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MCTB
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Appendix
A.1 Modbus Communications
The MCT B utilizes Modbus as its standard protocol and offers both Modbus RTU serial and Modbus TCP forms.
This section provides information on the supported functions and data available over the Modbus interface. It
is assumed that the user is already familiar with Modbus protocol and has a basic understanding of its
implementation and use. For additional information and support, modbus.org (
http://modbus.org/tech.php )
provides an abundance of technical documents, standards and support applications.
Listed below are a few of the more common software packages that claim to support Modbus protocol. This
list is provided as informational only. Contact the software manufacturer for more information on applying their
software.
LabView by National Instruments
Wonderware by Wonderware SpecView by SpecView Corporation
11500 N Mopac Expwy 26561 Rancho Pkwy. South 13409 53
rd
Ave NW
Austin, TX 78759-3504 Lake Forest, CA 92630 Gig Harbor, WA 98332
Phone 800-683-8411 Phone 949-727-3200 Phone 253-853-3199
http://www.natinst.com http://www.wonderware.com http://www.specview.com
A.1.1 Supported Function Codes
Modbus bases its data model on a series of tables, each of which defines a particular range of values. In the
case of the MCTB, all data is contained within the Holding Register data table. This means that all
communication transactions between the MCTB and the master (client) are performed using three command
types. These include Read Holding Registers, Write Single Register and Write Multiple Registers.
Function Codes
code (hex)
16 bits
access
(word)
Read Holding Registers 03 03
Write Single Register 06 06
Write Multiple Registers 16 10
Function code 03 (read holding registers) is used to read the contents of a contiguous block of data registers
from the MCTB. The request from the master specifies the starting register address and the number of
registers. The response message from the MCTB contains the value of each register requested (two bytes per
register).
Function code 06 (write holding register) is used to write to a single data register in the MCTB. The request
from the master specifies the address of the register to be written and the value. The response message from
the MCTB is an exact duplicate of the sent message (if completed successfully).
Function code 16 (write multiple registers) is used to write to a contiguous block of data registers in the MCTB.
The request from the master specifies the starting register address to be written, the number of registers to
write and two bytes of data for each register. The normal response message from the MCTB returns the
function code, starting address and quantity of data registers written.