Comtech EF Data SDM-300A - Manual (Page 292)

SDM-300A Satellite Modem Revision 6
ASYNC/AUPC Interface MN/SDM300A.IOM
11–10
11.3.8 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:
• 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)
11.4 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.
• 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.
• 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.