Open Journal of Applied Sciences, 2013, 3, 18-23
doi:10.4236/ojapps.2013.32B004 Published Online June 2013 (http://www.scirp.org/journal/ojapps)
A Realization Method of Protocol Conversion Between
Modbus and IEC61850*
Fan Zhang1, Yongli Zhu1, Chunyu Yan2, Jiangang Bi2, Haijun Xiong1, Shuai Yuan2
1School of Control and Computer Engineering, North China Electric Power University, Baoding, China
2China Electric Power Research Institute, Beijing, China
Received 2013
ABSTRACT
In order to adapt to the construction needs of the smart grid, smart substation need to solve the problem of protocol
conversion between the conventional non-standardized condition monitoring equipment and the standardized monitor-
ing system. This paper proposed a realization method of conversion method between Modbus and IEC61850. Ob-
ject-oriented technology is used for information model on Modbus. After the analysis of IEC61850 and MMS informa-
tion and service model, to establish the model mapping relationship between IEC61850, MMS and Modbus based on
the principle of minimum information point which is one to one correspondence. Combined with the implementation of
SISCO MMS-EASE LITE software development kit programming, giving a realization method using QT programming
techniques based on the mapping model. Finally, the establishment the interval controller as an example of protocol
scheme verifies the correctness and the feasibility of protocol conversion method.
Keywords: Modbus; IEC 61850; Protocol Conversion; Model Mapping; Consolidated Monitoring Unit
1. Introduction
With the gradual popularization and application of intel-
ligent substation, IEC61850 standard is widely used in
substation automation system [1]. State Grid Corporation
developed the substation equipment online monitoring
system technical guidance in the process of building a
strong and smart grid by 2011 [2]. It defined that condi-
tion monitoring system is structured into the process
layer, the spacer layer and the station layer. The commu-
nication between the layers is used IEC61850 protocol.
To achieve the goal of intelligent substation, on the one
hand to develop the standard specification condition
monitoring equipment to replace aging seriously equipment,
on the other hand, the condition monitoring equipment
which need not replace should be completed the stan-
dardized transformation to adapt to the current grid de-
velopment needs. The conventional condition monitoring
equipment mainly used in Modbus, IEC60870-5-103
(103), IEC60870-5-104(104), CAN bus standard as well
as a large number of proprietary protocols nowadays
[3,4]. Modbus whose structure is simple optimization and
support a variety of characteristics of electrical interface
and transmission medium is a protocol supported by
most condition monitoring devices. Therefore, It is key
to study the protocol conversion between Modbus and
IEC61850 for the substation condition monitoring
equipment standardization.
Model mapping and protocol conversion research be-
tween the IEC61850 standard and MMS, 103, 104 stan-
dard has being focused on nowadays. The research on the
model mapping between Modbus and IEC61850 standard
is less [5]. [6] gives a model mapping method between
Modbus and IEC61850 using the System Configuration
Description Language schema file for expanded imple-
mentation of the model, but does not give the specific
engineering part of the protocol conversion module.[7]
gives a CORBA-based the IEC61850 protocol conver-
sion device. The method can be flexible and facilitate the
realization of the protocol conversion, but due to the
complexity of the CORBA ability, there are some limita-
tions to realize it. IEC61850-8-1 has given the method to
the implementation issues for IEC61850 which is to map
the abstract information and services model to the
Manufacturing Message Specification (MMS) [8]. Due to
the complexity of the MMS, most of the current domestic
manufacturers achieve the mapping of the communica-
tion services by SISCO MMS-EASE LITE tool package
in the development of IEC61850 substation product.
[9,10] summarizes the information mapping correspon-
dence between MMS and IEC61850 and gives the ser-
vices mapping correspondence between ACSI and MMS
*ProjectSupported by the science and technology projects of State Grid
Corporation (GYl7201200047)

F. ZHANG ET AL. 19
service, but it cannot be used in the standardization of
equipment condition monitoring access issues.
2. The Information Model and Mapping
2.1. The information model of IEC61850 and
MMS
The actual communication functions and device has been
described in an abstract way in IEC61850.It defines the
five basic class model which include the Server, the
logical device(LD),the logical node(LN),the data(Data),
the data attribute(DA).A Server includes multiple LD,
and each LD contains multiple LN, as well as each LN
contains multiple Data. It can be seen that IEC61850
information model is a hierarchical and three-dimen-
sional object-oriented structure.MMS is kind of specific
communication service of IEC61850. The information
Model of MMS is consistent with IEC61850. Each MMS
application must contain at least one VMD object. VMD
is the root in the object structure of the entire MMS, and
other objects are included in the VMD object. Some
types are included in the other sub-objects and become
deeper sub-object.
