WirelessHART Fieldgate SWG70 Function and system design
Endress + Hauser 15
3.3.2 Cache
The WirelessHART Fieldgate stores information received over the WirelessHART network
and makes it available to the host for further processing. This ensures that information is
available immediately for the host system without having to send a request to the device and
wait for the response. The following commands and answers to requests are cached in the
Fieldgate.
Information cached in the
WirelessHART Fieldgate
Each listed command has its own cache memory. Static commands are stored in the cache
upon the first request. Dynamic variables are stored each time a field device sends a burst
message so that up-to-date values are available at all times.
With the exception of write commands 17, 18, 22 and 51, when the WirelessHART
Fieldgate receives a request from a host system which is embedded in Command 77, the
response is sent immediately (provided that the response is available in the cache).
Long Tag Emulation WirelessHART uses the long tag for addressing devices. Not every HART device supports
long tags, for example, older HART devices with HART Protocol Version 5 or less, do not sup-
port long tag addressing.
If a HART 5.0 device is connected to a WirelessHART Adapter (SWA70), the WirelessHART
Fieldgate emulates the long tag using the "Message" field. When a host system addresses a
HART 5 device, the emulation translates Command 20(22) directly into Command 12 (17)
which the HART 5 understands. The response is stored in the Fieldgate cache for CMD
12(17) and for CMD 20(22).
Cache HART Command Description
Static: cached upon
read
0, 11, 21 Read unique identifier (associated with tag or long tag)
Static: cached upon
read & write
confirmation
12, (17) Read (Write) Message
13, (18) Read (Write) Short Tag, Descriptor, Date
20, (22) Read (Write) Long Tag
50 (51) Read (Write) Dynamic Variable Assignments
Dynamic: cached on
publication only
1 Read Primary Variable
2 Read Current and Percentage
3 Read All Variables
9 Read Device Variables and Status
33 Read Device Variables
38 Read Additional Device Status
48 Reset Configuration Change Counter
93 Read Trend
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