Operating instructions

3 Function and System Design WirelessHART Fieldgate SWG70
12 Endress+Hauser
3.3.2 Cache
The WirelessHART gateway 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 gateway.
Information cached in the
WirelessHART gateway
Each listed command has its own cache memory. Static commands are stored in the cache upon the
first request. These contain device parameters, e.g. tags, that do not normally change. 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 commands the Write commands 17, 18, 22 and 51, when the WirelessHART
gateway 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 support long tag
addressing.
If a HART 5.0 device is connected to a WirelessHART adapter, the WirelessHART gateway 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 gateway 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
48 Read Additional Device Status
38 Reset Configuration Change Counter
93 Read Trend