Watlow Addendum to EZwarePlus - Manual (Page 6)

Chapter 3: Communications
Basics
The Silver Series EM OIT can communicate with Watlow controllers via Modbus TCP or Modbus RTU.
This section defines terms you will encounter and provides information about Modbus communications
that will help you create a user interface with the Silver Series EM OIT for equipment and machines that
include Watlow controllers.
About Gateways, Controllers and PLCs
The Silver Series EM OIT can communicate with a variety of devices. Because many of these devices
are Programmable Logic Controllers (PLCs), EZwarePlus refers to devices in many places generically
as “PLCs”. Don’t let this confuse you. When you are setting up the OIT to communicate with a Watlow
product, from the OIT’s point of view the controller is a PLC.
This addendum refers to configuring the Silver Series EM OIT to communicate with “controllers”, but
the instructions apply equally to limit controllers and gateways such as the EZ-ZONE
®
RUI Gateway.
Parameter Register Addresses
The Modbus RTU and Modbus TCP communications protocols assume that each device’s memory is
organized in blocks of like-data. Each data block contains either read-only data or read/write data and
either bit-size data or word-size (16-bit) data. For example, the Coils data block contains read/write bits
and the Input Register data block contains read-only words.
Within each of these data blocks there is a range of memory locations. The data blocks are often referred
to by the first digit of their address range. For example, the Holding Registers are often referred to as,
“the 4x registers”. The Modbus standard defines both a numbering scheme and an addressing scheme for
the memory locations in the data blocks. The table below illustrates this information for the four most
commonly discussed data blocks.
Name Number Access
Data
Size
Memory Location
Numbers
(Absolute)
Memory Location
Addresses
(Relative)
Coils 0x Read/Write 1 bit 1 to 65,536 0 to 65,535
Discrete Inputs 1x Read-Only 1 bit 100,001 to 165,536 0 to 65,535
Input Registers 3x Read-Only Word 300,001 to 365,536 0 to 65,535
Holding Registers 4x Read/Write Word 400,001 to 465,536 0 to 65,535
Because the location numbering scheme includes the data block number, it is helpful to think of it as an
absolute address. The absolute address specifies the address completely or absolutely in that it says
which data block and which address contains a piece of data. Because the location addresses do not
include the data block number, it is helpful to think of them as relative addresses. They specify where to
look relative to the starting point of the data block.
Some software and device manufacturers document the use of their products in terms of the numbering
scheme while others use the addresses. In either case the manufacturer typically calls the numbers they
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Silver Series EM OIT  6  Watlow Addendum