Octagon PC-510, Руководство пользователя

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5-11
PC-510 user’s manual Serial ports
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RS-485 network
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 communi-
cation, maintaining network registration, and providing housekeep-
ing tasks with other nodes. Remotes, however, cannot initiate a
communication. They can only respond to messages that are ad-
dressed to them from the host.
The following diagram demonstrates how a typical two-wire trans-
mission is implemented in the RS-485 network.
Figure 5-4 RS-485 two-wire (half duplex) transmission
RS-485
Node
T+
T-
R+
R-
Node 2
T+
T-
R+
R-
Node 3
T+
T-
R+
R-
Node 4
Not Terminated Not Terminated
PC-510
Terminated
Terminated
J5: COM6
Pin 11
Pin 17
RS-485 (+)
RS-485 (-)