Mocomtech CIM-550 - Manual (Page 111)

CiM-550 IP Enabled Satellite Modem Rev. 2
Theory of Operation CD/CIM550.IOM
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6.3.3.2 R ECEIVE C LOCKING
There are three receive clocking modes in the CiM-550.
6.3.3.2.1 Buffer Disabled
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, and in the case of Loop Timing, it is
essential that the buffer is disabled (which is done automatically).
6.3.3.2.2 Buffer Enabled, RX=TX
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. This is a relatively simple case, as the output clock for the buffer is
derived directly from either ST or TT.
6.3.3.2.3 Buffer Enabled, RX<>TX
This is an uncommon case, where the receive and transmit data rates are not equal. The
modem will generate a phase-locked buffer output clock which uses the transmit clock,
regardless of its frequency in relation to the receive data rate.
6.3.3.3 X.21 N OTES
For X.21 operation, use the EIA-422/530 pins, but ignore Receive Clock if the Modem is
DTE, and ignore Transmit clocks if the Modem is DCE.
6.3.3.4 L OOP T IMING W ITH S YNC EIA-232
The CiM-550 distinguishes between synchronous and asynchronous EIA-232 by
detecting clock activity on the TX Clock pin of the interface. If no clock is detected, it is
assumed that the mode is asynchronous. Therefore, if loop timing is employed in a
synchronous EIA-232 application, it is essential to provide an external loop between the
ST and TX clock pins. If this is not done, the modem will assume an async mode, which
is not compatible. The loop should be placed between pin 15 and pin 24 on the 25 pin ‘D’
type interface.