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8
Communication Guide
8.1 Overview of Communication Protocol
ASJ10L-LD1A/C residual current operated relay uses Modbus-RTU communication protocol.
The protocol defines check codes, data sequences, etc. in detail, which are necessary for specific
data exchange. The Modbus-RTU protocol uses a master-slave acknowledgment connection (half
duplex) on a communication line, which means that signals are transmitted in opposite directions on
a single communication line. First, the host computer's signal is addressed to the only terminal
device (slave), then the response signal from the terminal device is transmitted to the host in the
opposite direction.
The Modbus-RTU protocol only allows communication between the host (PC, PLC, etc.) and
terminal devices, and does not allow data exchange between independent terminal devices, so that
each terminal device does not occupy the communication line when they are initialized, but only
responds to query signals reaching the local computer.
8.1.1 Transmission Mode
The information is transmitted asynchronously and in bytes. The communication information
transmitted between the master and the slave is in 11-bit word format, which consists of 1 start bit, 8
data bits (the lowest valid bit is sent first), no check bit, and 1 stop bit.
8.1.2 Information Frame Format
Address
Function
data
CRC
1 byte
1 byte
n byte
2 bytes
Address code: the address code is composed of one byte (8-bit binary) at the beginning of the
frame and the decimal is 0-255. Only 1-247 is used in the residual current relay, and other addresses
are reserved. These bits indicate the address of the user-specified terminal device that will receive
the host data from it. The address of each terminal device must be unique. Only the terminal to
which it is addressed will respond to the query containing the address. When the terminal sends back
a response, the slave address data in the response tells the host which terminal is communicating
with it.
Function code: the function code tells the terminal to which it is addressed which function to
perform. The following table lists the function codes used in this series of instruments as well as
their significance and function.
function
definition
Function
03H/04H
Read
data
register
Get the current binary value of one or more
registers
10H
Modify
multi
register
Set binary values into a series of multiple
registers
Data area: the data area contains the data required by the terminal to perform specific functions
or the data collected when the terminal responds to the query. The contents of the data may be
values, reference addresses, or set values. For example, the function code tells the terminal to read a
register, and the data area needs to indicate which register to start from and how many data to read.
The embedded address and data vary according to the types and the different contents between the
slaves.
