Rockwell Automation 1761-HHP-B30 MicroLogix 1000 with Hand-Held Programmer (HHP) - Manual (Page 435)

Appendix D
Understanding the Communication Protocols
D–9
The information in this section describes the DH-485 network functions,
network architecture, and performance characteristics. It also helps you plan
and operate the MicroLogix 1000 on a DH-485 network.
Important: Only Series C or later MicroLogix 1000 discrete controllers and
all MicroLogix 1000 analog controllers support the DH-485
network.
DH-485 Network Description
The DH-485 protocol defines the communication between multiple devices
that co-exist on a single pair of wires. This 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 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 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.
DH-485 Token Rotation
A node holding the token can send any valid packet onto the network. Each
node is allowed only one transmission (plus two retries) each time it receives
the token. After a node sends one message packet, it attempts to give the
token to its successor by sending a “token pass” packet to its successor.
If no network activity occurs, the initiator sends the token pass packet again.
After two retries (a total of three tries), the initiator attempts to find a new
successor.
The allowable range of the node address of an initiator is 0 to 31. The
allowable address range for all responders is 1 to 31. There must be at least
one initiator on the network.
DH-485 Communication
Protocol