User`s manual

MGate MB3000 User’s Manual Modbus Overview
A-3
Modbus Ethernet vs. Modbus Serial
Although Modbus standard is intended as an application layer messaging protocol, the data format
and communication rules for Ethernet-based Modbus TCP are different from serial-based Modbus
ASCII and RTU.
The major difference between the Ethernet and serial Modbus protocols is the behavior of the
communication model. Modbus ASCII and RTU allow only one request on the network at a time.
Once a request is sent, no other communication on the bus is allowed until the slave sends a
response, or until the request times out. However, Modbus TCP allows simultaneous requests on
the network, from multiple masters to multiple slaves. TCP masters cannot send more than one
request at a time to a slave, but they can send requests to other slaves before a response is received.
The Modbus TCP standard recommends that slaves be able to queue up to 16 requests at a time.
The MGate MB3000 will queue up to 32 requests from each TCP master, for up to 16 TCP
masters.
Integrate Modbus Serial and Ethernet with Gateways
Ordinarily, Modbus TCP and Modbus ASCII/RTU are unable to communicate with each other.
However, with a Modbus gateway in between the Modbus serial network and the Modbus Ethernet
network, TCP masters are able to communicate to serial slaves and serial masters are able to
communicate to TCP slaves.