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PPC-2000 User’s Guide Appendix A: Modbus Protocol
Doc.# 30002-00 Rev 2.3 Watlow Anafaze 273
Data Field
The contents of the data field varies depending on whether
messages originate from a master or slave. Data fields in slave
messages consist of hexadecimal values.
Data fields of master messages contain additional information
which the slave must use to take the action defined by the
function code. This can include items like digital and register
addresses, the quantity of items to be handled, and the count of
actual data bytes in the field.
For example, if the master requests a slave to read a group of
holding registers (function code 03), the data field specifies the
starting register and how many registers are to be read.
If no error occurs, the data field of a response from a slave to a
master contained the data requested. If an error occurs, the
field contains an exception code that the master application can
use to determine the next action to be taken.
The data field can be nonexistent (of zero length) in certain
kinds of messages, where the function code alone specifies the
action.
Error Checking Field
The error checking field contains a 16-bit value implemented
as two 8-bit 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.
Field Format
When messages are transmitted on standard Modbus serial
networks, each character or byte is sent in this order (left to
right):
Least Significant Bit (LSB)........Most Significant Bit (MSB)
The bit sequence is:
• With Parity Checking
• Without Parity Checking
Start 1 2 3 4 5 6 7 8 Parity Stop
Start 1 2 3 4 5 6 7 8 Stop Stop