Premier Mounts CDM-600 Receive Clocking, Buffer Disabled RX Satellite, 10.3 X.21 Notes

CDM-600 Satellite Modem	Revision 7
Clock Modes and Drop and Insert (D&I)	MN/CDM600.IOM

10.2Receive Clocking

There are three receive clocking modes in the CDM-600, plus an additional setting used for Drop and Insert only – see later section.

10.2.1Buffer Disabled (RX Satellite)

When the buffer is disabled, the receive clock (Receive Timing, or RT) is derived directly from the demodulator, and hence will be subject to plesiochronous and Doppler offsets. In certain instances, this may be acceptable. There is still a minimum buffer in use to de- jitter the effects of removing overhead framing.

G.703: Applicable.

10.2.2Buffer Enabled, TX=RX (TX Terrestrial or External Clock)

In this instance, it is required that buffer be enabled, so that the clock and data appearing on Receive Timing and Receive Data (RT and RD respectively) are synchronous with the transmit clock or the external reference input. This is a relatively simple case, as the output clock for the buffer is derived directly from either ST, TT or the external source.

G.703: Applicable.

10.2.3Buffer Enabled, RX<>TX (TX Terrestrial or External Clock)

This is an uncommon case, where the receive data rate does not equal the transmit or external reference. The modem will generate a phase-locked buffer output clock which uses the selected reference, regardless of its frequency in relation to the receive data rate.

G.703: Applicable.

10.3X.21 Notes

For X.21 operation, use the RS-422 pins, but ignore Receive Clock if the Modem is DTE, and ignore Transmit clocks if the Modem is DCE.

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