byte received will be an address. The follow command message structure is used, where T is the required character delay. Response from the instrument is based on the command.
T1,T2,T3 ADDRESS | FUNCTION | DATA | CHECKSUM | T1,T2,T3 |
N X |
|
Address Field
The address field immediately follows the beginning of the frame and consists of
Each slave must be assigned a unique address and only the addressed slave will respond to a query that contains its address. When the slave sends a response, the slave address informs the master which slave is communicating.
Function Field
The Function Code field tells the addressed slave what function to perform. MODBUS function codes are specifically designed for interacting with a PLC on the MODBUS industrial communications system. Command codes were established to manipulate PLC registers and coils. As far as the Transmitter is concerned, they are all just memory locations, but the response to each command is consistent with Modbus specifications.
The high order bit in this field is set by the slave device to indicate an exception condition in the response message. If no exceptions exist, the
Data Field
The data field contains information needed by the slave to perform the specific function or it contains data collected by the slave in response to a query. This information may be values, address references, or limits. For example, the function code tells the slave to read a holding register, and the data field is needed to indicate which register to start at and how many to read.
Error Check Field (CRC)
This field allows the master and slave devices to check a message for errors in transmission. Sometimes, because of electrical noise or other interference, a message may be changed slightly while it is on its way from one device to another. The error checking assures that the slave or master does not react to messages that have changed during transmission. This increases the safety and the efficiency of the MODBUS system.
The error check field uses a
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