PL-565 Accumass BW500 Page 121
Communications
Modbus RTU/ASCII Protocol
Modbus is an industry standard protocol owned by Schneider Automation Inc.
1
and is
used throughout process control industries for communication between devices.
Modbus RTU and Modbus ASCII are both master-slave type protocols. BW500’s
Modbus is a slave unit.
BW500 supports both the RTU and ASCII version of Modbus and attempts to
automatically detect the type when a connection is made.
A brief description of Modbus RTU and Modbus ASCII is given in this manual. For a
full description of the Modbus protocol, contact your local Schneider representative.
Also you may try their web site at:
http://www.modicon.com
At the time of publication of this manual, the Modbus Protocol was located under
products / technical publications / communications products / Modbus protocol.
Note:
Milltronics does not own the Modbus RTU protocol. All information regarding that
protocol is subject to change without notice.
How Modbus Works
As mentioned above, Modbus is a master-slave type protocol. This can also be
referred to as a query-response protocol. What both of these terms mean is that on
the network, there is one master which requests information from multiple slave
devices. The slave devices are not permitted to talk unless they have been asked for
information. When responding, the slaves will either give the information that the
master has requested or give an error code consisting of why it can not give the
information or that it did not understand the request. refer to Error Handling on page
135
All BW500 information is mapped into the Modbus holding registers so that Modbus
function code 03 can read from them and Modbus function code 06 and 16 can write
to them.
1. Modicon is a registered trademark of Groupe Schneider.
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