Modbus RTU/ASCII: emerged in the mid-1970s for linking terminals with Modicon PLC’s using a master/slave protocol. The original Modbus serial line specification included two transmission modes: RTU and ASCII. The Modbus RTU uses binary coding and CRC error checking. Modbus ASCII mode is more readable, but less efficient because each byte is represented by two ASCII code and it uses less effective LRC error checking. The communications are initiated by Modbus masters using polling, query/response protocol. The master can send broadcast messages, using a unit ID of 0, which all slaves accept, but do not reply to. Normally the master polls individual slaves sequentially.

Modbus TCP: is designed to allow Modbus protocol to be carries over TCP/IP based networks. Unlike Modbus RTU/ASCII, which a master can communicate to multiple slaves using UID, Modbus TCP sets up a point to point connection. To communicate with multiple slave devices, different TCP/IP connection is needed for each slave device. Also multiple polling/queries messages can be pipelined or queued. The response messages could be replied out of order by the slave devices. Therefore a transaction ID is assigned to each polling message to avoid mixing up query and response message.

The MODport-10x, MODBUS gateway allows the legacy MODBUS RTU/ASCII devices to operate on a MODBUS TCP network. It allows either MODBUS serial master or slave to communicate with MODBUS TCP’s slave or master.

Modbus TCP masters to Modbus RTU/ASCII slaves: MODPort 10X allows multiple MODBUS TCP masters to communicate with a MODBUS serial network. Since MODBUS serial network can only handle one query at one time, queries from different masters are pipelined and processed one by one.

Modbus RTU/ASCII master to Modbus TCP slaves: When MODport-10X function as MODBUS RTU/ASCII master to MODBUS TCP slave gateway, MODport-10X can connect up to 8 MODBUS TCP slave. User can specify a UID range for each MODBUS TCP slace.

Inactive timeout: MODport-10X provides inactive timeout that allows user to specify a time period to disconnect TCP/IP connection if there is no activity in the network. It could be the peer of the connection is down if the connection does not tear down it would occupy one connection slot and prevent any another connection again.

Configuration tools: User friendly MODport management software provides an easy way to configure MODport-10X. It can search all MODport within a local area network independent of its subnet. It can also find a MODport of a specific IP address over wide area network. MODport-10X can also be configured by its console port or through telnet connection. MODport-10X also provides web interface for user to configure it by a web browser.

MODport-10X will work at Loop back Mode, which all data is sent back immediately for easy connection test.

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