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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
8-BITS
Address Field
The address field immediately follows the beginning of the frame and consists of 8-bits.
These bits indicate the user assigned address of the slave device that is to receive the
message sent by the attached master.
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 high-order bit is maintained as zero in
the response message.
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 CRC-16 check in the RTU mode.
11/14/2006
FUNCTION DATA
8-BITS
N X 8-BITS 16-BITS
Page 10 of 23
CHECKSUM
T1,T2,T3
Rev. 14
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