Comtech EF Data CDM-550T - Manual (Page 31)

CDM-550T Satellite Modem Revision 3
Physical Description MN/CDM550T.IOM
4.3 Rear Panel
External cables are attached to connectors on the rear panel of the CDM-550T. These comprise
the IEC line input connector, the Rx and Tx IF connectors, the Data connector, Alarms
connector, Remote Control connector, and Auxiliary Serial connector.
The IEC line input connector contains the ON/OFF switch for the unit. It is also fitted with two
fuses - one each for line and neutral connections (or L1, L2, where appropriate). These are
contained within the body of the connector, behind a small plastic flap.
• For 230 volt AC operation, use T0.5A, (slow-blow) 20mm fuses.
• For 115 volt AC operation, use T1A fuses, (slow-blow) 20mm fuses.
IMPORTANT
For continued operator safety, always replace the fuses with the correct type
and rating.
The IF port connectors are both a 50
Ω BNC female type. 75Ω cable connectors (male) will have
no problem mating with this 50
Ω type.
The Data connector is a 25 pin ‘D’ type female (DB25-F). This connector conforms to the RS-
530 pinout, which allows for connection of different electrical standards, including RS-422, V.35,
and RS-232. Note that it is the responsibility of the user to provide the appropriate cables to
connect to this RS-530 connector. A shielded 25 pin ‘D’ type provides a very solid solution to
EMC problems, unlike the sometimes used V.35 Winchester connector. The pinout for the RS-
530 connector is provided in the next section.
Note that the currently selected interface type is indicated by a small orange LED which is
located immediately above the connector. This provides an easy visual indication to anyone
mating a connector at the rear of the unit.
The Alarms connector is a 15 pin 'D' type male (DB15-M). This provides the user with access to
the Form-C relay contacts which indicate the fault status of the unit. These are typically
connected to an external fault monitoring system, often found in satellite earth stations. In
addition, the receive I and Q demodulator samples are provided on this connector. Connecting
these signals to an oscilloscope in X,Y mode will provide the receive signal constellation
diagram, which is a useful diagnostic aid. A pin is also provided which can mute the transmit
carrier. This requires that the pin be shorted to ground, or a TTL ‘low’, or an RS-232 ‘high’
signal be applied.
As an aid to antenna pointing, or for driving step-track equipment, an analog AGC signal is
provided on a pin of this connector. The demodulator incorporates three separate AGC control
loops, one of which is analog, and two of which are entirely digital. The first of these loops keeps
the signal level constant at the input to the flash A/D converter in the final IF stage. This loop has
a limited dynamic range (~ 35 dB) and operates on the total power within the IF bandwidth
(which varies with data rate). The characteristics of this control voltage are shown at the rear of
the specifications section.
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