EGW1-IA3-MB-DF1 – Modbus TCP a DF1 User’s Manual
Exemys
www.exemys.com
Rev. 1
13
3.5
Translation Tables page.
In these sections, some basic aspects on Modbus TCP and DF1 protocols are outlined.
Modbus TCP data are classified into four types, such as defined in Modbus ASCII/RTU.
These areas are known as:
Input Registers
Holding Registers
Input Status
Coil Status
Input Registers and Holding Registers are 16-bit words, while Input Status and Coils Status are mapped to
discrete 1-bit data. By the other hand Input Registers and Input Status are read-only areas, used to read
input data such as digital inputs and statistics. Holding Registers and Coils Status areas may be both read
and written.
PLCs compatible with DF1 protocol usually map addressable data into Files and Elements. A group of
data with similar characteristics is called a File and each datum within a File is an Element. Files are
organized into File Types, according to their different purposes. For example, N Files are used to store 16-
bit integers, B Files store bit variables and the O File is used to write a value to a PLC output. Each File is
associated to a unique File Number.
1.
N Files: This table provides the capability to map N Files, which usually store program variables.
Up to thirty-two N Files will be allowed, thus covering a wide range of typical situations. Each Element in
the File is mapped into Holding Registers, allowing both reads and writes (16 bits wide). The only entry to
know before adding an N File is its File Number. (Usually, File Number 7 is associated with a default N
File, though other N Files could be configured using the application software provided with your PLC).
2.
B Files: This table stores the Bit File, used to keep 1-bit variables. Up to two B Files can be added
to the table. This might seem a scarce resource, but actually it is not. Have in mind that each 1-bit
location is mapped to a different Modbus TCP Coil Status, thus consuming the addressing space very
quickly. Also consider that these two B Files lets you map 8192 individual bits, providing an adequate
capability for most applications.
3.
S File: This is the only fixed (non-configurable) table. The mapping (Input Registers 30192-30447)
provides reads of internal statistics.
In order to work accurately, I/O expansion boards and other Files in general have to be configured with
the application software provided by the PLC vendor. Otherwise, the Files will be unavailable to the
EGW1-MB-DF1, which will return Modbus exceptions (if enabled) upon unsolved requests. Contact you
dealer for specific information about your PLC and configuration.
Many newer PLCs (such as FlexLogix and ControlLogix) do not provide (as a factory default) compatibility
with the type of request issued by the EGW1-MB-DF1. However, they do provide a mechanism to make
them downwards-compatible with other PLCs and with the EGW1-MB-DF1. Please refer to Appendix E if
you are attempting to connect one of the PLCs above to the EGW1-MB-DF1.
You can insert a record into an intermediate position by selecting the appropriate row. After insertion,
rows below the insertion point will be shifted downwards. If you insert erroneous data, delete the record
by selecting it and then clicking on Delete.
