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Modbus Functions 

 

FP Web-Server V2.11 

148 

Either the PLC or a Modbus-RTU master unit can control the Modbus-TCP client functions of 
the FP Web-Server. 

•  For PLC control a shared buffer in the PLC's memory is used. The PLC program places 

the server IP address and Modbus commands in this buffer. The FP Web-Server reads 
the buffer and executes the commands. Additionally, the PLC can also access a 
Modbus-RTU slave unit via the 9-pin RS232C interface. 

•  Any Modbus-RTU master connected to the 9-pin RS232C of the FP Web-Server can 

access a remote Modbus-TCP server. This gateway routes all Modbus-RTU function 
requests to the Modbus-TCP server. Optionally the Modbus-RTU master can address 
different Modbus-TCP servers (IP address and/or unit ID is calculated by the accessed 
RTU slave address) or the local PLC directly. 

 

The Modbus-TCP communication can also be done via modem. For this the client (FP 
Web-Server controlled by the PLC, see PLC sample program "Dialup-StayOnline.asc") has to 
dial up the Modbus-TCP server to establish a modem connection before Modbus-TCP 
communication can take place. 

The PLC can also control continuous, automatic data exchange via Modbus protocol. See PLC 
sample program "Modbus-TCP-Client.asc", which is installed along with the FP 
Web-Configurator. 

•  This function uses the configurable Modbus-to-PLC address translation table. 
•  The gateway routes all Modbus function requests. 
•  The Modbus-TCP client can also handle UDP broadcast messages (all responses from 

the servers are ignored). 

•  An integrated setup wizard helps you to configure the Modbus-TCP client. 
•  NumberMaxOfClientTransactions = 6 (if all are busy, then the oldest is disconnected). 
•  SO-RCVBUF, SO-SNDBUF = 4kB with TCP and 2kB with UDP (other IP Stack 

parameters can be defined).