Advantech AMT-73 Installation And Operation Manual page 52

should be greater than if it is being used as a Doppler Buffer to reduce the periodic clock slip
occurrences.
The length of the buffer is related to latency and data rate. It may not be appropriate,
for example, to choose a buffer length of 64 kbits if the data rate is 64 kbps, unless a
latency of 1 second is acceptable.
The Doppler buffer can be occasionally centered by invoking the IFBUFRESET command.
THIS COMMAND MAY CAUSE A MOMENTARY DISRUPTION OF MODEM TRAFFIC.
The flexibility of the AMT-70/73 clocking provides the following options:
TRANSMIT CLOCK OPTIONS
•
Transmit Internal Clock (SCT) – This is the normal operation, where the modem provides ST
to the DTE device, and the DTE provides TT in return (not always).
•
External Mode (Transmit Terrestrial Clock) (SCTE) – In this mode the DTE provides the
transmit clock, and the modem synchronizes to that clock.
•
RX Loop-Timing (SCR) – When selected the modem synchronizes its transmit clock with the
clock it receives from the demodulator. This avoids the possibility of clock slips of dissimilar
clocks (near end/far end).
•
RS530TFD – This option using clock recovery is used when the DTE does not return TT, and
therefore the modem has to recreate the modem's internal transmit clock from the data. This is
standard for the AMT-73L modem. It only operates on RS530/EIA449 data.
RECEIVE CLOCK OPTIONS
•
Doppler Buffer Mode – This Doppler buffer can be switched between AUTO mode and
Manual Mode. In AUTO mode the buffer selects what is considered a reasonable delay. In
Manual mode the user can specify a buffer length between 32 and 65536 bytes. To effectively
disable the buffer select RX Timing to DEMOD, select Manual mode for the buffer and enter a
32byte delay. NOTE after every demodulator reconfigure (RRESET command) the
IFBUFRESET command should be issued to reset the Doppler buffer fill level to mid-point.
•
Buffer Enabled, LOCAL, (SCT) Synced to Internal TX ST Clock – In this mode, the buffer is
enabled, the data from the demodulator is clocked into the buffer by the Demod Clock, but is
clocked out by the ST clock.
•
Buffer Enabled, DTE_TT, (SCTE) Synced to External TT Clock – In this case the TT clock is
used to transmit the data out of the Doppler Buffer.
•
Buffer Enabled, DEMOD, (SCR) Synced to Demodulator derived Reference – In this mode
the demodulator clock is de-jittered to remove high frequency jitter before clocking out of the
Doppler buffer. Low frequency jitter will not be filtered.
AMT-70/AMT-73/AMT-75 Installation and Operation
NOTE:
52