KROHNE CORIMASS MFC 85 Interface EN - Manual (Page 28)

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If the instrument is connected correctly, it should now communicate with an external master.
Please note, that all devices on the bus (including the master) must be set to the same
communications protocol, baudrate and format, but with a different address.
6. Modbus 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.
6.1 RTU Frame Format
The format of the Query and Response frames vary slightly depending upon the command
function. The basic form is outlined below.
Silent
Period
Slave
Address
Function
Code
Register Start Address
or
Byte Count when
required
No.of.Points
or
Data bytes
when required
CRC
3.5 T 8 bits 8 bits 8 bit byte count
16 bit address
n x 8 bits 16 bits
Silent period.
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 us.
Slave Address.
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.
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.
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
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.
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.
CRC
This field contains a 16 bit CRC which is calculated on all the data bits of the message bytes.