Spectrum Controls 1734sc-OE2CIH - Manual (Page 38)

4-2 Chapter 4: OE2CIH and HART
User’s Manual Pub. 0300272-01 Rev. A.0
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).
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 (0 to 1 or 1 to 0).
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 (0 to 1 or 1 to 0). If the bit 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