Allen-Bradley micrologix 1500, Руководство пользователя

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Publication 1764-UM001A-US-P
E-8 Understanding Communication Protocols
Line Drivers
Line drivers, also called short-haul “modems”, do not actually modulate the serial
data, but rather condition the electrical signals to operate reliably over long
transmission distances (up to several miles). Line drivers are available in Full- and
Half-Duplex models. Allen-Bradley’s AIC+ Advanced Interface Converter is a
Half-Duplex line driver that converts an RS-232 electrical signal into an RS-485
electrical signal, increasing the signal transmission distance from 50 to 4000 feet
(8000 feet when bridged).
DH-485 Communication
Protocol The information in this section describes DH-485 network functions, network
architecture, and performance characteristics. It will also help you plan and operate
the MicroLogix controllers on a DH-485 network.
DH-485 Network Description
The DH-485 protocol defines the communication between multiple devices that
coexist on a single pair of wires. DH-485 protocol uses RS-485 Half-Duplex as its
physical interface. (RS-485 is a definition of electrical characteristics; it is
not a
protocol.) RS-485 uses devices that are capable of co-existing on a common data
circuit, thus allowing data to be easily shared between devices.
The DH-485 network offers:
•
interconnection of 32 devices
•
multi-master (peer-to-peer) capability
•
token passing access control
•
the ability to add or remove nodes without disrupting the network
•
maximum network segment of 1219 m (4000 ft)
The DH-485 protocol supports two classes of devices: initiators and responders.
All initiators on the network get a chance to initiate message transfers. To
determine which initiator has the right to transmit, a token passing algorithm is
used.
The following section describes the protocol used to control message transfers on
the DH-485 network.