Octagon Systems XE-700, Справочное руководство

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Figure 4–3 Typical RS–422 four-wire interface circuit
Receiver
Xmitter
110
Ω
100
Ω
TX –
RX +
TX +
RX –
Gnd
100
Ω
Receiver
Xmitter
110
Ω
RX –
TX +
TX –
RX +
RS–485
An application may implement a node as either the “host” node or as a “remote”
node in an RS–485 network. There can be as many as 32 nodes without any bus
repeaters in the network. A host is referred to as the node that initiates
communication; a remote is referred to as a node that is addressed by the host.
In any given communication sequence in an RS–485 network, there can only be one
host. The host is responsible for initiating communication, maintaining network
registration, and providing housekeeping tasks with other nodes. Remotes,
however, cannot initiate a communication. They can only respond to messages that
are addressed to them from the host.
The devices at each end of an RS–485 network must be terminated. Any node
located between the end points should not be terminated. The XE–700 SBC
optionally terminates with a 110 ohm resistor. Refer to Table 4–5 on page 43.
Figure 4–4 shows a typical RS–485 network. Note that for 2-wire RS–485 networks
the transmit and receive pairs must be connected together external to the XE–700
(TXD+ tied to RXD+, TXD– tied to RXD–).
The RTS* signal is used to control the transmitter and receiver in RS–485 mode.
The RTS* signal is controlled by the Modem Control Register bit 1 (MCR[1], which
is offset 0x04 from the UART base address). Writing MCR[1] to 0 (default state)
sets RTS* to an inactive state (RTS* = logic high) and DISABLES the RS–485
Transmitter and ENABLES the Receiver. Writing MCR[1] to 1 sets RTS* to an
active state (RTS* = logic low), and ENABLES the RS–485 Transmitter and
DISABLES the RS–485 Receiver.
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