Chapter 11: Communications; Modbus Remote Terminal Unit (Rtu) And Modbus Tcp - Watlow EZ-ZONE RM User Manual

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Chapter 11: Communications

EZ-ZONE RMA & Communications
With the introduction of the first Programmable
Logic Controllers (PLCs) in the early to mid 1970s
it quickly became apparent that there was a need
to communicate between one PLC and another, and
then on a wider scale, between PLCs and other com-
puters within the company infrastructure. Some of
those needs involved applications with interlinking
processes, such as batch processes or assembly lines
utilizing multiple controls that required better syn-
chronization and control.
Over time the scope of the requirements for indus-
trial communications broadened and became better
defined, with specific needs being addressed. Those
requirements and specifications centered on collect-
ing data, configuring controls, and controlling a pro-
cess.
Protocols
Protocol describes how to exchange data. It also
prevents two machines from attempting to send data
at the same time. There are a number of different da-
ta communications protocols in use today. The proto-
col part of Watlow communications is very important,
because it gives us a quality of communication that
others often don't have. The EZ-ZONE RM family of
controls provides several different protocols (Modbus
RTU & TCP, EtherNet/IP, and DeviceNet) meeting
today's communication needs across many industrial
applications.
In information technology, a protocol is the spe-
cial set of rules that end points in a telecommunica-
tion connection use when they communicate. Proto-
cols exist at several levels in a telecommunication
connection. For example, there are protocols for the
data interchange at the hardware device level and
protocols for data interchange at the application pro-
gram level. In the standard model known as Open
Systems Interconnection (OSI), there are one or more
protocols at each layer in the telecommunication ex-
change that both ends of the exchange must recog-
nize and observe. Virtually all networks in use today
are based in some fashion on the OSI standard. OSI
was developed in 1984 by the International Organi-
zation for Standardization (ISO), a global federation
of national standards organizations representing
approximately 130 countries. As can be seen below
the OSI model is a set of seven layers that define the
different stages that data must go through to travel
from one device to another over a network.
®
Wat lo w EZ-ZO NE R M S ys t em
Figure 11.1
Modbus Remote Terminal Unit (RTU) and Modbus
TCP
Gould Modicon, now called AEG Schneider, cre-
ated the protocol refered to as "Modbus" used in pro-
cess control systems. Modbus provides the advantage
of being extremely reliable in exchanging informa-
tion, a highly desirable feature for industrial data
communications. This protocol works on the prin-
ciple of packet exchanges. The packet contains the
address of the controller to receive the information,
a command field that says what is to be done with
the information, and several fields of data. Reading
from these registers retrieves all information in the
controller. Each of these registers are listed in this
user's manual (Operations, Setup, Profiling, & Facto-
ry Pages). You will need this list to determine where
the data is located. The last item sent in the packet
is a field to ensure the data is received intact. This is
called a cyclical redundancy check-sum (CRC). All in-
formation exchanged is in hexadecimal numbers.
Many parameter values within the RM system
are four bytes in length and require two Modbus reg-
isters. By default, the low register number contains
the two lower bytes and the high register number
contains the two higher bytes. If it makes your pro-
gramming easier you may reverse this Modbus de-
fault where the low register number contains the two
higher bytes and the high register number contains
the two lower bytes. This setting can be modified in
the RM Access Setup pages under the "Com" menu.
Modbus RTU is typically deployed over serial con-
nections where Modbus TCP is deployed over the
Ethernet physical layer. If it is desired to acquire
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