Rockwell Automation 1770 DH,DHII,DH485 CABLE - Manual (Page 13)

DH and DH+ Overview
Chapter 1
1-3
Unlike a master/slave relationship, a floating master relationship does not
require the current master to poll each node to grant permission to transmit
information. This results in less overhead per transaction and a more efficient
network.
On DH, 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 network can communicate directly
with nodes on a DH+ network. Nodes on DH networks, however, cannot
communicate directly with nodes on DH II networks.
To communicate between nodes on a DH network and a DH+ network, use the
1785-KA Data Highway/Data Highway Plus Communication Adapter Module.
Refer to the Data Highway Local Area Network Overview (publication
1770-2.39) 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.