KROHNE MFC 300 Modbus - Manual (Page 8)

6
MODBUS PROTOCOL
8
MFC 300
www.krohne.com 11/2009 - 4000744801 - AD Modbus MFC 300 R01 en
Modbus protocol
6.1 General information concerning the protocol
Using RTU (Remote Terminal Unit) format, data is transmitted as 8 bit binary characters. There
are no special characters to determine the start and end of a message frame.
Synchronization is achieved by a minimum silent period of at least 3.5 character times before the
start of each frame transmission and a maximum silent period of 1.5 character times between
characters in the same frame.
6.2 RTU frame format
The format of the query and response frames vary slightly depending upon the command
function. The basic form is outlined below.
Command function Frame format Description
Silent period 3.5 x T All transmissions must be preceded by a minimum silent
period of 3.5 x T, where T is the transmission time of a
single character. This can be calculated from the baud
rate, e.g. at 19.2 kb no parity with 1 stop bit (10 bits),
T = 520 µs.
Slave address 8 bits This is a single byte slave address which is transmitted
first and must be in the range of 1...247. Address 0 is
reserved for a broadcast address which all slaves should
recognize, and therefore requires no response.
Function code 8 bits This is an eight bit code in the range of 1...255 although
only 126 functions exist as the codes 129...255 represent
an error condition. An error condition occurs when the
addressed slave does not accept the command, in which
case it responds with the function code + 128, i.e. with its
MSB set to 1.
Register start address or
byte count when required
8 bit byte count
16 bit address
Register start address: Register start address: Register start address: Register start address: for a query command that requires
data to be returned, this field will contain the 16 bit start
address of the register (or data) to be returned.
Note that the converter uses protocol addresses.
Therefore the register address listed is the actual number
required in the Modbus command.
E.g: E.g: E.g: E.g: to access input register 30006, the register start
address is 30006dec = 7536hex.
Byte count: Byte count: Byte count: Byte count: In general this is only present in frames that
are transferring data, and has a value equal to the number
of bytes contained in the data field. The data field is limited
to a maximum of 250 bytes.
Number of points or data
bytes when required
n × 8 bits Number of points: Number of points: Number of points: Number of points: for a query command that requires data
to be returned, this field will contain the number of
registers to be returned regardless of their bit size.
Data bytes: Data bytes: Data bytes: Data bytes: contains the data requested. The converter can
use big endian format (MSB first) or little endian
format (LSB first).
CRC 16 bits This field contains a 16 bit CRC which is calculated on all
the data bits of the message bytes.
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