1766-RM001A-EN-P MicroLogix 1400 Programmable Controllers ...

588 Protocol ConfigurationDH-485 CommunicationProtocolThe information in this section describes the DH-485 network functions,network architecture, and performance characteristics. It also helps youplan and operate the controller on a DH-485 network.DH-485 Network DescriptionThe DH-485 protocol defines the communication between multipledevices that coexist on a single pair of wires. DH-485 protocol usesRS-485 Half-Duplex as its physical interface. (RS-485 is a definition ofelectrical characteristics; it is not a protocol.) RS-485 uses devices that arecapable of co-existing on a common data circuit, thus allowing data to beeasily shared between devices.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.)The DH-485 protocol supports two classes of devices: initiators andresponders. All initiators on the network get a chance to initiate messagetransfers. To determine which initiator has the right to transmit, a tokenpassing algorithm is used.The following section describes the protocol used to control messagetransfers on the DH-485 network.DH-485 Token RotationA node holding the token can send a message onto the network. Eachnode is allowed a fixed number of transmissions (based on the TokenHold Factor) each time it receives the token. After a node sends amessage, it passes the token to the next device.The allowable range of node addresses is 0…31. There must be at leastone initiator on the network (such as a MicroLogix controller, or an SLC 5/02 or higher processor).Publication 1766-RM001A-EN-P - October 2008