In view of the MMS and IEC61850 model structure
and its modeling method can carry out a similar analysis
of the information structure of the Modbus protocol to
complete the Modbus information modeling.
2.2. Modbus and its Information Model
GB/T 19582-2008 national standard gives two commu-
nication protocols about the Modbus Application proto-
col and service standards which included the Modbus
serial link norms based on the TIA/EIA standard and the
Modbus TCP/IP protocol based on the RFC793 and
RFC791 standard [11,12]. And the Modbus serial link
norms can divided into two communication modes of
Modbus RTU and Modbus ASCII. Modbus RTU and
Modbus TCP are mostly supported in substation condi-
tion monitoring equipment with its simple optimization
feature. The Modbus parameter type is less, mainly in-
cludes coil, discrete, input registers and holding registers.
The length of the first two is 1 bit, as the latter two is 2
bytes.
The Modbus data structure of the information is dif-
ferent from the IEC61850 and MMS which is a linear
plane. The information data is defined by a correspond-
ing Application Service Data Unit(ASDU). Figure 1
shows the ASDU structure of two types of Modbus. In
order to more clearly describe it contains information, we
will divide the Protocol Data Unit(PDU) to three parts
which contains the data starting address(DSA), data
size(DS) and data content(DC). DBA and DS here refer
to the read or write the starting address and the length of
the parameter. The specific parameter type is determined
by the function code(FC). We can complete the informa-
tion model of one ASDU by the object-oriented model-
ing technology easily.
2.3. The Mapping Model between Modbus, MMS
and IEC61850
After the above analysis, the mapping from Modbus to
MMS and IEC61850 is same to the flat-screen informa-
tion points mapping to three-dimensional data objects.
Figure 2 gives the mapping relationship between Mod-
bus, MMS and IEC61850. The correspondence is sepa-
rated by dotted lines. In the figure, the address field is the
slave address of the Modbus RTU as well as the unit
identifier of Modbus TCP.
 The Information Mapping
Modbus address field is used to identify a condition
monitoring device, so the Modbus address field is
mapped to logical device of IEC61850 and the domain
model of MMS.DSA can be uniquely identifies a device
measuring point, so DSA is mapped to DA of IEC61850
and the simple named variables. DS is represented the
number of the registers or coils and so on, therefore it
will be mapped to the basic data types of IEC61850 and
that of MMS.
Modbus RTUModbus TCP
A
S
D
U
Slave Address(1 byte)
Transaction identifier
(2 bytes)M
B
A
P
Protocol identifier (2
bytes)
Byte Count (2bytes)
Unit identifier (1 byte)
Function Code (1byte)Function Code
(1 byte)
P
D
U
Data Starting
Address (2 bytes)
Data area
Data Starting Address
(2 bytes)
Data area
Data Size (2 bytes)Data Size (2 bytes)
Data ContentData Content
CRC error check code
(2 bytes)
Figure 1. The ASDU structure of Modbus.
Figure 2. The mapping relationships between Modbus,
MMS and IEC61850.

F. ZHANG ET AL.
20
 The Service Mapping
The function code specifies the different Modbus type
of service, so it should be established the mapping from
Modbus FC to the ACSI services or MMS services. The
relationship is shown in Table 1.
3. The Realization of Protocol Conversion
System Based on the Mapping Model
3.1. Design of System Architecture
We designed the following protocol conversion mode
based on the mapping model. Firstly, the point data
model of Modbus map to the variable of MMS, and then
to finish the mapping from MMS to ACSI. Finally, to
complete the Modbus protocol conversion to IEC61850.
Figure 3 is a system architecture block diagram.The sub-
station online monitoring system Technical Guidelines
require the protocol conversion module should be deco-
rated in comprehensive monitoring unit(CMU) to support
multiple protocols translation of the state devices. In or-
der to complete the following Modbus status monitoring
device communication, we need to design the modbus
master model in protocol conversion module.
Table 1. The mapping table of modbus function code.
FC ACSI Service
MMS
Service
Functional
Description
0x01,0x02 GetDataValues Read
Read the single
point status data
0x03,0x04 GetDataValues Read
Read the Integers,
floating point data
0x05,0x0F SetDataValues Write
Set the single point
of state control
0x06,0x10 SetDataValues
SetSGValues Write Set the control operation status
Figure 3. The system architecture block diagram.
