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Appendix B: Modbus Map
Modbus communications uses master-slave communication in which only the master can
initiate a transaction, called a query. The slave addressed by the query will respond by
either supplying the requested data to the master or by performing the requested action. A
slave device never initiates communications, and will always generate a response to the
query unless certain error conditions occur. The GSC400 is designed to communicate on
the Modbus only as a slave device.
10.1
Communication Details
A master can query slaves individually or query all slaves collectively by initiating a
broadcast message. A slave does not send a response message to a broadcast query. If a
query requests actions unable to be performed by the slave, the slave response message
will contain an Exception Response Code defining the error detected. Query and response
messages share the same message structure. Each message is comprised of four message
fields: the Device Address, the Function Code, the Data Block, and the Error Check field.
Query / Response Message Structure:
* Device Address
* Function Code
* Eight-Bit Data Bytes
* Error Check
Device Address Field
The Device Address field contains the unique Modbus address of the slave being queried.
The addressed slave will repeat its address in the Device Address field of the response
message. This field is 1 byte.
Function Code Field
The Function Code field in the Query message defines the action to be taken by the
addressed slave. This field is echoed in the Response message, and will be altered by
setting the MSB of the field to "1" if the response is an error response. This field is 1
byte.
Data Block Field
The query Data block contains additional information needed by the slave to perform the
requested function. The response Data block contains data collected by the slave for the
queried function. An error response will substitute an Exception Response Code for the
Data Block. The length of this field varies with each query.
Error Check Field
The Error Check field provides a method for the slave to validate the integrity of the
query message contents and allows the master to confirm the validity of response
message contents. This field is 2 bytes.
Summary of Contents for GSC400 Series
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Page 16: ...Figure 5 Digital IO Example ...
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