Rockwell Automation 1770 DH,DHII,DH485 CABLE - Manual (Page 75)

DH II Overview
Chapter 4
4-4
DH II uses token-passing protocol to allow nodes on the network to transmit
messages over the cables. With token-passing protocol, only the node
possessing the token can transmit messages. As long as a node possesses the
token, it is the master.
When a node has sent all of its messages or used all of its token-hold time, it
passes the token to the node with the next highest address. Token passing
continues in this manner until the token is passed to the node with the lowest
address. When the node with the lowest address is finished with the token, the
cycle begins again.
On DH II, the interface modules control access to the network locally. This
means if one module faults, the other modules continue to communicate on the
network.
Through interface modules, nodes on a DH II network can communicate with
nodes on a DH+ network.
To communicate between nodes on a DH II network and a DH+ network, use
1779-KP5 Data Highway II/Data Highway Plus Interface Module. Refer to the
Data Highway II Local Area Network Overview (publication 1779-2.10) for
more information.
A message from a node on one network to a node on another network uses
message transmission time in both networks. To achieve the best network
performance, configure each network so that a minimum of messages cross
between them. You can do this by grouping stations that need to frequently
communicate with each other on the same network.
In many applications, nodes on a DH II network need to communicate with
devices such as:
computers
color graphic terminals
dumb terminals
robots
computerized numerical controls (CNC)
motion controllers
How Nodes Communicate
on DH II
Communication Between
Networks
Communication with
Synchronous and
Asynchronous Devices