User`s manual

MGate 4101-MB-PBS UM Quick Configuration
A-8
Response Timeout
The master sends a request. The slave is unresponsive for the amount of time specified by the response
timeout function. The master gives up on the request and resumes operation, allowing another request to be
initiated.
To allow for a wide range of devices, baudrates, and line conditions, actual response timeout values are left
open for manufacturers to determine. This allows the Modbus protocol to accommodate a wide range of devices
and systems. However, this also makes it difficult for system integrators to know what response timeout value
to use during configuration, specially with older or proprietary devices.
The MGate MB3000 provides a patent-pending function that tests all attached devices and recommends a
response timeout value. This function saves considerable time and effort for system integrators, and results in
more accurate timeout settings.
Modbus Ethernet vs. Modbus Serial
Although Modbus 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
7
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 with serial slaves and serial masters are able to communicate with TCP
slaves.