Watlow EZ-ZONE RMA Modul - Manual (Page 52)

Watlow EZ-ZONE ® RMA Module • 49 • Chapter 7 RMA Communications
EZ-ZONE RMA & Communications
With the introduction of the first Programmable
Logic Controllers (PLC's) in the early to mid 1970's
it quickly became apparent that there was a need
to communicate between one PLC and another, and
then on a wider scale, between PLC's 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 in-
dustrial communications broadened and became
better defined, with specific needs being addressed.
Those requirements and specifications centered on
collecting data, configuring controls, and controlling
a process.
Protocols
The Protocol describes how to exchange data. Due
to the volume of traffic (limited bandwidth) and sen-
sitivity to disturbances on the network the protocol
will define the number of bits in a packet of informa-
tion, the speed of the data transfer, whether or not
error checking is done, etc... There are a number of
different data communications protocols in use to-
day. The EZ-ZONE RMA module can be optionally
equipped with the following protocols:
- Modbus ® RTU & TCP
- Profibus ® DP
- EtherNet/IP™
- DeviceNet™
Each of these protocols are heavily in use today with-
in a wide array of industrial applications.
Modbus
Introduction to the Modbus Protocol
Gould Modicon, now called AEG Schneider, first cre-
ated the protocol referred to as "Modbus RTU" used
in process control systems. Modbus provides the ad-
vantage of being extremely reliable in exchanging
information, a highly desirable feature for industrial
data communications. This protocol works on the
principle of packet exchanges. The packet contains
the address of the controller to receive the informa-
tion, a command field that says what is to be done
with the information, and several fields of data. Each
RM module User's Guide has a comprehensive list-
ing of these registers found in the Operations, Setup,
Profiling, and Factory Pages.
Many parameter values within the various RM
modules are four bytes in length and require two
Modbus registers. By default, the low order word con-
tains the two lower bytes of the 32 bit parameter and
the high register number contains the two higher
bytes. If it makes your programming easier you may
reverse this Modbus default when using RM mod-
ules 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 Communica-
tions [Com] Menu.
If it is desired to acquire more information on Mod-
bus RTU or Modbus TCP direct your browser to:
http://www.modbus.org .
User Programmable Memory Blocks
The RMA module equipped with the Modbus proto-
col features a block of 40 contiguous Modbus regis-
ters that can be configured by the user to reflect the
parameters of their choice. This assembly allows for
direct read/write (depending on actual parameter) ac-
cess in one contiguous block. When the RMA is used
in conjunction with other RM modules it is important
to know the assembly sizes for each. The list below
reflects the size for each module as of this revision.
• RMC (Control) equals 40 members
• RME (Expansion) equals 40 members
• RMA equals 40 members
• RMH (High Density) equals 80 members
• RML (Limit) equals 80 members
• RMS (Scanner) equals 80 members
To acquire a better understanding of the tables found
in the back of this guide please read through the text
below which defines the column headers used. (See
Appendix: Modbus Programmable Memory Blocks )
Assembly Definition Addresses
Fixed addresses used to define the parameter that
will be stored in the "Assembly Working Addresses";
may also be referred to as a pointer. The value stored
in these addresses will reflect (point to) the Modbus
address of a parameter within an RM module.
Assembly Working Addresses
Fixed addresses directly related to their associated
"Assembly Definition Addresses" (e.g., Working Ad-
dresses 200 & 201 will assume the parameter point-
ed to by definition addresses 40 & 41). Take a look at
the section entitled " Modbus Default Assembly Struc-
ture 40-119 " found in the Appendix. The RMC as-
sembly can be seen where the first member is identi-
fied as "Control Loop Set Point 1". This is a writable
Chapter 7: RMA Communications
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