Schneider Electric 174 CEV - Manual (Page 9)

Modbus Plus to Ethernet Bridge
4
1.2
Bridge Message Transactions and Paths
Two types of messages can be initiated to Modbus Plus nodes using Modbus
commands. Programming messages are used for logging into a node and
controlling it, as in load/record/verify operations. Data access messages are used
to read or write data at the node, or to get statistics from the node.
When a message transaction is initiated, a logical path is established between the
originating node and destination node. The path is maintained until the transaction
is completed. The type of path that is set in each device is determined by the
Modbus command embedded in the message.
Table 1 summarizes the four possible types of Modbus Plus paths. The bridge
supports up to eight paths of each type.
Table 1 Modbus Plus Path Types
Path Type Opened in Node Opened for Modbus Command
Data Master (DM) Originator Data access
Data Slave (DS) Destination Data access
Program Master (PM) Originator Programming
Program Slave (PS) Destination Programming
The maximum amount of concurrent TCP/IP connections available in the bridge is
between 8 and 16, depending on the usage of paths in host applications.
For example, up to 16 data connections can be active between TCP/IP hosts and
Modbus Plus nodes (a maximum of 8 active DM paths and 8 active DS paths). Up
to 16 programming paths can be active (8 PM and 8 PS).
If Modsoft software is being used, it sets both a PM and a DM path in the bridge
for each active connection it maintains to a Modbus Plus destination node.
Modsoft could be running on a single host, or simultaneously on multiple hosts.
Thus up to eight TCP/IP connections can be maintained with Modsoft running on
one to eight hosts.
Table 2 summarizes the Modbus function codes which open DM and DS paths in
the respective nodes. For complete information about Modbus commands, see
the Modbus Protocol Reference Guide, publication PI--MBUS--300.