Modbus TCP Client for NJ (NX) Controller

The FB Modbus TCP Client provides some read/write features in accordance with the specifications defined by the Modbus organization.

List of read/write functions provided :
 

The option Keep-Alive should be fixed to a short time to enable quick re-connection when server was disconnected roughly.
       

MTCP_Client_Connect

Input variables

Output variables

Input variables of MTCP_Client_Fnxx
       

Output variables of MTCP_Client_Fnxx

Error code returned

Precautions in Using Socket Services

 Precautions for UDP and TCP Socket Services

• Communications processing are sometimes delayed when multiple functions of the built-in Ether-

Net/IP port are used simultaneously or due to the contents of the user program.

• Communications efficiency is sometimes reduced by high communications traffic on the network line.

• The close processing for a close request instruction discards all of the buffered send and receive data

for the socket. For example, send data from a send request instruction immediately before the close

processing is sometimes not sent.

• After a socket is open, the built-in EtherNet/IP port provides a receive buffer of 9,000 bytes per TCP

socket and 9,000 bytes per UDP socket to enable data to be received at any time. If the receive buffer

is full, data received by that socket is discarded. Make sure that the user application always executes

receive requests to prevent the internal buffer from becoming full.

 Precautions for UDP Socket Services

• The destination IP address can be set to a broadcast address for a UDP socket to broadcast data to

all nodes on the network. However, in this case, the maximum length of send data is 1,472 bytes.

Data lengths broken into multiple fragments (1,473 bytes or more in UDP) cannot be sent.

• For UDP socket, controls to confirm the reliability of communications, such as the confirmation of

send data, are not performed. To improve the reliability of communications when you use UDP sockets,

make sure the user program confirms that data is sent and resends data when necessary.

 Precautions for TCP Socket Services

• If the TCP socket is closed on the remote node without warning during communications (i.e., if the

connection is closed), the socket at the local node must also be closed. You can use the Read TCP

Socket Status instruction (SktGetTCPstatus) to see if the connection is closed. Immediately close the

socket at the local node if the TCP socket at the remote node is closed.

• If the remote node’s TCP socket closes without warning, the data to send may remain in the buffer at

the local node. The remaining data is discarded in the local node’s TCP close processing. The steps

that are required in applications to avoid this include sending data from the sending node that permits

closing and closing the socket only after checking the remote node.

• While open processing is performed for a TCP socket, a port that was closed first cannot be opened

again for 60 seconds from the time the close processing is performed for the remote socket. However,

this is not true if you specified 0 (automatic assignment by the Unit) as the port for the SktTCPConnect

instruction.

• You can use Connect from another socket to open a connection to a socket that was opened with

Accept. A connection is not opened if you try to use Connect from another socket to open a connection

to a socket that was opened with Connect. Also, a connection is not opened if you attempt to use

Accept from another socket to open a socket that was opened with Accept. Furthermore, you cannot

use Connect from more than one other node to establish multiple connections with a single TCP

socket that was opened with Accept on the built-in EtherNet/IP port.

• You can use the keep-alive function for TCP sockets at the built-in EtherNet/IP port. The keep alive

function checks whether a connection is normally established when no data is sent or received for a

certain period on the communications line where the connection was established. The built-in Ether-

Net/IP port responds to checks from other nodes even if keep alive is not specified.