BusWorks® Model 989EN U
ser’s Manual Modbus TCP/IP Digital I/O & Counters
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Note that with TCP, the transmitter expects the receiver to acknowledge
receipt of the data packets. Failure to acknowledge receipt of the packet will
cause the transmitter to send the packet again, or the communication link to
be broken. Because each packet is numbered, the receiver can also
determine if a data packet is missing data, or it can reorder packets not
received in the correct order. If any data is detected as missing, all
subsequent received data will be buffered. Complete data will be passed up
the protocol stack to the application, but only when it is complete and in the
correct order.
TCP/IP refers to Transmission Control Protocol and Internet Protocol, and
serves as the foundation for the world-wide web. TCP/IP allows blocks of
binary data to be reliably exchanged between computers. The primary
function of TCP is to ensure that all packets of data are received correctly,
while IP makes sure that messages are correctly addressed and routed. We
see that the TCP/IP combination does not define what the data means or
how the data is to be interpreted, it is only concerned with transporting and
delivering the data.
Again, referring to the OSI Network Model, the Network Layer or Internet
Layer resides just below the Transport Layer and is responsible for routing
the packets to the network. There are many network layer protocols such as
ICMP, IGMP, ARP, RARP, but our focus here is on IP.
An IP packet is a chunk of data transferred over the Internet using standard
Internet Protocol (IP). The Internet Protocol (IP) is responsible for the actual
addressing and delivery of the data packets. IP packets vary in length
depending on the data being transmitted, but each packet begins with a
header containing addressing and system control information. Similar to
UDP, IP is a connectionless and unacknowledged method for sending data
packets between two devices on a network. IP does not guarantee delivery
of the data packet, but relies on a transport layer protocol (like TCP), or
application layer protocol (like Modbus) to do that. The IP addressing
scheme also makes it possible to assemble an indefinite number of
individual networks into a larger overall network, without regard to the
physical implementation of the sub networks. The data can then be sent
from one network station to another, regardless of these differences.
The IP specific header (MBAP of the Modbus TCP/IP specification) is the
same for Modbus/UDP as for Modbus/TCP, and is 7 bytes long and
comprised of the following fields:
•
Invocation Identification/Transaction Identifier (2 bytes).
•
Protocol Identifier (2 bytes) - Set to 0 for Modbus by default and
other settings are reserved for future extensions.
•
Length (2 bytes) - Represents a byte count of all following bytes.
•
Unit Identifier (1 byte) - Used to identify a remote unit located on a
non-TCP/IP network.
Modbus uses TCP/IP Ethernet to carry the data of the Modbus message
structure between devices. That is, Modbus TCP/IP combines a physical
network (Ethernet), with a networking standard (TCP/IP), and a standard
method of representing data (Modbus).
ABOUT MODBUS
TCP/IP
