Automatically Gathering Additional Device Status; Automatically Gathering Hart Device Information - Spectrum 1734 User Manual

At the recommendation of the HART Foundation, a method to force the
module to use Command 3 regardless of HART Revision is also added.

4.2.2 Automatically Gathering Additional Device Status

Most devices support a HART command that allows additional device status to
be read. For HART Revision 5 and 6 devices, Command 48 is optional but in
HART 7 it is required to be supported by the device. Command 48 returns up to
25 bytes of status information. This group of 25 bytes includes both predefined
status bytes and device-specific status.
The module updates its internal data cache of Command 48 data under the
following conditions:
1. Initial device connection.
2. Detection of More Status Available bit change (
3. Every two minutes, if bit remains set.
The module keeps an internal cache of each channel's Command 48 status data
and makes that data available through a CIP unconnected message to provide
easy access to this data without the need for you to explicitly construct and
execute a pass-through message. This data is kept up-to-date by the module by
monitoring the More Status Available bit in the device's Field Device Status byte
which is returned in the reply to every HART command sent to a device and re-
sending Command 48 to the device if this bit changes (
remains set, then the module sends a Command 48 to the device every two
minutes.
A tag in the Input table will notify you that the additional status for one of the
channels has been refreshed by the module, and you should re-send the Read
Additional Status CIP message to the class instance (
channels at once. (Sending a request for an individual channel does not reset the
tag.)
The data returned by the Read Additional Status is in the original order as
received from the HART device. This means any multi-byte elements in the data
will not be in the correct order for use in RSLogix and will be reversed in the
PLC. HART format is most-significant bit (MSB) first, while RSLogix is least-
significant bit (LSB) first for multi-byte variables. The module is unable to re-
order the bytes because it does not have knowledge of where the multi-byte
elements are within the data, and this can vary from device to device. Refer to the
HART device's documentation for the location of each multi-byte element and
reverse the order of the bytes in ladder to interpret.

4.2.3 Automatically Gathering HART Device Information

Upon first connection with a HART device, the module sends a series of HART
commands to retrieve information about that device. The module will store this
information so that it can be retrieved at any time (by the PLC) using a CIP
unconnected message.
Chapter 4: OE2CIH and HART
0 to 1
0 to 1
Instance=0
User's Manual Pub. 0300272-03 Rev. B
4-3
1 to 0
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) that returns all