Watlow CLS200, MLS300 and CAS200 User Manual download pdf (Page 30)

24 Communications Speciﬁcation

Chapter 2: Modbus-RTU Protocol

When a message is sent from a master to a slave device, the function

code ﬁeld tells the slave what kind of action to perform. Examples are to

read the On/Off states of a group of discrete coils or inputs; to read the

data contents of a group of registers; or to read the diagnostic status of a

slave.

When the slave responds to the master, it uses the function code ﬁeld to

indicate either a normal (error-free) response or that some kind of error

occurred (called an exception response). For a normal response, the

slave simply echoes the original function code. For an exception

response, the slave returns a code that is equivalent to the original

function code with its most signiﬁcant bit set to a logic 1.

For example, a message from the master to slave to read a group of

holding registers would have the following function code:

0000 0011 x3

If the slave device takes the requested action without error, it returns the

same code in its response. If an exception occurs, it returns:

1000 0011 x83

In addition to its modiﬁcation of the function code for an exception

response, the slave places a unique code into the data ﬁeld of the

response message. This tells the master what kind of error occurred, or

the reason for the exception.

The master device’s application program has the responsibility of

handling exception responses. Typical processes are to post subsequent

retries of the message, to try diagnostic messages to the slave, and to

notify operators.

Contents of the Data Field

The data ﬁeld is constructed using sets of two hexadecimal numbers, in

the range of x00 to xFF.

The data ﬁeld of messages sent from a master to slave devices contains

additional information that the slave must use to take the action deﬁned

by the function code. This can include items like descrete 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.

1 2 ... 25 26 27 28 29 30 31 32 33 34 35 ... 91 92

No comments

Watlow CLS200, MLS300 and CAS200 User Manual Page 30