Watlow CLS200, MLS300 and CAS200 User Manual download pdf (Page 28)

22 Communications Speciﬁcation

Chapter 2: Modbus-RTU Protocol

The Query-Response Cycle

The Query

The function code in the query tells the addressed slave device what

kind of action to perform. The data bytes contain any additional

information that the slave will need to perform the function. For

example, function code 03 will query the slave to read holding registers

and respond with their contents. The data ﬁeld must contain the

information telling the slave which register to start at and how many

registers to read. The error check ﬁeld provides a method for the slave to

validate the integrity of the message contents.

The Response

If the slave makes a normal response, the function code in the response

is an echo of the function code in the query. The data bytes contain the

data collected by the slave, such as register values or status. If an error

occurs, the function code is modiﬁed to indicate that the response is an

error response, and the data bytes contain a code that describes the error.

The error check ﬁeld allows the master to conﬁrm that the message

contents are valid.

Serial Transmission

Each 8-bit byte in a message contains two 4-bit hexadecimal characters.

This high character density allows better data throughput than ASCII for

the same baud rate. Each message must be transmitted in a continuous

stream.

Coding System

• 8-bit binary, hexadecimal 0 to 9, A to F

• 2 hexadecimal characters contained in each 8-bit ﬁeld of the message

Bits per Byte

• 1 start bit

Device Address

Function Code

8-Bit

Data Bytes

Error Check

Query Message from Master

Response Message from Slave

Device Address

Function Code

8-Bit

Data Bytes

Error Check

1 2 ... 23 24 25 26 27 28 29 30 31 32 33 ... 91 92

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Watlow CLS200, MLS300 and CAS200 User Manual Page 28