Modbus Protocol part 3

More into Modbus message structure

Address field

Address field contains two ASCII characters or 8 bits (RTU).Also we know that the valid slave addresses are in 0-247 range decimal.

·         In the response slave puts its own address in the address field to let the master know which slave is responding.

Function filed

The function field consists of two ASCII characters (ASCII) or 8 bits (RTU).In error free response the function code is similar to receive query. But if there is some error, the most significant bit of query function code is set to 1 in the response function field.

Ø  Example

Query function code=0000 0011

Error free response function code=0000 0011

If there is an error, response function code=1000 0011.

In erroneous situation slave puts unique code in to data field which describes the error occurred.

Data field

The data field is constructed using sets of two hexadecimal digits in the range of 00 to FF hexadecimal (pair of ASCII characters or one RTU character).In query, this data field contains discrete and register addresses, the quantity of items to be handled, the count of data bytes to follow in the data field and the data to be written in to registers.

If no error occurs response data field contains the requested data else it contains unique code which describes the error.

Error checking field

How Modbus does detect errors?

Standard Modbus serial networks use two kinds of error checking.

1.      Parity checking (even or odd) can be optionally applied to each character.

2.      Frame checking (LRC or CRC) is applied to the entire message.

Both the character check and message frame check are generated in the master device and applied to the message contents before transmission. The slave device checks each character and the entire message frame during receipt.

1.      Parity Checking

Users can configure controllers for Even or Odd Parity checking, or for No Parity checking. This will determine how the parity bit will be set in each character. If either Even or Odd Parity is specified, the quantity of 1 bits will be counted in the data portion of each character (seven data bits for ASCII mode, or eight for RTU).

The parity bit will then be set to a 0 or 1 to result in an Even or Odd total of 1 bit.

2.      Frame checking

IN ASCII mode there are two ASCII characters in the error checking field. The error check characters are the result of a Longitudinal Redundancy Check (LRC) calculation that is performed on the message contents, exclusive of the beginning ‘colon’ and terminating CRLF characters.

What is LRC?

The LRC field checks the contents of the message, exclusive of the beginning ‘colon’ and ending CRLF pair. It is applied regardless of any parity check method used for the individual characters of the message. The LRC field is one byte, containing an 8–bit binary value. The LRC value is calculated by the transmitting device, which appends the LRC to the message. The receiving device calculates an LRC during receipt of the message, and compares the calculated value to the actual value it received in the LRC field. If the two values are not equal, an error results.

In RTU mode error checking field has two bytes. The error check value is the result of a Cyclical Redundancy Check calculation performed on the message contents. The CRC field is appended to the message as the last field in the message. When this is done, the low–order byte of the field is appended first, followed by the high–order byte. The CRC high–order byte is the last byte to be sent in the message.

For more details refer Modbus official site.