ABB AWIN GW120, User Manual

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WIRELESS HART FIELD DEVICE TECHNO LOG Y O VER VIEW
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connected to the network. Given this logic, devices could be one or more hops from the ac-
cess point as long as they have neighbors (or their neighbors have neighbors) which can
communicate directly with the access point. Messages may be passed along from device to
device in the wireless mesh until they reach their destination, either the host or a remote
field device. In this way, the messages follow paths from the source device to the host and
vice versa.
2.6 WirelessHART Field Device
The WirelessHART field device can connect into a plant process and can send and receive
data over the mesh network. The WirelessHART field device typically consists of a sensor, a
transducer, and a transceiver.
2.6.1 Sensors
The sensor connects into the plant process and measures (or controls) some aspect of the
analog process, such as temperature, pressure, level, flow, position, etc. The sensor may also
provide a discrete connection to the process as is the case with remote I/O field devices.
2.6.2 Transducers
The transducer converts or scales the measurement, so it may be used by a host system in
the context of the process. Proportional-Integral-Derivative scaling or conversion takes place
in the transducer portion of the field device.
2.6.3 Transceivers
The WirelessHART field device transmits or “hops” HART messages wirelessly to other de-
vices or directly to the access point using an IEEE 802.15.4 compliant radio transceiver.
2.7 HART Field Devices and Wired 4… 20 mA Field Devices
Traditional HART capable “smart” 4… 20 mA devices are similar to WirelessHART field de-
vices except that they use a Frequency Shift Keyed (FSK) HART modem to communicate over
a wired loop instead of communicating via a radio transceiver.
2.8 WirelessHART Adapter
In many cases, it is desirable to use existing HART field devices as part of a WirelessHART
network. This is possible through the use of a WirelessHART adapter, which bridges the con-
nection between the wired and wireless transmission mediums. WirelessHART adapters con-
tain a WirelessHART transceiver for connection to the WirelessHART network, as well as a 4…
20 mA FSK HART modem, which is used to communicate directly with a HART device via a
wired loop.
2.8.1 Connecting to a HART Device
A WirelessHART adapter may be used with a HART field device to collect messages received
over the HART 4… 20 mA loop and retransmit these messages via its WirelessHART trans-
ceiver. In the case of HART devices that transmit their Primary Variables (PVs) digitally, multi-
ple HART devices may be connected in the same wired loop as a single WirelessHART