1.1 Master/Slave RS422 or V.35
* Clock must be looped externally at slave end.
RS422 Clock Loopback V.35 Clock Loopback
Join RT-A to TT-A Join SCR-A to SCTE-A
(pin 8 to pin 17) (pin V to pin U)
Join RT-B to TT-B Join SCR-B to SCTE-B
(pin 26 to pin 35) (pin X to pin Y)
Note: By wiring the interface for clock turnaround, the impedance will be reduced. This
generally will cause no problem providing the cable length to the final terminal
equipment is not excessive. Selecting no loading at the terminal equipment will ensure
correct line matching.
Modem Settings (Master)
TX_CLOCK TX_TERR
BUF_CLK TX_TERR
RX_CLOCK NORMAL