3.2. Realization of System Software
 Design of the Software Architecture
QT as the software development tools is used in the
system. Figure 4 is the design of software structure dia-
gram. The system overall is design for three layers which
include the top-level for MMS service layer, the mid-
dle-level for protocol conversion layer and the low-level
for Modbus service layer. The top-level provide MMS
services to communicate with the station monitoring unit
(SMU) or the standard client of IEC61850. If the service
request which get from SMU is correct, it will send the
request to the middle-level. The MMS requested service
data in MMS-EASE LITE is encapsulated in the struc-
ture MVLU_XX_VA_CTRL, if the service is read, XX
is RD, otherwise XX is WR. The low-level receive or
send Modbus messages as a Modbus Master to commu-
nicate with the condition monitoring devices(CMD). The
Modbus response data information from the correct re-
sponse will be sent to the middle layer. The middle-level
will complete the analysis of MMS service data informa-
tion provided by upper and construct the corresponding
Modbus request protocol frame to send to lower. And it
will complete the analysis of Modbus response data from
lower and construct the corresponding MMS response
service to send to upper. The signals and slots mecha-
nism of QT has good characteristics of inter-thread
communication, so the information exchange of system
between the three layers can utilize the mechanism to
complete. Figure 6 shows the signals and slots function
of the software design and identifies the relationships
between the different signals and slots. The solid line
represents the system receives signals and slots from the
Modbus Response from CMD and the data flow. The
broken line represents the association of the signals and
slots after receipt of the MMS request and data flow.
 Design of Modbus class diagram
Signal and Slot FunctionSignal/SlotDescription
mms_service_rd(MVLU_RD_VA_CTRL*) SIGNAL
Top-layer
mms_s ervice_wr(MVLU_WR_VA_CTRL*)
on_response_rd(MVLU_RD_VA_CTRL*) SLOT
on_response_wr(MVLU_WR_VA_CTRL*)
onWriteDataReady (QByteArray)
SIGNAL
Midd l e-
layer
onRdResponse(MVLU_RD_VA_CTRL*)
onWrResponse(MVLU_WR_VA_CTRL*)
onReq u estRd(MVLU_RD_VA_CTRL*)
SLOTonRequestWr(MVLU_WR_VA_CTRL*)
onRecieveData (QByteArray)
device_data_ready (QByteArray)SIGNALLow-layer
write_data_device (QByteArray)SLOT
Modbus
Response
MMS
Response
MMS
Request
Modbus
Request
Figure 4. The design of software structure diagram.

F. ZHANG ET AL. 21
We complete the Modbus information modeling with
the object-oriented approach. Figure 5 shows the Mod-
bus class diagram design. The ModbusFrame class is
used to describe the parameters of the Modbus ADU.
Establishing ModbusServer abstract class as an abstract
of the Modbus function can be inherited by different in-
terface communication protocols for service. Modbus-
ServerTCP and ModbusServerRTU inherited from Mod-
busServer what achieve the service of Modbus TCP and
Modbus RTU respectively. We established two subclasses
for ModbusServerRTU. The two subclasses are respon-
sible for building and parsing of the response or request
of the Modbus RTU. In addition, we create an abstract
class FrameBase. FrameBase can be inherited by differ-
ent subclass which is decided by function code and can
complete the building or parsing of different Modbus
frame.
 The process of software
Figure 6 shows the flowchart of the system software.
The system can be designed two threads, one thread is
used for receiving the MMS requests from SMU or
IEC61850 client, and another r is used for receiving the
Modbus response from CMD. As a CMU should connect
to multiple CMD, we provide a thread reincarnation
scanning up to four serial ports or network ports in order
to ensure the overall operating efficiency of the system.
It is to say we should increase the number of threads to
appropriate the number of devices. In addition, taking
into account the MMS communication frequency of the
upper and lower Modbus communication frequency in-
consistency, we establish the policy of the pipe.
+bool InitialModbus()
+bool IsActiveModbus()
-quint8 SlaveAddress
-quint8 FC
-quint16 BeginAddress
-QByteArray Data
-quint16 CRC
-QByteArray FrameData
+virtual QByteArray CreateModbusRequest()
+virtual void NormalResponseProcess()
+virtual void ExceptionResponseProcess()
+QByteArray CreateModbusRequest()
+void NormalResponseProcess()
+void ExceptionResponseProcess()
+QByteArray CreateModbusRequest()
+void NormalResponseProcess()
+void ExceptionResponseProcess()
+QByteArray CreateModbusRequest()
+void NormalResponseProcess()
+void ExceptionResponseProcess()
+QByteArray CreateModbusRequest()
+void NormalResponseProcess()
+void ExceptionResponseProcess()
-ModbusFrame * m_pframe
-FrameBase * m_pbaseframe
+bool initialModbusFrame()
+QByteArray getRequestADU()
+int exeActionFC()()
+void closeServer()
+bool initialModbusFrame()
+QByteArray getRequestADU()
+int exeActionFC()
+void closeServer()
+bool initialModbusFrame()
+int exeActionFC()
+void closeServer()
+bool initialModbusFrame()
+QByteArray getRequestADU()
+void closeServer()
Figure 5. Modbus class diagram.
