Modbus: always assume RTU for serial devices (#16134)

meter/mbmd.go

@@ -53,8 +53,9 @@ func NewModbusMbmdFromConfig(other map[string]interface{}) (api.Meter, error) {
 
 	// assume RTU if not set and this is a known RS485 meter model
 	if cc.RTU == nil {
-		b := modbus.IsRS485(cc.Model)
-		cc.RTU = &b
+		if rtu := modbus.IsRS485(cc.Model); rtu {
+			cc.RTU = &rtu
+		}
 	}
 
 	modbus.Lock()

util/modbus/modbus.go

@@ -47,7 +47,7 @@ func (s Settings) Protocol() Protocol {
 	switch {
 	case s.UDP:
 		return Udp
-	case s.RTU != nil && *s.RTU:
+	case s.Device != "" || s.RTU != nil && *s.RTU:
 		return Rtu
 	default:
 		return Tcp

util/modbus/modbus_test.go

@@ -3,6 +3,7 @@ package modbus
 import (
 	"testing"
 
+	"github.com/samber/lo"
 	"github.com/stretchr/testify/require"
 )
 
@@ -23,3 +24,20 @@ func TestParsePoint(t *testing.T) {
 		require.Equal(t, tc.ops, ops)
 	}
 }
+
+func TestSettingsProtocol(t *testing.T) {
+	tc := []struct {
+		Settings
+		res Protocol
+	}{
+		{Settings{UDP: true}, Udp},
+		{Settings{RTU: lo.ToPtr(true)}, Rtu},
+		{Settings{Device: "foo"}, Rtu},
+		{Settings{URI: "foo"}, Tcp},
+		{Settings{}, Tcp},
+	}
+
+	for _, tc := range tc {
+		require.Equal(t, tc.res, tc.Protocol(), tc)
+	}
+}