Modbus+ uses a proprietary 1 Mbit/second physical layer based upon RS-485, HDLC as the data link layer, and standard Modbus messaging protocol. The higher speed of the protocol makes it a cost-effective choice for certain applications. Hundreds of devices exist for Modbus+, available from a variety of manufacturers.What Can AB Tech Solution Do for Your Modbus+ Product Development?
PROFINET has three distinct manifestations: PROIFINET I/O and PROFINET CBA, which utilize standard Ethernet hardware, and PROFINET IRT, which requires special hardware. PROFINET CBA is primarily used for non-real-time systems, whereas PROFINET I/O is used for "soft" real-time plants. PROFINET IRT (Isochronous Real Time) used where tight, "hard" real-time deadlines are required, such as in an interpolated multi-axis motion application.
Because it is intended for process automation environments, PROFIBUS-PA is not as widely used as PROFIBUS-DP. PROFIBUS-PA also utilizes device profiles, like PROFIBUS-DP, but the profiles are more likely to be process device configuration profiles.
PROFIBUS-DP is currently the most popular fieldbus in Europe. It is primarily utilized by Siemens, but PROFIBUS-DP controllers and devices are available from several manufacturers. A variety of automation device profiles for equipment such as encoders, robots, and the like are available from PI to encourage "plug and play" interchangeability between different vendors' equipment.
EtherNet/IP also standardizes human-element factors by standardizing such things as visual indicators, connectors, and labeling.
The DeviceNet protocol utilizes the producer-subscriber model for data interchange, allowing for multiple communication hierarchies to be implemented. Messages can be prioritized, and networks can be set up using master-slave or peer-to-peer models. Because it implements CIP, it interoperates seamlessly with other CIP-based protocols such as EtherNet/IP.
The CAN protocol specifies only a physical and link layer. A variety of higher-level protocols, such as DeviceNet, CANopen, J1939, and untold proprietary messaging protocols successfully run over CAN.
The CiA also publishes a safety version of the CANopen protocol, and provides a piece of special hardware which will allow CANopen devices to be used in Safety Shutdown Systems.
DNP operates on a variety of physical media. Typically, modem communications over telephone lines are used, but the DNP also specifies a protocol for Internet communications, and DNP has been implemented successfully on corporate networks, as well as cellular, licensed and unlicensed radio systems.
Modbus Serial Protocol
The Modbus serial protocol can be implemented using point-to-point physical layers, such as RS-232 and RS-422, or it can be implemented in multi-drop bus configurations using RS-485.
Modbus/TCP is supported by hundreds of manufacturers worldwide, including Schneider Electric and Allen-Bradley. It is maintained by the Modbus Organization.
Modbus/TCP has recently been incorporated by the ODVA into the EtherNet/IP protocol. This allows intelligent controller and field devices to utilize the strengths of both protocols in the same hardware.
Because of the large installed base (85%) of 4-20 mA field devices, HART is one of the most popular fieldbuses today. Because the modem signals which ride on top of the 4-20 process variable signal have an average voltage of zero, HART does not affect legacy 4-20 mA signal conditioners, which means HART-compliant field devices can replace standard field devices. Handheld HART communicators can be used to configure a field device or to gather diagnostics from the device while the plant process is still running.
Foundation Fieldbus H1 also provides for supplying power to devices through the cable, allowing for intrinsically safe operation in explosive environments.
Foundation Fieldbus uses client/server and publisher/subscriber communications methods. Communications between devices and controllers can be scheduled or unscheduled, event-driven messages.
ASi uses a continuous, polarized two-wire cable that is chained between all the devices in a system. ASi devices connect to the wire with piercing "vampire" taps, which greatly simplifies the wiring process. The special, self-healing quality of the ASI cable allows for an IP67 rating, even after a device has been removed from the cable. The ASi cable also delivers power (24 VDC at 8 amperes) to the field devices.
In addition to a simple communications protocol, LonWorks provides a signaling protocol, a network operating system, a web interface, and several utilities for installation and management. LonWorks can be used with wireless communications protocols such as Zigbee.
BACnet, the Building Automation and Control network protocol, was developed by the American Society of Heating, Refrigeration, and Air conditioning Engineers (ASHRAE) to provide standard messaging for air handling equipment, building lighting systems, and the like. The BACnet protocol is maintained by the BACnet International organization, which is made up of several BACnet equipment and system vendors to promote BACnet technology and conformance.