D-Link DE-805TP - 10Mbps Ethernet Mini Hub User Manual download pdf (Page 20)

Modbus for Field Technicians

Page 20

10. THERE CAN ONLY BE ONE MASTER ON A MODBUS SERIAL

TRUNK

Modbus is a poll-response type of protocol. A master issues a message. If the

address in the message matches the address of a server device it will respond

(if it can). All other devices remain quiet all the time until they are sent a

message with a matching address. The master must wait long enough to

process the response before sending the next message. If it doesn’t then its

next poll and the response from the previous may overlap.

When Modbus over Ethernet is used, more than one master can poll a server

device for data. The number of queries that a server can process

simultaneously is dependent on several factors – does the vendor support

multiple simultaneous socket connections and how many do they allow.

Vendors hardly ever publish this information.

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D-Link DE-805TP - 10Mbps Ethernet Mini Hub User Manual Page 20