chore(inputs.modbus): Improve deprecation documentation (#13832)

plugins/inputs/modbus/README.md

@@ -104,7 +104,8 @@ See the [CONFIGURATION.md][CONFIGURATION.md] for more details.
   ## data_type   - INT8L, INT8H, UINT8L, UINT8H (low and high byte variants)
   ##               INT16, UINT16, INT32, UINT32, INT64, UINT64,
   ##               FLOAT16-IEEE, FLOAT32-IEEE, FLOAT64-IEEE (IEEE 754 binary representation)
-  ##               FLOAT32, FIXED, UFIXED (fixed-point representation on input)
+  ##               FIXED, UFIXED (fixed-point representation on input)
+  ##               FLOAT32 is a deprecated alias for UFIXED for historic reasons, should be avoided
   ## scale       - the final numeric variable representation
   ## address     - variable address
 
@@ -383,6 +384,9 @@ Directly jump to the styles:
 This is the original style used by this plugin. It allows a per-register
 configuration for a single slave-device.
 
+> [!NOTE]
+> _For legacy reasons this configuration style is not completely consistent with the other styles. Especially `FLOAT32` which suggests a floating point representation is actually a_ ___Fixed Point___ _data type and should be considered_ ___deprecated___
+
 #### Usage of `data_type`
 
 The field `data_type` defines the representation of the data value on input from
@@ -414,7 +418,7 @@ modbus data source. For _coil_ and _discrete_ registers only `UINT16` is valid.
 Use these types if your modbus registers contain a value that is encoded in this
 format. These types always include the sign, therefore no variant exists.
 
-##### Fixed Point: `FIXED`, `UFIXED` (`FLOAT32`)
+##### Fixed Point: `FIXED`, `UFIXED`, (`FLOAT32` - _deprecated_)
 
 These types are handled as an integer type on input, but are converted to
 floating point representation for further processing (e.g. scaling). Use one of
@@ -430,7 +434,7 @@ Select the type `FIXED` when the input type is declared to hold signed integer
 values. Your documentation of the modbus device should indicate this with a term
 like 'int32 containing fixed-point representation with N decimal places'.
 
-(FLOAT32 is deprecated and should not be used. UFIXED provides the same
+(`FLOAT32` is deprecated and should not be used. `UFIXED` provides the same
 conversion from unsigned values).
 
 ---

plugins/inputs/modbus/configuration_register.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 
 	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/models"
 )
 
 //go:embed sample_register.conf
@@ -176,12 +177,12 @@ func (c *ConfigurationOriginal) newFieldFromDefinition(def fieldDefinition, type
 func (c *ConfigurationOriginal) validateFieldDefinitions(fieldDefs []fieldDefinition, registerType string) error {
 	nameEncountered := map[string]bool{}
 	for _, item := range fieldDefs {
-		//check empty name
+		// check empty name
 		if item.Name == "" {
 			return fmt.Errorf("empty name in %q", registerType)
 		}
 
-		//search name duplicate
+		// search name duplicate
 		canonicalName := item.Measurement + "." + item.Name
 		if nameEncountered[canonicalName] {
 			return fmt.Errorf("name %q is duplicated in measurement %q %q - %q", item.Name, item.Measurement, registerType, item.Name)
@@ -258,6 +259,14 @@ func (c *ConfigurationOriginal) validateFieldDefinitions(fieldDefs []fieldDefini
 }
 
 func (c *ConfigurationOriginal) normalizeInputDatatype(dataType string, words int) (string, error) {
+	if dataType == "FLOAT32" {
+		models.PrintOptionValueDeprecationNotice(telegraf.Warn, "input.modbus", "data_type", "FLOAT32", telegraf.DeprecationInfo{
+			Since:     "v1.16.0",
+			RemovalIn: "v2.0.0",
+			Notice:    "Use 'UFIXED' instead",
+		})
+	}
+
 	// Handle our special types
 	switch dataType {
 	case "FIXED":

plugins/inputs/modbus/sample_register.conf

@@ -32,7 +32,8 @@
   ## data_type   - INT8L, INT8H, UINT8L, UINT8H (low and high byte variants)
   ##               INT16, UINT16, INT32, UINT32, INT64, UINT64,
   ##               FLOAT16-IEEE, FLOAT32-IEEE, FLOAT64-IEEE (IEEE 754 binary representation)
-  ##               FLOAT32, FIXED, UFIXED (fixed-point representation on input)
+  ##               FIXED, UFIXED (fixed-point representation on input)
+  ##               FLOAT32 is a deprecated alias for UFIXED for historic reasons, should be avoided
   ## scale       - the final numeric variable representation
   ## address     - variable address