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2.5.1.4 External Clock for more than one Sub-channel
This option enables connection of more than one sub-channel to the line from the P.T.T. service.
Generally, the P.T.T. modems supply clock to every line – i.e. the P.T.T. has to provide the
clock. Since the modems are all from the same service, the clocks are synchronized. The
external buffer will compensate for phase differences between the clocks received from the
sub-channels. However, the external buffer option will not compensate for the clock differences,
if different clock sources are used.
2.5.2 RSD-20/X.21
The X.21 standard states that DCEs and DTEs will only have one clock signal (signal timing).
The direction of this clock is always from the DCE to DTE. This signal timing is used for the
transmit clock and receive clock.
According to this standard, four alternatives are available for the synchronization of the system
when working with the RSD-20/X.21:
a) Main channel is DTE (attached to a modem).
b)Main and all sub-channels are DCE (attached to DTEs).
c) Main channel is DCE (attached to a DTE and all sub-channels are DTE (attached to modems).
d) Main channel is DCE and sub-channels are mixed (DTEs and DCEs).
2.5.2.1 Main channel is DTE
(attached to a modem)
In this mode the RSD operates according to the signal timing of clock of the main channel
modem. DTEs attached to the RSD-20 sub-channels are synchronized to the mains clock.
DCEs attached to sub-channels are synchronized on their own signal timing. A special buffer,
integrated in the RSD-20/X.21, compensates for the phase difference between the modems of
the sub-channels and the main channel modem.
2.5.2.2 Main and all sub-channels are DCE
(attached to DTEs)
In this mode the RSD provides the clock to all ports attached to the main and sub-channels. The
RSD’s baud rate should be strapped to the appropriate system baud rate.
2.5.2.3 Main channel is DCE (attached to a DTE) and all sub-channels are DTE
(attached to modems)
In this mode the RSD’s baud rate is strapped to CLK-1. The DTE attached to the main channel
is synchronized to the RSD’s clock, the modems attached to sub-channels 2, 3, 4 should be
strapped to external clock, except for the modem attached to sub-channel 1 which should be
strapped to internal clock.
NOTE It is also possible to work with the RSD’s internal clock in this mode, but
CLK-1 is preferred.
INSTALLATION RSD-20 OPERATOR'S MANUAL
2.8