MBTCP Protocol PLX3x Series
User Manual Ethernet and Serial Gateways
Page 96 of 218 ProSoft Technology, Inc.
May 26, 2016
Modbus TCP/IP Client Access to Database
The Client functionality exchanges data between the PLX3x gateway's internal
database and data tables established in one or more Quantum processors or
other server based devices. The command list, defined in the user configuration,
specifies what data is to be transferred between the gateway and each of the
servers on the network. No ladder logic is required in the processor (server) for
Client functionality, except to assure that sufficient data memory exists.
The following illustration describes the flow of data between the Ethernet Clients
and the internal database.
Multiple Server Access to Database
The MBTCP gateway provides server functionality using reserved Service Port
502 for Modbus TCP/IP MBAP messages, as well as Service Ports 2000 and
2001 to support the TCP/IP Encapsulated Modbus version of the protocol used
by several HMI manufacturers. Server support in the gateway permits Client
applications (for example: HMI software, Quantum processors, etc) to read from
and write to the gateway’s database. This section discusses the requirements for
attaching to the gateway using Client applications.
The server driver is able to support multiple concurrent connections from several
Clients. Up to five (5) Clients can simultaneously connect on Service Port 502
and five (5) more can also simultaneously connect on Service Port 2000. Service
Port 2001 is used by the MBTCP protocol to pass Encapsulated Modbus
commands through from the Ethernet port to the gateway’s serial port.
When configured as a server, the internal database of the MBTCP gateway is
used as the source for read requests and the destination for write requests from
remote Clients. Access to the database is controlled by the command type
received in the incoming message from the Client. The following table specifies
the relationship of the gateway’s internal database to the addresses required in
the incoming Modbus TCP/IP requests.
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