Mocomtech CIM-550 - Manual (Page 110)

CiM-550 IP Enabled Satellite Modem Rev. 2
Theory of Operation CD/CIM550.IOM
84
6.3.2.2 A
SYNC
C
HARACTER
F
ORMATS
U
SING
1.5 S
TOP
B
ITS
For situations where an async character format using 1.5 stop bits cannot be avoided, set
the modem data rate to exactly twice the incoming baud rate. This uses twice as much
bandwidth, but will permit this mode of operation.
6.3.3 CLOCKING
When dealing with satellite modems, the subject of clocking can be a complex issue. This
section describes the various clocking options which are available with the CiM-550.
6.3.3.1 T RANSMIT C LOCKING
There are four transmit clocking modes in the CiM-550. EIA-422/449 signal mnemonics
will be used for illustration, but the description applies equally to V.35, and synchronous
EIA-232.
6.3.3.1.1 Internal Clock
In this mode, the modem, assumed always to be the DCE, supplies the clock to the DTE.
(The EIA-422/449 name for this signal is Send Timing, or ST.) The DTE then clocks
from this source, and gives the modem transmit data (Send Data, or SD), synchronous
with this clock. It is optional whether the DTE also returns the clock (Terminal Timing,
or TT) - the modem can accept if it is present, but uses ST if is not.
6.3.3.1.2 External Clock
In this mode, the modem expects to see the DTE provide the clock, so that it can phase-
lock its internal circuits. In this case, the modem does not provide any signal on ST, but
instead requires a clock signal on Terminal Timing (TT), synchronous with the data. If no
clock is present, an alarm will be generated.
6.3.3.1.3 Loop-Timed, RX=TX
In certain circumstances, a terminal at the distant-end of a satellite link may be required
to provide a clock to the DTE equipment which is locked to the receive satellite signal.
This is similar to Internal Clock mode, in that the modem will source Send Timing (ST)
to the DTE, but now the timing is derived from the demodulator. The DTE then clocks
from this source, and gives the modem transmit data (Send Data, or SD), synchronous
with this clock. It is optional whether the DTE also returns the clock (Terminal Timing,
or TT) - the modem can accept if it is present, but uses ST if it is not. If the demodulator
loses lock, the modem’s internal clock will be substituted, so an accurate and stable clock
is present on ST, rather than a clock which may jitter and wander in a random fashion.
6.3.3.1.4 Loop-Timed, RX<>TX (Asymmetric Loop Timing)
The CiM-550 incorporates circuitry which permits loop timing when the TX and RX data
rates are not the same. In this case the clock frequency appearing at ST will be whatever
the TX data rate is programmed to, but phase-locked to the demodulator’s receive symbol
clock. In all other respects the operation is the same as for ‘standard’ loop timing.