Interface Module XC Modbus
Protocol Converter / User Manual
Page 3 from 10
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Communication (Modbus RTU, Protocol Converter)
The interface port RS485 is conforming to
“Modbus Application Protocol V1.1b”: Modbus RTU
(Remote Terminal Unit).
Fixed baud rate from the 2
nd
electrical interface (see EMH COMBI-TOOL-Software: communication
interfaces, set please the data transmission at a fixed baud rate without baud rate changeover), data
format: 8N1;
The following rules define the protocol for information transfer between a Modbus MASTER and the
Interface Module XC Modbus
:
The module
supports only the “Modbus RTU Mode of Transmission”: Read Holding Registers.
Transmission settings: 8N1 (1 start bit, 8 data bits, no parity, 1 stop bit).
All communications on the RS485 bus conform to a MASTER/SLAVE scheme. In this scheme,
information and data is transferred between a Modbus MASTER device (e.g. AMR Software) and
up to 32 SLAVE devices (Modbus Interface Module XC).
The MASTER initiates and controls all information transfer on the communications loop.
A SLAVE device never initiates a communications sequence.
All communications activity o
n the loop occurs in the form of “PACKETS”. A packet is a serial
string of 8 bit / byte. The maximum number of bytes contained within one packet is 255.
All packets transmitted by a MASTER are REQUESTS. All packets transmitted by a SLAVE are
RESPONSES.
At most one SLAVE can respond to a single request from a MASTER.
The Modbus module supports the
Function Code 3
(Read Holding Registers) with signed 32 bit
values (
Signed Int32, Type D
). All values includes 2 words (registers) with 16 bits each: first register
(even register addresses: 0,2,4,6...) for high-word and second register (odd register addresses:
1,3,5,7...) for low-word.
Example:
Value = 12345678
Hex
(32 bit := 4 Byte) start at register address 20:
Register
address
Value
Type of value
20
1234
Hex
(:= 4660
Dec
)
16 bit, High-Word
21
5678
Hex
(:= 22136
Dec
)
16 bit, Low-Word
:= 4660 × 65536 + 22136 = 305419896
The Modbus module uses for
ID-Address
the first data value in the service data list (TS): register
addresses 0 and 1. For setting the value use please the EMH COMBI-TOOL-Software.
Possible value range: 1 to F7
Hex
:= 1 to 247
Dec
/ Value outside of this range: ID-Address := 1 (default)
Example:
Value (ID-Address) = 0000007B
Hex
(:= 00000123
Dec
) starts at register address 0:
Register
address
Value
0
0000
Hex
1
007B
Hex
(:= 123
Dec
)
