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Appendix B – Modbus Protocol
NMBA-01 Installation and Start-up Guide
B-7
RTU
When RTU mode is used for character framing, 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 per-
formed on the message contents.
The CRC field is appended to the message as the last field in the mes-
sage. 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.
Additional information about error checking is contained later in this
appendix.
How Characters are
Transmitted Serially
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)
With RTU character framing, the bit sequence is:
With Parity Checking
Without Parity Checking
Error Checking
Methods
Standard Modbus serial networks use two kinds of error checking. Par-
ity checking (even or odd) can be optionally applied to each character.
Frame checking (CRC) is applied to the entire message. Both the char-
acter 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 dur-
ing receipt.
The master is configured by the user to wait for a predetermined time-
out interval before aborting the transaction. This interval is set to be
long enough for any slave to respond normally. If the slave detects a
transmission error, the message will not be acted upon. The slave will
not construct a response to the master. Thus the time-out will expire
and allow the master’s program to handle the error. Note that a mes-
sage addressed to a nonexistent slave device will also cause a time-
out. A proper time-out value for the NMBA-01 is 100 ms.
Start
1
2
3
4
5
6
7
8
Par
Stop
Start
1
2
3
4
5
6
7
8
Stop
Stop
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