Message Connection Types

Introduction

The transmission of most messages require a point-to-point connection between a transmitter and receiver.

For all types of explicit messages, the connection automatically closes when the communication ends, or is timed-out.

For implicit messages, keep the connection open. If the I/O connection—CIP for EtherNet/IP, TCP for Modbus TCP—the transmission stops. In this case, the scanner employs the TCP implicit messaging connection to dynamically re-establish the CIP connection.

Calculating the Connection Timeout

For CIP connections, you can control the connection timeout setting by specifying both the network multiplier and the requested packet interval (RPI in ms):

Timeout = Network Multiplier x RPI

NOTE: You can locate and configure these values in the Control Expert Ethernet Configuration Tool. Open the DTM Editor for the Ethernet communication module, then edit the following settings:

the network multiple is the Time-out Multiplier parameter found in the Device List → <device> → <connection> → Connection Settings page, and
the RPI is the EM Connection RPI parameter found in the Channel Properties → EtherNet/IP page

A large timeout value may affect the ability of the network to optimize the availability of connection resources, re-establish connections, and update I/O data when the connection is lost.

A small timeout value may unnecessarily cause the frequent closing and re-establishing of connections.

It is preferable to use a larger timeout value for explicit messaging connections, and a smaller timeout value for implicit messaging connections. The specific value you employ depends on your application requirements.

Connection Types and Protocols

The connection type and transport protocol employed depends upon the message type and message protocol, as follows:

Message Type	Message Protocol	Connection Type	Connection Protocol
Explicit	EtherNet/IP	CIP, TCP	TCP/IP
	Modbus TCP	TCP	TCP/IP
	FTP	TCP	TCP/IP
	HTML (web)	TCP	TCP/IP
	SMTP	TCP	TCP/IP
	SNMP	N/A	UDP/IP
	SNTP	N/A	UDP/IP
	DHCP	N/A	UDP/IP
	BOOTP	N/A	UDP/IP
Implicit	EtherNet/IP	CIP, TCP	UDP/IP
	Modbus TCP	TCP	TCP/IP
	IGMP	N/A	IP
	RSTP	N/A	Ethernet

Connection- Overhead

Any message transmission includes overhead, which consumes network bandwidth and processing time. The smaller the size of the data transmitted, the relatively greater the portion of the message allocated to overhead.

Consequently, it makes sense to design your I/O messaging by consolidating data from multiple I/O devices—with similar processing capabilities and performance needs—and transmitting it through a single adapter. This design conserves bandwidth, stores network resources, and improves performance.