Watlow CLS200, MLS300 and CAS200 User Manual download pdf (Page 31)

Chapter 2: Modbus-RTU Protocol

Communications Speciﬁcation 25

The data ﬁeld can be nonexistent (of zero length) in certain kinds of

messages, where the function code alone speciﬁes the action.

Contents of the 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 (CRC) 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.

How Characters are Transmitted Serially

When messages are transmitted on standard Modbus-RTU 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:

CRC Error Checking

All messages include an error-checking ﬁeld that is based on a Cyclical

Redundancy Check (CRC) method. The CRC ﬁeld checks the contents

of the entire message. It is applied regardless of any parity check

method used for the individual characters of the message.

The CRC ﬁeld is two bytes, containing a 16-bit binary value. The CRC

value is calculated by the transmitting device, which appends the CRC

to the message. The receiving device recalculates a CRC during receipt

of the message, and compares the calculated value to the actual value it

received in the CRC ﬁeld. If the two values are not equal, an error

results.

The CRC is started by ﬁrst preloading a 16-bit register to all 1’s. Then a

process begins of applying successive 8-bit bytes of the message to the

current contents of the register. Only the 8 bits of data in each character

are used for generating the CRC. Start and stop bits do not apply to the

CRC.

During generation of the CRC, each 8-bit character is exclusive ORed

with the register contents. Then the result is shifted in the direction of

the least signiﬁcant bit (LSB), with a zero ﬁlled into the most signiﬁcant

bit (MSB) position. The LSB is extracted and examined. If the LSB was

a 1, the register is then exclusive ORed with a preset, ﬁxed xA001. If the

LSB was a 0, no exclusive OR takes place.

Start 1 2 3 4 5 6 7 8 Stop Stop

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Watlow CLS200, MLS300 and CAS200 User Manual Page 31