Communication protocols and architectures in iPMCC
An iPMCC configuration is made of lots of motor starters. In order to supervise the system, it is necessary to send information such as motor status, current values, alarms, etc… The traditional wire-to-wire connection is not an efficient and cost-effective way when there is a lot of data to be transmitted. Today, transmission via a communication network is the preferred way.
The communications need the support of a common language, which is the communication protocol. The following chart shows the protocols commonly used at different levels of an industrial communications networks. At the moment, the most popular device bus protocols are Modbus SL, Profibus-DP and DeviceNet, while Ethernet TCP/IP is growing very fast.
Fig. N75: Different communication protocols
Modbus
Modbus is a message handling structure introduced by Modicon in 1979. Modbus is an application level protocol based on the OSI model. It is independent of the physical layer.
Fig. N76: Modbus architecture
Modbus SL (Serial Line)
Modbus can be implemented on RS232, RS442 or RS485 links as well as other media like Ethernet. Modbus RS485 has been the most common protocol in the world. It supports communications speed up to 115kbps, but most devices support only communication up to 19.2 kbps.
Modbus RS485 is a low cost communication implementation, and it has the largest installation base and supplier network. The weak point of Modbus is the transmission speed (since it is limited by serial line speeds) and maximum number of devices. Modbus may face some problems in the application of very large industrial site, but it is still an economical and reasonable choice to the majority of motor protection systems.
Modbus is based on a Master/Slave concept. One device is the master and sends request to read or write data to each slave in turn. Slave answers to requests from the master. Even though you can have many devices connected to one serial line only one device can talk at a time.
Fig. N77: Modbus SL architecture
Modbus/TCP
Modbus/TCP is an excellent choice for large sites applications. Modbus/TCP uses the standard 10Mbps Ethernet media in physical layers to carry the Modbus message handling structure. It offers very fast speed and big number of devices in one network; it is easier to integrate MCC into the Local Area Network (LAN) of a company, so it is the choice of more and more customers.
Unlike Modbus SL, Modbus/TCP works on a Client/Server concept:
- A client initiates the requests and a server answers,
- Any device can be a client or a server,
- Many devices are both client and server at the same time,
- A network can consist of many clients.
Fig. N77a: Modbus SL architecture
Many clients can send requests at the same time and many servers can answer at the same time:
- A client can talk to multiple servers at the same time,
- A server can answer multiple clients at the same time,
- Ethernet switches take care of packet delivery to all a devices at the same time.
Fig. N78: Typical communication architecture
Differences between Modbus/TCP and Modbus SL:
- Devices can be a client and a server at the same time.
- Everyone can talk at the same time: multiple devices can initiate communications, not just one. Increases system response time by parallel communications.
- Multiple requests can be sent from one device to another without waiting for the first request to be answered. A new piece of data is added to the Modbus frame called the Modbus Transaction identifier to allow a response to be matched to a specific request.
- The Transmission speed is much increased:10Mb, 100Mb, 1Gb etc.
- The transmission media is much more flexible and costs are lower: fibre, radio etc.
- The number of nodes on a single network is almost unlimited: maximum recommended is around 300, but routers can be used to join several networks.
Modbus I/O Scanning
Modbus I/O Scanning is a feature in Schneider Electric Programmable Logic Controllers (PLC) which allows simple Modbus transactions with a simple setup screen. It is only requested to set the address, poll time and data to read and/or write.
Fig. N79: Modbus SL architecture
Profibus
Profibus is a protocol introduced by a fieldbus working group in 1987. This group consists of 13 industrial organizations and 5 research institutes. Now, Profibus is managed by a user group which includes manufacturers, users and researchers.
Profibus-DP is the version of Profibus used at device level. It has been a successful protocol in the last decades, especially in Europe. Profibus-DP is a protocol with high transmission speed. It supports the communication up to 12 Mbps, but actually 1.5 Mbps is the most practical maximum value in applications because it may need special transmission media and it should be implemented in a short distance to achieve the transmission speed up to 12 Mbps.
DeviceNet
DeviceNet is a protocol based on CAN, which is a protocol widely used in the automotive industry. ODVA (Open DeviceNet Vendor Association) takes now the responsibility to promote and provide technical support to DeviceNet specification.
ODVA is an international association comprised of members from the world's leading automation companies. Collectively, ODVA and its members support network technologies using the Common Industrial Protocol (CIP™). These currently include DeviceNetTM, EtherNet/IPTM, CompoNetTM and the major extensions to CIP — CIP SafetyTM, CIP SyncTM, and CIP MotionTM. ODVA manages the development of these open technologies and assists manufacturers and users of CIP Networks through tools, training and marketing activities.
DeviceNet provides communication with 3 possible speeds: 125, 250 or 500 kbps, which depends on the bus length and cable as well as product consumption. The maximum number of devices is 64, including master devices. The bus length is limited to 100m at 500 kbps.
Synthetic view
The following table shows a short (non-exhaustive) comparison of these protocols:
| Modbus SL RS485 | Profibus-DP | DeviceNet | Modbus/TCP | |
| Speed | up to 115 kbps | 9.6 kbps to 1 Mbps | 125, 250 or 500 kbps | 10 / 100Mbps / 1Gbps |
| Max. distance without repeaters | 1300 m | 100m at 12Mbps 1.2km at 10kbps |
100m at 500kbps 500m at 125kbps |
Twisted pair: 100m Optical fibre: 2000m |
| Max. number of devices | 32: 1 master and 31 slaves | 126: mono or multi-masters, 122 slaves max with 3 repeaters | 64: 1 master and 63 slaves | 64 with I/O scanning; no limit with others |
| Max. distance with repeaters | Depends on the type of repeater | 400 to 4800m according to speed | Depends on the type of repeater | 10km optical fibre |
Fig. N80: Comparison of communication protocols
