The Interface Solution Experts
11
HIM
Smart HART
Loop
Interface and Monitor
HART Parameters
To program the HART parameters, change the settings
in the
Communications Settings box, then press Quick
Set. See the descriptions below of the various
sections of the screen.
HART Address
The
HART Address is the address of the HART device
that the HIM will be monitoring.
Number of Retries
The
Number of Retries can be set between 1 and 9,
and will determine how many times the HIM will
attempt to poll the HART transmitter (without success),
before it indicates a HART Fault condition.
Normal/Burst/Listen (Passive) Modes
The HIM can operate in one of four modes:
Normal,
Burst, or Listen (Passive) and Listen (Specified Slave).
In each of these modes, the HIM attempts to find a
HART transmitter.
In
Normal mode, the HIM polls the HART loop for a
transmitter, then polls the HART instrument twice per
second, requesting the current process status and the
HART instrument’s diagnostic status. The HART
instrument responds with the requested data.
In
Burst mode, the monitored HART instrument is
programmed to continuously transmit its process
variable and health status. The HIM samples the
continuous HART data three times per second.
Listen (Passive) mode allows the HIM to operate on a
loop that already has primary and secondary HART
masters. In
Listen (Passive) mode, the monitor
connects passively, continuously sampling HART data
from a smart instrument without affecting normal loop
operation. When using this mode, either the monitored
smart HART instrument must be set in
Burst Mode or a
HART master must be continuously polling the smart
HART slave device.
The HART protocol allows for two communications
masters on the loop, a Primary and a Secondary.
Setting the HIM to function as the Primary HART
Master in the application means that any other HART
device in the loop must be configured either as a HART
Secondary Master (1 per loop) or as a HART Slave (up
to 16 per loop). Conversely, setting the HIM to function
as the Secondary HART Master allows other HART
devices to function either as a Primary Master or as
slaves. Configuring more than one device on a single
loop as a Primary or Secondary HART Master will
cause a communications failure.
Listen (Specified Slave)
When multiple HART instruments are present, the
Listen (Passive) mode cannot be used because data
returned from one instrument will overwrite the data
previously stored in the HIM. If multiple HART slave
instruments are communicating on the loop, the HIM
will overwrite its internal HART data sets with the latest
device read from the bus, no matter where the data
originates. To allow selective monitoring on a multi-
instrument loop, and to allow multiple HIMs to be used
on a digital loop, the
Listen (Specified Slave) feature is
used.
If the HIM is in “Listen” mode it is not a HART commu-
nication master and therefore is not in control of
communication. Other HART masters are controlling
the polling of slaves and the HIM can only “Listen”.
The HIM must be able to detect when it has not heard
from its assigned slave device so that it can declare a
“No HART” input and force outputs to the values
specified by the user. The HIM must therefore be told
to wait an appropriate amount of time for the slave
device to send its message before the HIM declares
“No HART”. This timing coordination is accomplished
with
Timeout Period.
Timeout Period is a value between 1 and 30sec and
must be greater than the period between polls by the
HART master of the Specified Slave. For example,
assume that there are five HART transmitters on the
same multi-drop loop as the HIM. The HART master is
configured to poll each device every second. It then
takes the HART master five seconds to repeat the poll
to the one device being monitored by the HIM. the HIM
Timeout Period must be set at a value greater than five
