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Technical changes reserved
The serial Transmission Modes:
Two different serial transmission modes are defined: The RTU mode
and the ASCII mode. It defines the bit contents of message fields
transmitted serially on the line. It determines how information is
packed into the message fields and decoded.
The transmission mode (and serial port parameters) must be the
same for all devices on a Modbus serial line. Although the ASCII
mode is required in some specific applications, interoperability
between Modbus devices can be reached only if each device has
the same transmission mode.
When devices communicate using the RTU (Remote Terminal Unit)
mode, each 8–bit byte in a message contains two 4–bit hexadecimal
characters. The main advantage of this mode is that its greater cha-
racter density allows better data throughput than ASCII mode for the
same baud rate. Each message must be transmitted in a continuous
stream of characters. (only RTU mode is used in HTG410H)
The format (11 bits) for each byte in RTU mode is:
- Coding System: 8–bit binary
Bits per Byte:
- 1 start bit
- 8 data bits, least significant bit sent first
- 1 bit for parity completion
- 1 stop bit
Even parity is required; other modes (odd parity, no parity)
may also be used.
The physical medium
A Modbus solution over serial line should implement an electrical
interface in accordance with EIA/TIA-485 standard also known as
RS485 standard. This standard allows point to point and multipoint
systems, in a "two-wire configuration". In addition, some devices
may implement a "Four-Wire" RS485-Interface.
In such a Modbus system, a master device and one or several slave
devices communicate on a passive serial line.
On standard Modbus system, all the devices are connected on a
trunk cable constituted by 3 conductors. Two of those conductors
the two-wire configuration form a balanced twisted pair, on which
bi-directional data are transmitted, typically at the bit rate of 9600
or 19200 bits per second, other baud rates may be possible : 1200,
2400, 4800, … 38400 bps, 56 Kbps, 115 Kbps. A third conductor
must also interconnect all the devices of the bus: the common.
The Query: The function code in the query tells the addressed slave
device what kind of action to perform. The data bytes contain any
additional information that the slave will need to perform the function.
The data field must contain the information telling the slave which
register to start at and how many registers to read. The error check
field provides a method for the slave to validate the integrity of the
message contents.
The Response: If the slave makes a normal response, the function
code in the response is an echo of the function code in the query.
The data bytes contain the data collected by the slave, such as regis-
ter values or status. If an error occurs, the function code is modified
to indicate that the response is an error response, and the data bytes
contain a code that describes the error. The error check field allows
the master to confirm that the message contents are valid.
Query message
from Master
Response message
from Slave
The Query–Response Cycle (QRC)
HTG410H
Modbus and RS485 specific wiring rules
Device Address
Function Code
Eight-Bit
Data Bytes
Error Check
Device Address
Function Code
Eight-Bit
Data Bytes
Error Check
