Modbus TCP Specification
22 (46)
5
Modbus TCP Specification
The Modbus protocol is a
single master protocol
. The Modbus TCP
client
(master) controls the
entire communication, and the connected Modbus TCP
servers
(slaves) are only allowed to
respond to its requests. The Anybus M-Bus to Modbus-TCP gateway is a Modbus TCP
server
.
Modbus TCP communication requires an established connection between the servers and the
client over a specified TCP port. If there is a network firewall between the servers and the client,
the specified TCP port must be opened in the firewall. The default port number is 502.
5.1
Function Codes
Supported Modbus Function Codes
Code
Name
Description
0x01
Read Coil
Not used
0x03
Read Holding Register
Reading of meter data
0x05
Write Single Coil
Not used
0x06
Write Single Register
Not used
0x10
Write Multiple Register
Not used
0x0F
Force Multiple Coil
Not used
0x2B
Read Device Identification
Reading of device data by MEI = 0x0E
Function codes marked “Not used” are replied with ILLEGAL DATA ADDRESS (0x02). Other
unsupported codes are replied with ILLEGAL FUNCTION (0x01).
If the function code 0x2B (Read Device Identification) is used with MEI=0x03, the gateway will
respond with identification data. The values 0x01 and 0x02 are supported as Device ID code,
allowing to retrieve basic and regular device identification data.
Identification Data
Code
Name
Data type
20 UL model
80 UL model
Type
0x00
VendorName
String
HMS Industrial Networks AB
Basic
0x01
ProductCode
String
1
2
Basic
0x02
MajorMinorRevision
String
001
Basic
0x03
VendorUrl
String
www.anybus.com
Regular
0x04
ProductName
String
Anybus M-Bus to Modbus-TCP Gateway
Regular
0x05
ModelName
String
Standard
Regular
0x06
UserApplicationName
String
Anybus M-Bus to
Modbus-TCP 20
Anybus M-Bus to
Modbus-TCP 80
Regular
5.2
Data Format
The arrangement of data in the Modbus registers corresponds to the usual structure. It uses big
endian representation. For the 16 bit registers, the higher byte is sent first, then the lower byte.
Example
: value:
0x1234
transmission order:
0x12
,
0x34
If number and data ranges go beyond 16 bits, representation is similar. Again, the most
significant 16 bit register is sent first and is addressed with the lowest register address.
Example
: value:
0x12345678
transmission order:
0x12
,
0x34
,
0x56
,
0x78
The byte order of 32 bit and 64 bit values can be changed within the system configuration file by
setting the parameter
MODBUS_SWAP
, see
Configuration Files, p. 39
.
M-Bus to Modbus-TCP Gateway User Manual
SCM-1202-0096-EN 2.1
