Octagon XE-800, Справочное руководство

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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–800 SBC
optionally terminates with a 100 ohm resistor. Refer to Table 5–4 on page 46.
Figures 5–4 and 5–5 show typical RS–485 networks. Note that for 2-wire RS–485
networks the transmit and receive pairs must be connected together external to the
XE–800 (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.
Figure 5–4 Typical RS–485 4–wire interface circuit
Xmitter
TX +
TX –
Receiver
RX +
RX –
DI
DE
RO
RE
100
Ω
100
Ω
100
Ω
Xmitter
Receiver
DI
RO
RE
Xmitter
Receiver
DI DE RO RE
Receiver
RX
RX
RO
RE
Xmitter
TX
DI
DE
100
Ω
TX +
Figure 5–5 Typical RS–485 2–wire half duplex interface circuit
DI
DE RO
RE
Xmitter
TX +
TX –
Receiver RX +
RX –
DI
DE
RO
RE
100
Ω
100
Ω
Receive
Receiver
Receiver
Xmitter
Xmitter
Xmitter
TX +
RX +
RX –
DI
DE
RO
120 RE
DI DE RO RE
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