User guide

Communication Capabiliites
890 USE 108 00 August 2001 89
Multiple Modbus Plus networks can be interconnected using a BP85 Bridge Plus
device.
The following figure shows multiple networks linked by Bridge Plus.
Each node within a network must have a unique address number in the range 1 ...
64. The node address of a 984 chassis mount controller is specified using a set of
DIP switches provided on the top front of the A984-145, E984-245/255 models. The
E984-265/275/285 uses the rotary switches.
Modbus Plus uses a proprietary protocol that delivers high performance intercom-
munication capabilities at a data transfer rate of 1 Mbit/s. The network medium is
twisted-pair shielded cable, laid out in a sequential multidrop path directly between
successive nodes.
Modbus Plus
Token Rotation
Each node on a Modbus Plus network functions as a peer on a logical ring, gaining
access to the network upon receipt of a token frame. The token is a bit grouping that
is passed in a rotating address sequence from one node to the next. While an
individual node holds the token, it may initiate data read/write and statistical
transactions with other nodes; when the node passes its token, it may write to a
global database that is maintained by all nodes on the network. Use of this global
database allows rapid updating of alarms, setpoints, and other data.
Network A (Up to 64 Nodes)
Node Node Node
BP85
Node 5 Node 6 Node 8
Node 7
Network B (Up to 64 Nodes)
Node Node Node
Node 5
Node 4Node 3 Node 6
= Terminating Connector
= Inline Connector