I/O Blocks > Analog Input (AI) function block

Analog Input function block execution

The following diagram shows the timed response of the Analog Input function block:

Figure: Analog Input function block timing diagram

You select the manner of processing the analog measurement value by configuring the I/O selection, signal conversion, and filtering parameters.

I/O selection

When you configure the Analog Input function block, you select the I/O channel associated with an analog measurement by configuring the Device Signal Tag (DST) of the IO_IN parameter. You select the Device Tag and the parameter the AI block accesses on that channel.

When the channel type is configured as Analog Input Channel, the only selectable channel parameter for IO_IN is:

  • FIELD_VAL_PCT — the 4 to 20 mA signal, in percent of range. You should configure XD_SCALE as 0 — 100%.

When the channel type is configured as HART Analog Input Channel, you can select one of the following parameters:

  • HART_DV_SLOT0 — the slot 0 device variable value. This value is read digitally.

  • HART_DV_SLOT1 — the slot 1 device variable value. This value is read digitally.

  • HART_DV_SLOT2 — the slot 2 device variable value. This value is read digitally.

  • HART_DV_SLOT3 — the slot 3 device variable value. This value is read digitally.

  • HART_FIELD_VAL — the 4 to 20 mA signal of the HART transmitter, in the range and engineering units defined by XD_SCALE or OUT_SCALE, depending on L_TYPE. HART Status is applied. The range and units are automatically sent to the transmitter to set its range and units. The specified units must be supported by the transmitter.

  • HART_PV — the Primary Variable of a HART transmitter, in engineering units. This value is read digitally.

  • HART_SV — the Secondary Variable of a HART transmitter, in engineering units. This value is read digitally.

  • HART_TV — the Tertiary Variable of a HART transmitter, in engineering units. This value is read digitally.

  • HART_FV — the Fourth Variable of a HART transmitter, in engineering units. This value is read digitally.

Note

The HART_DV_SLOT[0-3] channel parameters are not supported in devices implementing the HART Protocol prior to revision 7. Therefore, you should not configure one of these channel parameters for a device that implements an earlier level of the HART protocol.

You can configure anti-aliasing filtering, NAMUR limit detection, and overrange/underrange detection for the channel parameters. For information on these capabilities, refer to the I/O configuration topic.

Simulation

To support testing, you can enable simulation. This allows the measurement value and status to be supplied manually or from another block.

When the block is downloaded to a controller, enabling SIMULATE sets the Simulate Active bit in the BLOCK_ERR parameter. When the block is downloaded to a workstation the Simulate Active bit is not set.

During configuration, decide whether you want the simulated value/status to be entered manually during operation or you will use a value/status from another block for the simulated value/status.

When the value is entered manually:

Note

Use the SIMULATE parameter to enter values manually and make sure SIMULATE_IN is not connected. If SIMULATE_IN is connected or a value is entered manually into SIMULATE_IN, the SIMULATE_IN value overrides the SIMULATE value.

When the value/status from another block is used:

Note

Do not enter a value for the SIMULATE_IN parameter. If you do and the status of SIMULATE_IN is not Bad: NotConnected, the manually entered value for SIMULATE_IN overrides any value you enter in SIMULATE.

Signal conversion

Select one of the following signal conditioning options for the L_TYPE parameter:

  • Direct signal conditioning — simply passes through the accessed channel input value (or the simulated value when simulation is enabled). For direct conversion, OUT_SCALE is written to XD_SCALE in controller-resident blocks.

  • Indirect signal conditioning — linearly converts the accessed channel input value (or the simulated value when simulation is enabled) from its specified range (XD_SCALE) to the range and units of the PV and OUT parameters (OUT_SCALE).

  • Indirect square root signal conditioning — converts the accessed channel input value (or the simulated value when simulation is enabled) from its specified range (XD_SCALE) by taking the square root of the value and scaling it to the range and units of the PV and OUT parameters (OUT_SCALE). If the input value is less than zero, signal conditioning stops and the input value is passed directly to the output.

  • Direct independent signal conditioning — simply passes through the accessed channel input values (or the simulated value when simulation is enabled). OUT_SCALE is independent of XD_SCALE.

    You view the accessed value (in percent of XD_SCALE) through the FIELD_VAL parameter.

    When the converted input value is below the limit specified by the LOW_CUT parameter and the Low Cutoff I/O option (IO_OPTS) is enabled (True), a value of zero is used for the converted value (PV). This option can be useful with zero-based measurement devices, such as flowmeters.

Note

You can set the I/O option in Manual or Out of Service mode only.

Filtering

You apply filtering to the converted value (PV) by specifying the filter time constant (in seconds) in the PV_FTIME parameter. When you specify a value of zero, no filtering is applied.

Block errors

The following conditions are reported in the BLOCK_ERR parameter:

  • Block configuration error — The block is wired to a HART channel and the HART device indicates that the function block's units are not compatible with the device. In this case, the Out of Service block error is also set.

  • Simulate active — Simulation is enabled and the block is using a simulated value in its execution.

  • Input failure/process variable has Bad status — The source of the block's process variable is bad. Indicates a hardware failure, a non-existent Device Signal Tag (DST), or a Bad status on the SIMULATE parameter.

  • Out of Service — The block is in Out of Service (OOS) mode.

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