Figure 5–3
Typical RS–422 four-wire interface circuit
Receiver
Xmitter
100
Ω
100
Ω
TX –
RX +
TX +
RX –
Gnd
100
Ω
Receiver
Xmitter
100
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; while 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 2060 CPU Card
optionally terminates with a 100 ohm resistor. Refer to Table 5–4 on page 51.
Figures 5–4 and 5–5 show typical RS–485 networks.
RS–485 uses the RTS signal to control the transmitter. RTS must be set to “1” to
receive characters, and set to “0” to transmit characters.
Figure 5–4
Typical RS–485 4–wire interface circuit
Xmitter
TX +
TX –
Receiver
RX +
RX
–
DI
DE
RO
RE
100
100
Ω
100
Ω
Xmitter
Receiver
DI
DE
RO
R
Xmitter
Receiver
DI
DE
RO
RE
Receiver
RX
RX
RO
RE
Xmitter
TX –
DI
DE
100
Ω
TX +
54
