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Modbus protocol is divided into two types: Modbus RTU (or ASCII) based on RS485, and Modbus TCP based on Ethernet. Modbus gateway is a converter that converts the data of Modbus RTU device into Modbus TCP data. Due to the limitation of the RS485 bus, Modbus TCP and RTU have the following advantages: (1) the existing Ethernet network can be used to eliminate the wiring of RS485. (2) multiple hosts can be supported to access Modbus instruments simultaneously.
Modbus gateway is also available in two types: storage type and ordinary type corresponding to ZLAN5143B and ZLAN5142. Compared with the ordinary Modbus, Modbus storage type can store the content of reading register inside the gateway, so the Modbus TCP can greatly improve the query speed, support for performance more superior when multiple host access. This article focuses on the characteristics of storage Modbus.
As shown in the figure, the general Modbus TCP data flow direction is (1) to (2) to (3) to (4). First, the Modbus TCP command is transformed into the corresponding instruction of Modbus RTU, and then the device responds Modbus RTU instruction to Modbus gateway, and then Modbus gateway is converted again to Modbus TCP to be sent to the monitoring upper computer.
We know that Modbus TCP is a network communication, and the transmission speed is very fast. Generally, it can be answered in 3ms, while Modbus RTU is RS485, which generally only has a speed of 9600bps, and generally sends and returns an instruction at least 30ms. This common non-storage Modbus gateway has a longer query response time. In addition, if there are a lot of upper computer at the same time to query the data, then the serial port will be congested. If the network is compared to a highway, the serial port is a foot-bridge, then the original way is to carry the highway traffic on the foot-bridge.
The storage Modbus gateway solves these problems. It can query for the register data temporarily stored in the Modbus gateway inside, so the Modbus TCP queries, Modbus gateway can immediately return instructions, really the features of the Modbus TCP quick play out. On the other hand, the saved Modbus gateway can automatically update the contents of the current saved register data by sending instructions automatically from the serial port, and keep a new register value.
The storage Modbus gateway ZLAN5143B is a fully automatic Modbus gateway with no configuration required. The user does not need to configure the required register address, function code, slave station address and so on. ZLAN5143B will automatically identify and dynamically add these registers.
ZLAN5143B can show a good reaction speed in multiple computer monitoring, regardless of the baud rate of serial port, it is generally able to give the upper response data within 3ms. It also shows the speed of real-time update of serial data.
Register storage Modbus gateway is the real Modbus TCP to Modbus RTU, which really gives full play to the advantages of fast speed, multi-host simultaneous query of Modbus TCP.
