© July 2017 Van Essen Instruments. All rights reserved
.
4
The master issues a Modbus request to the slaves in two modes:
In unicast mode, the master addresses an individual slave. After receiving and processing
the request, the slave returns a message (a 'reply') to the master. In that mode, a Modbus
transaction consists of 2 messages: a request from the master, and a reply from the slave.
Each slave must have a unique address (from 1 to 247) so that it can be addressed
independently from other nodes.
In broadcast mode, the master can send a request to all slaves. No response is returned to
broadcast requests sent by the master. The broadcast requests are necessarily writing
commands. All devices must accept the broadcast for writing function. The address 0 is
reserved to identify a broadcast exchange.
The Modbus RTU frame format for the Diver-MOD is shown in Figure 4. The client that initiates a
Modbus transaction builds the Modbus Protocol Data Unit (PDU) represented by the green blocks in
Figure 4, and then adds the address and error check(CRC) to build the appropriate communication
PDU.
A master addresses a slave by placing the slave address in the address field of the message. When
the slave returns its response, it places its own address in the response address field to let the
master know which slave is responding.
The function code indicates to the server what kind of action to perform. The function code can be
followed by a data field that contains request and response parameters.
All Diver-MOD commands contain checksum information (CRC), to enable detection of transmission
errors. Error checking field is the result of a "Redundancy Checking" calculation that is performed on
the message contents. The Diver-MOD uses the RTU calculation method for the CRC.
Details about the different fields of the Modbus frame for the Diver-MOD can be found in Table 1. The
Diver-MOD interface supports the function codes listed in Table 2.
Figure 4
Modbus message frame for the Diver-MOD on a serial line.
Table 1
Diver-MOD frame format.
Name
Length (bits)
Function
Start
28
At least 3½ characters times of silence (mark condition)
Address
8
Slave address
Function
8
Indicates the function code; e.g., read coils/inputs
Data
N × 8
Data + length will be filled depending on the message type
CRC
16
Checksum
End
28
At least 3½ character times of silence between frames