In order for the HART field device to be compatible with the DeltaV system, it must be able to communicate using the HART commands listed in this section.
In the DeltaV system, HART link initialization is performed by the DeltaV AI-HART card and the controller function block.
I/O Card Initialization
When a HART card is plugged in, it assumes that all channels are analog only. When you configure the card, you can set the channel to HART_ANALOG_INPUT (AO also).
The card then tests for an open circuit. If no open circuit is detected, the card sends out HART Command 0 (Read Manufacturer and Device Type) followed by Command 59 (Write Number of Response Preambles).
Controller Function Block Channel Initialization
The DeltaV Controller also performs HART channel initialization. As soon as the controller is notified that an AI-HART card configuration is complete, it sends Command 0 to each HART channel.
If a function block is linked to the HART_FIELD_VAL parameter (for AI) or OUT parameter (for AOUT) of the HART channel, the first time the function block executes, it sends scaling information (high and low range and units) to the HART instrument for its analog output signal. The units code is sent via HART Command 44 (Write Primary Variable Units). The high and low range information is sent via HART Command 35 (Write Primary Variable Range Values). At this time, the HART instrument sets a bit saying that it has changed its configuration to the new values. The controller sends HART Command 15 (Read Primary Variable Output Information) to read the range and units information. This is followed by Command 38 (Reset Configuration Change Flag) to clear the change flag in the instrument. Note that if command 44 is not supported, command 15 is inserted before command 35 in the description above.
There are other scenarios that affect link initialization, including:
You might connect a handheld device to an instrument in the field to change its scale values. The controller detects this change in the instrument and sends Command 15 to read the new values followed by command 38.
During loop tuning, you can change the range values in an AI function block. These changes are sent to the HART instrument.
You might specify range values to a high level of precision and some HART instruments might not be able to support that precision. The controller sends Command 15 to detect the actual range precision the instrument is using in the process.
If a HART instrument does not support the HART Commands 44 and 35 (used to set the scale values), the values read by HART Command 15 are compared to those specified by the function block. If the values are the same, they are accepted and the link initialization continues. If the values are not the same (or if the HART instrument is write-protected or blocking access to that configuration information), the controller stops the HART link initialization for 5 seconds, restarts it with cmd 0, and the initialization repeats.
HART Special Units Handling
The HART protocol allows units that might be unique to a particular HART instrument and unknown to the DeltaV system. To deal with this for a specific function block, select one of the following values for the Engineering unit descriptor: hyphen (-), blank space ( ), or no units. The DeltaV system then uses the units and scale information that are currently configured in the HART instrument. If the type of units are known to the DeltaV system, the units are displayed in the XD_SCALE parameter of the function block. However if they are unknown to the DeltaV system, the units field in XD_SCALE is blank.
HART Scan Update Rate (AI card)
After this card's initialization phase, the card sends out Command 3 (Read Dynamic Variables and Primary Variable) repeatedly to each HART channel. The HART scan update rate for each input channel on an I/O card configured for HART is 600 to 800 ms. The analog values for each input channel are scanned in much faster than the HART scan update rate.
Passthrough messages sent by AMS Device Manager can cause a card's HART scan rate to be twice the rate without AMS Device Manager running.
HART Scan Update Rate (AOUT card)
After this card's initialization phase, the card sends out Command 3 (Read Dynamic Variables and Primary Variable) repeatedly to each HART channel. The HART scan update rate for each output channel on an I/O card configured for HART is 600 to 800 ms.
If device variables (DV_SLOT) are configured for a channel, the card sends command 33 to read the configured device variables. The HART scan update rate for each channel configured for device variables is an additional 600 to 800 ms.
Passthrough messages sent by AMS Device Manager can cause a card's HART scan rate to be as much as twice the rate without AMS Device Manager running.
If valve diagnostics is running on a channel, the card's HART scan rate can be as much as twice the rate without diagnostics running.
As an example:
The scan rate for an 8 channel analog output card with all channels configured for HART is 4.8 to 6.4 seconds:
If the Device Variables screen is open, the scan rate can be 9.6 to 12.8 seconds.
If AMS Device Manager is running and the Device Variables screen is open, the scan rate can be 19.2 to 25.6 seconds.
If the AMS Device Manager is running, the Device Variables screen is open, and valve diagnostics are running, the scan rate can be 38.4 to 51.2 seconds.
HART Scan Update Rate (AI 4-20 mA CHARM)
The HART scan update rate for an AI 4-20 mA HART CHARM is typically less than one second but is dependent on HART communication loading and the capability of the HART device.
HART Scan Update Rate (AO 4-20 mA CHARM)
The HART scan update rate for an AO 4-20 mA HART CHARM is typically less than one second but is dependent on HART communication loading and the capability of the HART device.
HART Scan Update Rate (Redundant WirelessHART I/O card)
The HART scan update rate for a Redundant WirelessHART I/O card is 5 seconds.