MPCS: A Wireless Communication Protocol for Ubiquitous Industrial Environment - Advanced Research on Computer Education, Simulation and Modeling - page 406

the most important controller in industrial application, but different brands almost
have different inherent monitoring protocols. For example, Siemens PLC S7-200 uses
PPI, Siemens PLC S7-300 uses MPI, A-B PLC Micrologix and SLC series uses DF1,
MITSUBISHI PLC FX2N has different inherent protocols. It is common for a factory
to using different kinds of PLCs in their product lines. How to make these different
PLCs can communicate with each other is a key problem.
This paper addresses the MPCS protocol to solute this problem. MPCS is the ab-
breviation of MODBUS for Producer/Consumer Services. MPCS protocol applies
ZigBee as wireless protocol among wireless nodes. The core of MPCS is to use the
MODBUS protocol without polling to carry industrial equipments protocol. MOD-
BUS protocol is applied to an electronic controller on the lingua franca. The most
important is the protocol also must be supported by typical PLCs. But most of the
industrial monitoring systems adopt fixed period polling with less consideration about
dynamic period in using MODBUS. So in ubiquitous industrial environment, the
MODBUS protocol cannot satisfy the latency requirement of wireless protocol and it
cannot guarantee the real-time monitoring of industrial environment conditions. And
the polling method of MODBUS adds extra loads and burdens the wireless channel.
The MPCS protocol changes the polling mechanism of MODBUS and the slave
equipments can send the messages by themselves periodically without receiving
query command from the master equipment.
MPCS protocol applies ZigBee as wireless protocol among wireless nodes and
MODBUS with sending message periodically as industrial monitoring protocol. Ex-
periment shows that the combination of MPCS and ZigBee is a good way to solute the
interoperability in ubiquitous industrial environment. MPCS has the advantages in
saving bandwidth and lightening servers' load and enhances the real-time performance
of industrial wireless sensor networks.
This paper first introduces the experiment system. Secondly ZigBee and ZigBee
gateway are discussed. Thirdly it introduces MPCS protocol in different PLCs. There
is an experiment to test MPCS protocol in this section. Finally it is a conclusion.
2 System Overview
Wireless sensors provide the network with the ability to reconfigure on the fly without
being tied down by signal cables. The goal of the system is to implement such a net-
work using PLCs connected by Zigbee transceivers to a central computer that inter-
faces with a database accessible. The three major components consist of different
brands of PLCs, CC2530 Low Power Transceivers for ZigBee, and the SCADA soft-
ware hosted on the central computer. A block diagram of the high level design is
provided in Fig 1 below.
There are PLCs in this system, including Siemens PLC S7-224 and XINJIE PLC
XC3-24RT-E. S7-224 uses PPI protocol and XC3-24RT-E uses MODBUS protocol.
PLCs collect data from the industrial machinery and transmit them in MODBUS
format to ZigBee gateway in which data are processed. ZigBee gateway packed data
according to the ZigBee protocol, and transmitted them to via radio. Finally, data are
transmitted to ZigBee gateway of the central computer. At the central computer, in-
coming data from ZigBee gateway of it are received and processed by the SCADA
software which is developed in Kingview. [5]