HART is a bi-directional communication protocol that provides data access between intelligent field instruments and host systems. A host can be any software application from a technician's hand-held configurator to a plant's process control, safety or SCADA system and enterprise asset management or data historian applications. Communication occurs between two HART-enabled devices, typically a smart field device and a control or monitoring system. Standard 4-20mA wiring practices assure reliable communication.
HART provides two simultaneous communication channels, one analog, the other digital:
A 4-20mA signal communicates the primary measured value (PV) as an analog value of current using the wiring that provides power to the instrument. The host system then converts the current value to a physical value according to parameters defined by HART Software. For example, 7 mA = 80 degrees F.
Digital device information is communicated by encoding a digital signal, generally using a technique known as Frequency Shift Keying on the same 4-20mA wiring used for analog communications. The digital signal contains information from the device including PV, device status, diagnostics, and additional measured or calculated values, etc.
Together, the two communication channels provide a complete field communications solution that is easy to design, simple to use, low cost and extremely reliable.
WirelessHART is a wireless communications protocol for process automation applications. It adds wireless capabilities to HART technology while maintaining compatibility with existing HART devices, commands, and tools. WirelessHART uses mesh networking technology. Each device in a mesh network can serve as a router for messages from other devices. In other words, a device doesn't have to communicate directly to a gateway, but just forward its message to the next closest device. This extends the range of the network and provides redundant communication routes to increase reliability, particulalry in the difficult radio environment found in process facilities.
Each WirelessHART network includes three main elements:
Wireless field devices connected to process or plant equipment. This device could be a device with WirelessHART built in or an existing installed HART-enabled device with a WirelessHART adapter attached to it.
Gateways enable communication between these devices and host applications connected to a high-speed backbone or other existing plant communications network.
A Network Manager is responsible for configuring the network, scheduling communications between devices, managing message routes, and monitoring network health. The Network Manager can be integrated into the gateway, host application, or process automation controller.
For flexibility to meet different application requirements, the WirelessHART standard supports multiple messaging modes including one-way publishing of process and control values, spontaneous notification by exception, ad-hoc request/response, and auto-segmented block transfers of large data sets. These capabilities allow communications to be tailored to application requirements thereby reducing power usage and overhead.
Ethernet networks have been pervasive in office environments for decades. But in process facilities, 2-wire 4-20mA wiring has been the norm. Slowly, Ethernet and IP networks are growing towards the process plant floor. To address this growth FieldComm Group developed an internet protocol (IP) enabled version of HART named HART-IP.
HART-IP allows host level systems and asset management applications to access and integrate measurement and device diagnostics information from HART-enabled field devices through existing IP networks be they Ethernet, packet-radio, satellite or 3G/4G based.
If instrument information needs to be made available to an IP network, it can be done with HART-IP
HART-IP is a simple-to-use, high-level application technology that is independent of the underlying media. Thus HART-IP operates with redundant Ethernet media as well as mesh or ring topologies. Similarly, HART-IP can run over Power over Ethernet (PoE) for such infrastructure and devices. Speeds of 10 Mbit/s, 100 Mbit/s, and 1 Gbit/s etc. are supported.
Because the application layer is the same for HART field devices as HART-IP, time consuming and error-prone data mapping, for example with MODBUS RTU is eliminated, making HART-IP the most simple to use and suitable backhaul network for WirelessHART gateways and wired HART multiplexers and remote I/O.
HART-IP offers a straightforward method to access large amounts of all the standard HART information available in a HART device coming from products that concentrate many measurement points into a single output. It allows the information from these devices to be integrated with TCP/IP networks easily, without the need to go through any translation processes and with no loss of information.
|10 Mbit/s - 1Gbit/s support||Operates with existing infrastructure|
|HART Application Layer||Simple integration with existing control and asset management systems|
|Standard TCP/IP protocol||
Shared bandwidth with existing applications.
No requirement for customized Ethernet protocol