Black Box IC171C - Manual (Page 10)

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CHAPTER 2: Introduction
This interface is ideal for multi-drop or network environments. The RS-485
tri-state driver (not dual-state) will allow the electrical presence of the driver
to be removed from the line. The driver is in a tri-state or high-impedance
condition when not active. Only one driver may be active at a time and the
other driver(s) must be tri-stated.
Some communication software packages refer to RS-485 as RTS enable or
RTS block mode transfer.
RS-485 can be cabled in two ways, 2- and 4-wire mode. Two-wire mode does
not allow for full-duplex communication. It requires that data be transferred
in only one direction at a time and the two transmit pins be connected to the
two receive pins (Tx+ to Rx+ and Tx- to Rx-). Four-wire mode will allow full-
duplex data transfers. RS-485 does not define a connector pin-out, modem
control signals, or a physical connector.
Figure 2-2. RS-485 Two-Wire Connection.
TX+
TX-
RX+
RX- RX+
RX-
TX+
TX-
TX+
TX-
RX+
RX- RX+
RX-
TX+
TX-