Schneider Electric BM85 Modbus Plus Message Routing Paths, Routing to Programmable Controllers

2.1 Modbus Plus Message Routing Paths

A single Modbus Plus network can have up to 64 addressable node devices, with each device having a unique address of between 1 and 64. Multiple networks can be joined through Bridge Plus devices. Devices address each other across Bridge Plus devices by specifying routing paths of five bytes, with each byte representing an address on the next network. This routing method allows nodes in other networks to be addressed up to four networks away from the originating node.

The routing path is imbedded in the Modbus Plus message frame as it is sent from the originating node:

START

MODBUS PLUS

MESSAGE FRAME

ROUTING

PATH

END

ROUTING ADDRESS 1

ROUTING ADDRESS 2

ROUTING ADDRESS 3

ROUTING ADDRESS 4

ROUTING ADDRESS 5

EXAMPLE:

ROUTING ADDRESS 1 = 25 ROUTING ADDRESS 2 = 20

ROUTING ADDRESS 3 = 12

ROUTING ADDRESSES 4, 5 = 0

(NO FURTHER ROUTING)

Figure 4 Message Frame Routing Path Field

Figure 4 is an example of message routing to a programmable controller through three networks that are joined by Bridge Plus node devices. The message will first be routed to node 25, a Bridge Plus on the local network. That node forwards the message on to a Bridge Plus at address 20 on the second network. The second Bridge Plus forwards the message to its final destination, node address 12 on the third network. The zero contents of bytes 4 and 5 specify no further message routing.

2.1.1Routing to Programmable Controllers

For programmable controllers, the last non-zero byte in the message routing specifies the network node address of the controller (1 to 64).

18 Device Addressing and Message Routing

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