From cc6bef6b82dca6d4b848cafbedd7666c29bc7080 Mon Sep 17 00:00:00 2001
From: premultiply <4681172+premultiply@users.noreply.github.com>
Date: Sun, 19 Jul 2026 12:44:32 +0200
Subject: [PATCH] Modbus: handle NaN values (#31011)

---
 plugin/modbus.go             |  8 +++++++-
 util/modbus/functions.go     |  7 ++++---
 util/modbus/register.go      |  3 ++-
 util/modbus/register_test.go | 17 +++++++++++++----
 4 files changed, 26 insertions(+), 9 deletions(-)

diff --git a/plugin/modbus.go b/plugin/modbus.go
index 9eae8a9e4..bfc78e616 100644
--- a/plugin/modbus.go
+++ b/plugin/modbus.go
@@ -9,6 +9,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/evcc-io/evcc/api"
 	"github.com/evcc-io/evcc/util"
 	"github.com/evcc-io/evcc/util/modbus"
 	gridx "github.com/grid-x/modbus"
@@ -156,7 +157,12 @@ func (m *Modbus) FloatGetter() (func() (f float64, err error), error) {
 			return 0, fmt.Errorf("read failed: %w", err)
 		}
 
-		return m.scale * decode(bytes), nil
+		// a "*nan" sentinel decodes to NaN - the device reports no valid value
+		if res := m.scale * decode(bytes); !math.IsNaN(res) {
+			return res, nil
+		}
+
+		return 0, api.ErrNotAvailable
 	}, nil
 }
 
diff --git a/util/modbus/functions.go b/util/modbus/functions.go
index 867ad8746..faea9066e 100644
--- a/util/modbus/functions.go
+++ b/util/modbus/functions.go
@@ -5,6 +5,7 @@ import (
 	"encoding/hex"
 	"errors"
 	"fmt"
+	"math"
 	"slices"
 	"strconv"
 	"strings"
@@ -81,7 +82,7 @@ func decodeNaN16(f func(b []byte) float64, nan ...uint16) func(b []byte) float64
 	return func(b []byte) float64 {
 		u := binary.BigEndian.Uint16(b)
 		if slices.Contains(nan, u) {
-			return 0
+			return math.NaN()
 		}
 		return f(b)
 	}
@@ -91,7 +92,7 @@ func decodeNaN32(f func(b []byte) float64, nan ...uint32) func(b []byte) float64
 	return func(b []byte) float64 {
 		u := binary.BigEndian.Uint32(b)
 		if slices.Contains(nan, u) {
-			return 0
+			return math.NaN()
 		}
 		return f(b)
 	}
@@ -101,7 +102,7 @@ func decodeNaN64(f func(b []byte) float64, nan ...uint64) func(b []byte) float64
 	return func(b []byte) float64 {
 		u := binary.BigEndian.Uint64(b)
 		if slices.Contains(nan, u) {
-			return 0
+			return math.NaN()
 		}
 		return f(b)
 	}
diff --git a/util/modbus/register.go b/util/modbus/register.go
index 676fc2f82..4120b1209 100644
--- a/util/modbus/register.go
+++ b/util/modbus/register.go
@@ -249,7 +249,8 @@ func (r Register) Operation() (RegisterOperation, error) {
 func asFloat64[T constraints.Signed | constraints.Unsigned | constraints.Float](f func([]byte) T) func([]byte) float64 {
 	return func(v []byte) float64 {
 		res := float64(f(v))
-		if math.IsNaN(res) || math.IsInf(res, 0) {
+		// keep NaN so callers can map it to "not available"; sanitize Inf to 0
+		if math.IsInf(res, 0) {
 			res = 0
 		}
 		return res
diff --git a/util/modbus/register_test.go b/util/modbus/register_test.go
index a30740fba..ba1a030f4 100644
--- a/util/modbus/register_test.go
+++ b/util/modbus/register_test.go
@@ -71,10 +71,13 @@ func TestDecoding(t *testing.T) {
 		out float64
 	}{
 		{Register{Decode: "float32"}, []byte{0x4b, 0x3c, 0x61, 0x4e}, 12345678},
-		{Register{Decode: "float32"}, []byte{0xff, 0xff, 0xff, 0x7f}, 0}, // NaN
+		{Register{Decode: "float32"}, []byte{0xff, 0xff, 0xff, 0x7f}, math.NaN()}, // IEEE NaN preserved
 		{Register{Decode: "float32s"}, []byte{0x61, 0x4e, 0x4b, 0x3c}, 12345678},
-		{Register{Decode: "float32s"}, []byte{0xff, 0x7f, 0xff, 0xff}, 0},    // NaN swapped
-		{Register{Decode: "float32nans"}, []byte{0xff, 0xff, 0xff, 0x7f}, 0}, // NaN
+		{Register{Decode: "float32s"}, []byte{0xff, 0x7f, 0xff, 0xff}, math.NaN()}, // IEEE NaN swapped preserved
+		{Register{Decode: "float32nans"}, []byte{0xff, 0xff, 0xff, 0x7f}, math.NaN()},
+		{Register{Decode: "uint16nan"}, []byte{0xff, 0xff}, math.NaN()},
+		{Register{Decode: "uint32nan"}, []byte{0xff, 0xff, 0xff, 0xff}, math.NaN()},
+		{Register{Decode: "uint64nan"}, []byte{0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff}, math.NaN()},
 		{Register{Decode: "uint64"}, []byte{0x00, 0x04, 0x00, 0x03, 0x00, 0x02, 0x00, 0x01}, 0x0004000300020001},
 		{Register{Decode: "uint64s"}, []byte{0x00, 0x04, 0x00, 0x03, 0x00, 0x02, 0x00, 0x01}, 0x0001000200030004},
 	}
@@ -82,6 +85,12 @@ func TestDecoding(t *testing.T) {
 	for _, tc := range tc {
 		fun, err := tc.r.DecodeFunc()
 		require.NoError(t, err, tc)
-		require.Equal(t, tc.out, fun(tc.in), tc)
+
+		out := fun(tc.in)
+		if math.IsNaN(tc.out) {
+			require.True(t, math.IsNaN(out), tc)
+			continue
+		}
+		require.Equal(t, tc.out, out, tc)
 	}
 }