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A.1.4.3.8
Non-ASYNC Operation
The ASYNC interface has pass-through capability. If ASYNC is turned off in the
Configuration Interface menu, then a standard G.703, EIA-422, or V.35 interface is
selected. The modem will operate as a standard G.703, EIA-422, or V.35 interface with
no overhead. Instead of changing jumpers on the interface PCB to change polarities for
various signals, polarity inversion is available in the Utility Interface menu for the
following signals:
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A.1.4.3.9
ASYNC Channel EIA-485 2- and 4-Wire Operation
The ASYNC interface is compatible with either a 2- or 4-wire interface for the EIA-485
channel. The 2- or 4-wire operation is selected via the front panel.
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•
The receive modem controls the transmit and receive data in two wire operation. EIA-485
(2 wire) must be turned on in the UTILITY/INTERFACE menu.
The output driver will be turned on when the receive demux on the overhead card detects
ASYNC data. The input driver will be turned off at the same time. This prevents the
receive data from transmitting back to the originating modem.
For EIA-485 two wire, loop the highs (TX+, RX+) and then loop the lows (TX-, RX-) at
the connector for two wire operation. these loops are not accomplished inside the modem.
EIA-485 four wire circuits will become 2 wire when you attach any device that
is EIA-485 two wire.
For example: The modem Async overhead at a site that has an RF terminal, must be set to
TX EIA-485 (two wire) because the RF terminal is only EIA-485 two wire.
A–26
Send Data (SD)
Terminal Timing (TT)
Request to Send (RS)
Receive Data (RD)
Receive Timing (RT)
Receiver Ready (RR)
Data Mode (DM)
Monitor and Control (MC)
Send Timing (ST)
In the 2-wire mode, the EIA-485 receivers are disabled whenever the data is to be
transmitted down the 2-wire interface.
In the 4-wire mode, the receiver is always on.
SDM-300A Satellite Modem
Rev. 4
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