ALLEN BRADLEY 1747-L551 PLC Processor

The information in this appendix will help you plan, install, and
operate the SLC 500 in a DH-485 network. This chapter also
contains information that describes the DH-485 network functions,
network architecture, and performance characteristics. It also
covers:
• DH-485 network description
• DH-485 network protocol
• DH-485 token rotation
• DH-485 network initialization
• devices that use the DH-485 network
• 1747-AIC isolated link coupler for DH-485
• example system configuration
(includes 1761-NET-AIC advanced interface converter)
• important planning considerations
• installing the DH-485 network
DH-485 Network
Description
We have designed the DH-485 network to pass information between
devices on the plant floor. The network monitors process
parameters, device parameters, device status, process status and
application programs to support data acquisition, data monitoring,
program upload/download and supervisory control.
The DH-485 network offers:
• interconnection of 32 devices
• multi-master capability
• token passing access control
• the ability to add or remove nodes without disrupting the network
• maximum network length of 1219 m (4000 ft)
DH-485 Network Protocol
The following section describes the protocol used to control message
transfers on the DH-485 network. The 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.
Publication 1747-6.2