Practical Industrial Data Networks:Design, Installation and ...

9Data Highway Plus/DH485 overviewObjectivesWhen you have completed study of this chapter, you will be able to:• Describe the main features of Data Highway Plus• Fix problems with:• Cabling• Grounding/earthing• Shielding• Terminations• Token passing timing9.1 Allen Bradley Data Highway (Plus) protocol9.1.1 Overview of Allen Bradley protocolThere are three main protocol standards used in Allen Bradley data communications:The Data Highway protocolThis is a local area network (LAN) that allows peer-to-peer communications up to 64nodes. It uses a half-duplex (polled) protocol and rotation of link mastership. It operatesat 57.6 kbaud.The Data Highway Plus protocolThis is similar to the Data Highway network although designed for fewer PCs andoperates at a data rate of 57.6 kbaud. This has peer-to-peer communications with a tokenpassing scheme to rotate link mastership among the nodes connected to that link.Note that both protocol standards implement peer-to-peer communications through amodified token passing system called the floating master. This is a fairly efficientmechanism as each node has an opportunity to become a master at which time it can