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7ML19985CN01
Milltronics SF 500 - INSTRUCTION MANUAL
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Communicat
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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. SF 500’s Modbus is a slave
unit.
SF 500 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 website at http://www.modicon.com.
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 90.
All SF 500 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.
Modbus RTU vs. ASCII
There are two main differences between Modbus RTU and Modbus ASCII. The first is
that Modbus RTU encodes the message in 8-bit binary, while ASCII encodes the message
in ASCII characters. Therefore, one byte of information would be encoded into 8 bits for
RTU and into two ASCII characters for ASCII (which would be two 7-bit units). The second
difference is that the error checking method is different (see below).
Modbus RTU has the advantage that it has a much greater data throughput than ASCII.
Modbus ASCII has the advantage that it allows time intervals of up to one second to
occur between characters without causing an error. Either protocol works with the SF
500.
1.
Modicon is a registered trademark of Groupe Schneider.
Note:
Siemens Milltronics does not own the Modbus RTU protocol. All information
regarding that protocol is subject to change without notice.