Modbus Rtu Vs. Ascii; Modbus Format; Modbus Register Map - Siemens Milltronics BW500 Operating Lnstructions

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
BW500 and BW500/L.

Modbus Format

When using a commercial Modbus driver all of the message details are
Note:
handled for you.
To give you a better idea of how a Modbus message works, a master on network would
send a message in a format similar to this:
Station
address
Where:
Station address
Function Code
Information
Error Check
There is more to the frame than is described above, this is shown to give the user a
general idea of what is going on. For a full description, refer to the Modbus
specifications.

Modbus Register Map

The memory map of the BW500 and BW500/L occupies the Modbus holding registers
(R40,001 and up).
The BW500 and BW500/L was designed to make it easy for users to get useful
information via Modbus. The following chart gives an overview of the different sections.
7ML19985DK05Milltronics BW500 and BW500/L - OPERATING INSTRUCTIONS
Function
code
the network address of the slave being accessed
number that represent a Modbus command, either:
03 read function
06, 16 write functions
depends on function code
Cyclical Redundancy Check (CRC) for RTU and Longitudinal
Redundancy Check (LRC) for ASCII
Information
Error
check
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