CTI 2572-B / 2500C-2572-B Installation and Operation Manual 4
PLC Start
Using this method, PLC logic is used to set the IP address of the module during startup. The Start Network
Services command, triggered by PLC logic, sets the IP parameters based on data stored in V memory. Once the
IP parameters are successfully set, the network TCP/IP services are started
An important advantage to this method is that it eliminates the need to manually configure the module during
the initial installation or during a replacement operation. In addition, since the network parameters are stored
in PLC memory, the module IP address remains associated with the PLC even when modules are swapped. The
primary disadvantage is that it requires a small amount of additional ladder logic. In addition, if the PLC is not
in run mode (not executing logic) the module will not start up. The PLC logic to perform this function is
described in CHAPTER 8. START NETWORK SERVICES COMMAND.
Choosing between TCP and UDP Protocols
Many HMI drivers and OPC servers will allow you to select either TCP (Transmission Control Protocol) or UDP
(User Datagram Protocol) for delivering messages. The 2572-B client functions also allow you to choose
between the two. The following information is provided to help you determine which is right for your
situation.
UDP Protocol
The UDP protocol allows you to send a message to another node without previously establishing a logical
connection to the other node. This method, called connectionless delivery, uses less network bandwidth,
consumes fewer module resources, and usually results in faster response times. Although the delivery of UDP
messages is not acknowledged by TCP/IP, higher level protocols, such as CAMP, confirm that the message was
received and that the requested service performed.
NOTE:
In most installations, UDP is the preferred protocol for HMI applications that continuously read data
from the PLC.
TCP Protocol
The TCP protocol requires that a logical connection be established between two points before messages can
be sent. This method, called connection-oriented services, provides delivery acknowledgement and message
flow control within the TCP/IP protocol. If a message is not delivered correctly, the TCP protocol will cause the
message to be re-transmitted.
Although TCP may be slightly slower than UDP because additional acknowledgement messages must be
transmitted and evaluated, there are several instances in which it should be used:
1. If you have a network whose quality requires a significant amount of message re-transmission, TCP can
provide better overall performance because the TCP is quicker at delivery acknowledgement and retry
than the application level, which typically relies on a message timeout measured in seconds.
2. If you are using the Internet to access the module, you may want to consider TCP. When traversing the
Internet, some gateways may break a packet up into smaller chunks, a process called fragmentation. The
smaller packets may be sent via different paths and could arrive out of order. TCP will ensure that the
fragmented packets will be re-assembled in the correct order, while UDP will not.
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