Siemens Milltronics BW500, Operating Instructions Manual

Page: 135 / 220

Communications
11.5 Modbus RTU/ASCII Protocol
BW500 and BW500/L
Operating Instructions, 12/2016, A5E33482052-AD
133
11.5 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 and BW500/L’s Modbus is a slave
unit.
BW500 and BW500/L supports both the RTU and ASCII version of Modbus and attempts to
automatically detect the type when a connection is made.
Note
•
host should wait at least 500 ms between a message polls to the BW500 and BW500/L
• for Modbus RTU, the host should wait at least 1000 ms for a response from the BW500
and BW500/L. In Modbus ASCII mode the suggested timeout is 1500 ms.
1)
Modicon is a registered trademark of Groupe Schneider.
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/ (
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
Siemens does not own the Modbus RTU protocol. All information regarding that protocol is
subject to change without notice.
11.5.1 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 cannot give the information or that it did not understand the request. Refer to Error Handling
(Page 149).
All BW500 and BW500/L 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.