The scaling for AOUT is the same as for AIN, except that it does not have LTYPE or OUT_SCALE.
If the units selected are Special Units ("", "-", "no units"), then the scale and the units information are uploaded from the device and saved in the function block.
Once the function block is running and the units have been synchronized between the DeltaV system and the device, anytime the units or scale information is changed in the device by AMS Device Manager or a handheld device, the units are uploaded to the controller.
An exception to this occurs if, during scaling, the controller receives a nonrecoverable error from the device, in which case it continues attempting to recover the error indefinitely by reinitializing the HART link. During this time, it must ignore configuration change flags from the device (It might have already changed the units but not the scale, so we cannot read the device's values and replace the controller's.). Therefore, the user cannot change the scaling information in the device and expect it to be uploaded to the controller. The user must recover the error by changing the scale information in the function block. The user can use one of the Special Units to upload the scaling information from the device if needed.
When the IO_IN parameter of an Analog Input function block references the HART_FIELD_VAL parameter, the AI function block downloads scaling values and units to the HART transmitter to provide the correct translation between transmitter units and process units. You select the type of scaling by configuring the linearization type parameter (L_TYPE):
Direct signal conditioning — The values for 0% and 100% of OUT_SCALE are downloaded to the transmitter and the Analog Input function block OUT parameter is scaled using these values.
Indirect signal conditioning — The values for 0% and 100% and the units of XD_SCALE are downloaded to the transmitter. The channel input value is scaled using a linear interpolation between the range values of XD_SCALE and the range values of OUT_SCALE.
Indirect square root signal conditioning — The values for 0% and 100% and the units of XD_SCALE are downloaded to the transmitter. The normalized channel input value has a square root applied before it is scaled using a linear interpolation between the range values of XD_SCALE and the range values of OUT_SCALE.
Direct Independent signal conditioning — Passes through the accessed channel input values (or the simulated value when simulation is enabled). OUT_SCALE is independent of XD_SCALE.
For example, if you have a transmitter with a range of x°C to y°C and you want to use the range 100°C to 300°C for control, configure L_TYPE = Direct and OUT_SCALE = 100°C to 300°C. The range 100°C to 300°C is downloaded to the transmitter, and the channel input value of 100°C to 300°C is displayed in the HART_FIELD_VAL parameter.
If you have a transmitter with a range of x inches to y inches and you want to use the range 0 inches to 150 inches for the transmitter and the range of 0 gallons to 3000 gallons per minute for control, configure L_TYPE = Indirect square root, XD_SCALE = 0 to 150 in., and OUT_SCALE = 0 to 3,000 gallons per minute. The range 0 to 150 inches is downloaded to the transmitter, and the channel input value of 0 to 150 inches is displayed in the HART_FIELD_VAL parameter.
Because these values are downloaded to the transmitter during link initialization, make sure you configure the correct values in XD_SCALE and OUT_SCALE. Many HART devices have different rules on initialization values.