User guide

Communication Capabiliites
890 USE 108 00 August 2001 99
Modbus Plus Communication Paths
Modbus Plus
Message
Processing
With multiple devices processing messages asynchronously on a Modbus Plus
network, an individual device can have several concurrent transactions in process.
The A984-145, E984-245/255/265/275/285 Controllers open a communication path
when a transaction begins, keep it open during the transaction, and close it when
the transaction terminates. After the path is closed, it is available for another
transaction.
Four Types of
Communication
Paths
A984-145, E984-245/255/265/275/285 Controllers maintains four types of
communication paths
Data master paths-For read and write data or get and clear remote statistics
operations originated by a MSTR block in the A984-145, E984-245/255/265/275/
285 Controllers going to a destination device on the network. A 984-145, E984-
245/255/265/275/285 supports up to five data master paths-paths DM01... DM04
for processing up to four concurrent MSTR blocks, and path DM05 that may be
used for data master transactions via the Modbus port. Design your application
to use a maximum of four MSTR data master paths at any one time.
Data slave paths-For data reads and writes received over the network. The A984-
145 supports up to four data slave (DS) paths handling up to four concurrent
network transactions.
Program master paths-For sending programming commands from the local
controller to the Modbus Plus network. Program master paths can handle all
Modbus commands-i.e., function codes. If a Modbus master is connected to the
Modbus port on the A984-145, E984-245/255/265/275/285, it may be used for
either programming or monitoring functions. A984-145, E984-245/255/265/285
supports one program master (PM) path.
Program slave paths-For accepting programming commands received over the
network. A984-145, E984-245/255/265/275/285 supports one program slave
(PS) path.
Both the originating and destination devices open paths and maintain them until
the transaction completes. If the transaction passes through one or more Bridge
Plus devices to access a destination across multiple networks, each bridge opens
and maintains a path at each of its two network ports. Thus, a logical path is
maintained between the originating and destination devices until the transaction
is finished.
All paths are independent of one another, and activity on one path does not affect
the performance of the other paths.