User Manual
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6. Modbus TCP/RTU gateway
Modbus is one of the most popular automation protocols in the world, supporting traditional RS-232/485 devices and recently
developed Ethernet devices. Many industrial devices, such as PLCs, DCSs, HMIs, instruments, and meters, use Modbus as their
communication standard. In industrial applications, the most common protocol conversion is Modbus RTU to Modbus TCP conversion,
and it is usually required when legacy devices such as meters, mostly using Modbus RTU, need to be integrated with SCADA systems,
mostly using Modbus TCP.
Most modern PLCs and host computers support Modbus TCP over Ethernet. In order to access discrete Modbus RTU devices for data
collection and control, they can rely on the M100 Modbus gateway.
6.1. Ethernet masters with serial slaves
When the host computer or PLC is Modbus TCP Master, the Modbus TCP function must be enabled, the remote device must work in
Modbus RTU Slave mode. The M100(TCP Server mode) supports Modbus TCP with up to 16 simultaneous connections. The serial
interface supports both RS-232 and RS-485, selectable through software. Each serial port can be connected to one RS-232 serial
device, or to 32 RS-485 serial devices. When connected with more than one RS485 devices,please refer to section 7.5 for bus
collision detection.
Fig. 69 Ethernet masters with multiple serial slaves
The device supports multiple hosts to query data from the slave in Q&A mode. Please select modbus polling function as shown in
Fig.70. When USR-M100 gateway does not receive a response from the serial port after a response timeout, the device replies with an
acknowledgement and then processes the next host request.
When you need to use long frame data frequently or have high data transmission requirements, please adjust the baud rate and
lengthen the sending interval appropriately to prevent the slow serial port from causing messy codes or packet loss.
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