Media Access Method; Bridges To Other Data Highway Ii Links - Allen-Bradley 1779-KFMR User Manual

Chapter 2
Introducing the 1779 KFM Interface

Media Access Method

Bridges to Other Data Highway II
Links
2 10
Because the Data Highway II link has a carrier-band bus medium, only
one node can transmit at a time. However, each node in a Data
Highway II link has a regularly scheduled opportunity to access the
medium to transmit messages to other nodes.
This peer-to-peer communication between nodes is provided by the Data
Highway II link through token passing. The token passing is
self-initiating. The nodes form a logical ring. The token is passed around
the ring to each node on a regular schedule, thereby making throughput
and response time predictable. While a node possesses the token, it is the
master. Only the master can initiate communication. After a node has
had the opportunity to transmit messages for the prescribed period of
time, it passes the token on to the next node.
At this time, the Data Highway II network does not have bridges to join
Data Highway II links. However, the Data Highway II network was
designed with the flexibility to accommodate bridges at a later data. Any
discussion of bridges in this manual is only in regard to future product
development.
You could join two Data Highway II links with a bridge. The bridge
becomes a node on each of the two links. The two Data Highway II links
and their host/interface links combine to form a single Data Highway II
network (Figure 2.5).
Actually, you could configure a Data Highway II network of many Data
Highway II links joined together with bridges. A node can address up to
254 links.
The advantage of configuring a network of several small links instead of
one large link is that separate message transmissions can take place within
each of the links all at the same time. Each link has its own token which
it passes around to its nodes. For example, in Figure 2.5, the computer
could send a message to the PLC-3 processor at the same time that the
PLC-2/30 processor is sending a message to the PLC-2/15 processor.
However, a message from a node on one link to a node on another link
would use up message transmission time in both links. By configuring
links within a network so that most messages do not cross from one link
to another, you can improve the performance of the network.