4-2
Chapter 4: OE2CIH and HART
User’s Manual Pub. 0300272-03 Rev. B
Section 4.2
Detecting HART
Devices
If the channel is HART-enabled, to determine if a HART device is present on a
given channel, the module continuously sends out HART Command 0 (Read
Unique Identifier, which returns the expanded device code type, revision levels,
and device identification number). Short frame addressing is used on Command 0
and the polling address is incremented in the following manner:
0, 1-15, 16-63, 0
If the module receives a reply to Command 0, it starts its connection sequence to
populate its internal cache of device data for that channel, and then begins
dynamic variable scanning and processing of pass-through requests.
4.2.1 Auto-Scanning of Dynamic HART Variables (PV, SV, TV,
FV)
The module continuously sends Command 3 or Command 9 to the HART device
connected on a channel. Which command is sent is determined by the HART
Universal revision number of the HART device. If the device has a revision of 6
or greater; then Command 9 is used. Otherwise, for HART revision 5 devices,
Command 3 is used. The advantage of Command 9 is that it contains a Health
Status byte for each of the 4 dynamic variables that it returns. The dynamic
variables and the status bytes are published in the Input tags if a profile is
selected that contains both the analog and HART variables. This same data is
also made available through a CIP unconnected message to the module.
•
If Command 9 is being used then the module also sends out Command 2
(Read Loop Current) after every 10 Command 9 sequences are sent. This
is because Command 9 does not return the loop current, and the module
needs to cache this information to return in the CIP message Read all
Dynamic Variables and Current, and for the Current Fault detection.
•
If Command 3 is used the PV Status, SV Status, TV Status, and FV
Status are set to Good (
0×C0
) if any value is received, and Bad (
0×00
) if
no value is received (either due to lack of response or truncated
response), or the value is not a number (
NaN
).
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