Figure 6. The flow chart of software.
System need tree files at initialization time which in-
clude the condition monitoring device description file,
the device IED configuration file and the mapping asso-
ciated file. The device description file is given by the
production manufacturers. The file should contain the
representation method of the device data information in
Modbus PDU and so on.The device IED file is generated
by SCL configuration tools and should follow the SCL
modeling rules.The mapping associated files need manu-
ally or point table tool to carry out.In order to achieve the
procedures of the system mapping relationship conven-
iently and efficiently,A embedded relational database
which is SQLite is needed to establish three relational

F. ZHANG ET AL.
22
tables as the mapping model. Table 2 shows the basic
information mapping table for storage Modbus informa-
tiong and MMS variable mapping relationship. Table 3
shows the writing services relationship and Table 4
shows the reading services relationship. The initialization
of database is to resolve the mapping associated file
which establish by manual.
Table 2. Basic information mapping table.
Field Type Description
MMSVariable VarChar(65) MMS variable name
SlaveAddress Int Modbus address area
StartAddress Int Modbus data starting address
DataSize Int Modbus data size
Table 3. The service relationship of read.
Field Type Description
MMSVariable VarChar(65) MMS variable name
FunctionCode Int Modbus Function Code
(0x01~0x04)
Table 4. The service relationship of write.
Field Type Description
MMSVariable Varchar(65) MMS variable name
FunctionCode Int Modbus Function Code
(0x05,0x06,0x0F,0x10)
Table 5. A part of the data object of interval controller
mapping relationship.
MMS Modbus
MMS objects Slave
address FC DSA DS Description
MMXU1$MX$A$phs
A$cVal$mag$f 01 04 0001 2
A phase
current
MMXU1$MX$A$phsB
$cVal$mag$f 01 04 0003 2
B phase
current
MMXU1$MX$A$phsC
$cVal$mag$f 01 04 0005 2
C phase
current
MMXU1$MX$PPV$p
hsAB$cVal$mag$f 01 04 0007 2
AB line
voltage
LLN0$ST$Mod$stVal 01 0406 0011 2
Device mode
of operation
4. Case Study
MMS-EASE LITE gives the corresponding IED con-
figuration file of the interval controller E1Q1SB1. We
will complete the mapping model of E1Q1SB1 by the
protocol conversion method. First, MMS objects should
be created according to the IED file to complete the
mapping from MMS to IEC61850 standard. Then the
mapping model should be established from MMS objects
to Modbus information model after the analysis of MMS
objects. In the example, we test a part of data objects and
properties of logical Node MMXU1 and LLN0. The Ta-
ble 5 shows the testing objects. Finally, we will read or
write the phase current, line voltage and so on from the
analog interval controller by the IEC61850 standard cli-
ent.
By simulation, we can convert the ACSI or MMS ser-
vice request to corresponding Modbus request correctly
and return the corresponding Modbus request by the
analog sub-station of Modbus. The system can parse the
information data integrally and return the response of
ACSI or MMS correctly. In addition, the pipeline strat-
egy makes the condition monitoring equipment and sta-
tion-side monitoring device to achieve asynchronous
communication and solves the inconsistent problem of
the communication frequency between MMS and Mod-
bus. The process and results of the test results show that
the system is stable and reliable.
5. Conclusions
The paper researches the information of MMS, IEC61850
and Modbus and their mapping relationship between
their models firstly. The model can accurately describe
the point information and can be more easily achieved
with MMS and IEC61850 mapping. According to the
Modbus information model, a protocol conversion
method is given by QT technology and MMS-EASE
LITE tools package then. The program not only facili-
tates the realization of the protocol conversion, but also
provides a guideline to the private protocol which has a
linear plane features mapping with IEC61850. Finally,
Interval controller as an example verifies the feasibility
and correctness of the established mapping model and
protocol conversion program. The testing results are sat-
isfactory.
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