Milltronics ACCUMASS BW500, Instruction Manual

Page: 122 / 141

Page 122 Accumass BW500 PL-565
Communic
a
tion
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.
Modbus Format
Note:
When using a commercial Modbus driver all of the message details are 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
Function
code
Information
Error
check
Where:
Station address
the network address of the slave being accessed
Function Code number that represent a Modbus command, either:
03
read function
06, 16 write functions
Information depends on function code
Error Check Cyclical Redundancy Check (CRC) for RTU and
Longitudinal Redundancy Check (LRC) for ASCII
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.