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P300H P300 Series Modem Installation and Operating Handbook Page 63
Tx clock mode: [?]
1=Tx Clock In 2=Internal 3=Rx ref(=Sat)
Change, Tx, Clocking Menu
6.6.7 Change, Tx, CLOCKING Menu
Paradise Datacom products have always provided a `Data Marginal` warning where the incoming data
phase is monitored with respect to the clock. The Modem shows a `Data Marginal` warning on the front
panel if the data changes at the instant where it should be stable (and is sampled by the modem) which
would otherwise cause data errors with no fault indication.
On the P300 this is taken a stage further and if the data phase is incorrect with respect to the normal clock
edge, then the modem automatically switches to the other clock edge to sample the data. A Data Marginal
warning is only issued momentarily as the phase switches. If regular Data Marginal warnings appear (for
example in the traffic log) then the Tx data and clock phase are drifting (causing the modem to switch
repeatedly) and you will be getting regular data errors. This is not a modem fault, it is a warning of a
situation causing data errors of which you would otherwise be unaware ! For this reason it is not
possible to simply disable the Data Marginal alarm, instead you must investigate the external equipment
to find why the Tx clock/data phase is changing.
The Tx clocking menu is as follows:
This selects the clock source used to generate the signal towards the satellite. The options are:
TX CLOCK The external clock supplied on the interface `Clock In` line will be used (see
interface card details in appendix A for line names). Should this clock input fail the
Modem will switch to an internal backup clock (as used when INTERNAL is
selected) to maintain the carrier, distant demodulator lock, and if a framing
overhead is active, the Backward Alarm, Aux, and ESC channels.
INTERNAL The Modem outputs a clock on the `Int Tx clock out` line (see interface card
details in appendix A for line names) for use by the external equipment. Unlike the
previous Paradise products, there is no requirement to loop this clock back into
the `Clock In` line, on the P300 Modem it is routed internally.
The clock is normally generated from the internal frequency reference of ±1PPM.
However if a Station Clock of 10MHz (only) is applied to the unit, then under the
Rx Clocking screens it is possible to force the modem to replace the internal
1PPM reference with the 10MHz Station Clock as the reference for all clocks
and IF synthesizers which normally use the internal reference. If this is the case
then the Internal Clock would be generated from the 10MHz station clock.
RX REF(=???) As INTERNAL except the Tx clock is generated from the Rx output clock.
Ref(=???) shows the current Rx clock setting from which the Rx output clock is
itself being generated. This option is only of any practical use when the Rx
Clocking is set to Satellite or Station Clock. When the Rx Clocking = Satellite
this is commonly called `Loop Timing`.
Note: it uses the Rx output clock, not necessarily the clock from the satellite,
ie the clock used depends on the Rx Clocking setting. This means for example
that if the Rx Clocking = Station Clock, then setting Tx Clocking =Rx ref(=Stn)
would slave the Tx Clock ultimately from the Station Clock (whatever the Station
Clock frequency). Similarly setting Tx Clocking = Rx ref(Sat) (ie with Rx Clocking
= Satellite) would slave the Tx Clock to the incoming satellite clock (which is what
most other modem manufacturers means by Tx=Rx). In all cases if the clock
selected by the Rx Clocking fails, then the backup receive clocks (as defined in