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Introduction
The CDM-500 includes, as standard, a universal data interface which eliminates the need
to exchange interface cards for different applications. The interfaces offered include
RS422 (EIA530) DCE, V.35 DCE, synchronous RS232 DCE, and X.21 DTE and DCE.
The internal software is both powerful and flexible, and permits the user to store, and
then retrieve, up to 10 different modem configurations. The unit includes many test
modes and loopbacks for rapid verification of the correct functioning of the unit. Of
particular note is the IF loopback, which permits the user to perform a quick diagnostic
test without having to disturb external cabling. During the loopback, all of the receive
configuration parameters are temporarily changed to match those of the transmit side.
When normal operation is again selected, all of the previous values are restored.
A perennial problem is that of the IF port impedance. Whilst many users select the
universal standard of 50 ohms, there are many users who are still using 75 ohms. In the
past this has presented difficulties for the manufacturer, who has traditionally had to offer
both options. The CDM-500 eliminates this problem, by offering both impedances in the
same unit. The user simply selects from the front panel which impedance he wishes to
use.
In order to facilitate network management, the CDM-500 incorporates EDMAC, an
acronym for Embedded Distant-end Monitor And Control. In this mode, an additional 5%
overhead is combined with the traffic data, which permits M&C information to be added
(transparently to the user), allowing access to the distant-end modem.
This mode does not require any additional cabling at either the local or distant-end
Modems - access to EDMAC is via the standard M&C control port. Full monitor and
control is possible, and importantly, the on/off status of the carrier at the distant-end
carrier can be controlled.
The CDM-500 has been designed as a companion product for the Comtech EFData line
of RF Transceivers. As such, the Modem incorporates an FSK serial link which can be
activated on the Receive IF port, for the purpose of communicating with a Transceiver, if
connected. In this manner, a user may monitor, configure, and control the Transceiver,
using the front panel display and keypad of the Modem. The EDMAC channel may also
be used to convey M&C data to a Transceiver at the distant end of a satellite link, if it is
connected to a CDM-500.
1–2
CDM-500 Closed Network Satellite Modem
Rev. 1.3
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