Communication protocols and architectures in iPMCC

Lots of data are managed in an iPMCC application. An iPMCC application is typically made of 50 to 1000 motor starters. In order to supervise the system, it is necessary to send the motors’s information such as motor status, current value, alarm, 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, communications via a network is the preferred way.

The communications need the support of a common language, which is the communications protocol. The following chart shows the protocols most commonly used in different levels of industrial communications networks. At the moment, the most popular device bus protocols are Ethernet TCP/IP, Modbus-RTU, Profibus-DP and DeviceNet.

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-RTU (a.k.a 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 the relatively small number of devices that can be connected to one network. However, Modbus-RTU 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 requests to read or write data to each slave in turn. Slaves answer 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-RTU architecture

Modbus/TCP

Modbus/TCP is an excellent choice for large sites applications. Modbus/TCP uses the standard 100 Mbps 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-RTU, 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/TCP 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 to multiple clients at the same time,
• Ethernet switches take care of packet delivery to all a devices at the same time.

Fig. N78 – Typical communications architecture

Differences between Modbus/TCP and Modbus-RTU:

• 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 200, but routers can be used to join several networks.
• Gateways/Proxies allow transparent communications between Ethernet Modbus/TCP and Modbus-RTU devices.

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.

After configuration the communications system manages automatically all Modbus exchanges with scanned devices.

Fig. N79 – I/O Scanning architecture

Profibus

Profibus (PROcess Filed BUS) is a protocol introduced by a fieldbus working group in 1987. It is supported by PI (Profibus & Profinet International).

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. It supports communications up to 12 Mbps it, but actually 1.5 Mbps is the most practical maximum value in applications. In order to achieve a transmission speed up to 12 Mbps it requires additional constraints such as the suppression of bus’ derivations.

The network topology is a bus. The number of devices in a bus is limited. The use of specific repeaters may be required in order to achieve the theoretical maximum number.

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 DeviceNet™, EtherNet/IP™, CompoNet™ and the major extensions to CIP — CIP Safety™, CIP Sync™, and CIP Motion™. ODVA manages the development of these open technologies and assists manufacturers and users of CIP Networks through tools, training and marketing activities.

The network topology is a bus. The number of devices in a bus is limited.

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 RTU	Profibus-DP	DeviceNet	Ethernet Modbus TCP/IP
Speed	up to 115 kbps	9.6 kbps to 12 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: 2000 m (multi-mode) >2 km (mono-mode)
Max. number of devices	32 without repeater: 1 master and 247 slaves	126: mono or multi-masters, 122 slaves max with 3 repeaters	64: 1 master and 63 slaves	128 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 communications protocols