feat(rigctld): split that actually reaches the radio, or an honest refusal

set_split_vfo and set_split_freq both answered RPRT 0 and did nothing. WSJT-X
and JTDX in "Split Operating: Rig" send exactly that pair, believed both, and
transmitted on the RECEIVE frequency — on a pileup, straight onto the DX, while
showing the operator precisely what they had asked for. A lie that leaves no
trace in any log is the worst kind of bug this program can have.

The two commands are honoured as a PAIR. Arming alone does nothing on the radio,
because WSJT-X sends the frequency second and split armed on whatever the
transmit VFO happened to hold is worse than no split at all: it transmits
somewhere the operator never chose. The request is remembered and set_split_freq
does the work.

Kenwood gains SetSplit — FB to place the dial, then FR0/FT1 to arm, in that
order for the same reason. It writes what State() already knows how to read.

Everything else REFUSES, and that is the feature, not a shortfall. Only Flex and
Icom could even toggle split before, neither could set the transmit frequency,
and Yaesu, TCI and OmniRig have nothing at all. A refusal WSJT-X can report —
and act on, by falling back to Fake It — is worth far more than a success it has
no way to check.

Both paths are pinned: split reaching the rig as one armed call with the right
frequency, and a backend that cannot do it producing an error rather than RPRT 0.

internal/cat/cat.go

@@ -200,6 +200,31 @@ func (m *Manager) SetPTT(on bool) error {
 	return m.exec(func(b Backend) error { return b.SetPTT(on) })
 }
 
+// splitSetter is implemented by the backends that can arm split AND place the
+// transmit frequency. Both together: arming without setting the dial transmits
+// on whatever the transmit VFO happened to hold, which is worse than refusing.
+type splitSetter interface {
+	SetSplit(on bool, txHz int64) error
+}
+
+// SetSplit arms or clears split on the rig, with the transmit frequency.
+//
+// Returns a plain error on a backend that cannot do it, and that is the point.
+// The shared-CAT server used to answer "done" to WSJT-X's split commands while
+// doing nothing at all — the software then believed it was transmitting up the
+// band when it was transmitting on the DX's own frequency. A refusal WSJT-X can
+// report is worth far more than a success it cannot check.
+func (m *Manager) SetSplit(on bool, txHz int64) error {
+	return m.exec(func(b Backend) error {
+		s, ok := b.(splitSetter)
+		if !ok {
+			return fmt.Errorf("cat: this radio's backend cannot set split from software — " +
+				"use Split Operating: Fake It in WSJT-X/JTDX, or set split on the radio itself")
+		}
+		return s.SetSplit(on, txHz)
+	})
+}
+
 // SpotInfo is one cluster spot to render on a backend that supports a spot
 // overlay (the FlexRadio panadapter). Color is an optional "#AARRGGBB" string;
 // the backend picks a default when it's empty. (Status-based colouring can be

internal/cat/flex.go

@@ -59,15 +59,15 @@ type Flex struct {
 	meterRawLogged bool         // log the first raw meter-definition status once
 	txRawLogged    bool         // log the first raw transmit status once (field-name audit)
 
-	spotsEnabled bool           // push cluster spots + manage the panadapter overlay
-	spotIdx      map[int]bool   // panadapter spot indices currently known to the radio
+	spotsEnabled    bool           // push cluster spots + manage the panadapter overlay
+	spotIdx         map[int]bool   // panadapter spot indices currently known to the radio
 	pendingSpot     map[int]string // seq → callsign, awaiting the spot index in the R response
 	pendingSpotMode map[int]string // seq → ADIF mode, paired with pendingSpot
 	pendingSplit    map[int]bool   // seq → awaiting the new TX slice's index (split create)
 	spotCall        map[int]string // spot index → callsign (to fill the call on a panadapter click)
 	spotMode        map[int]string // spot index → ADIF mode, so a click can also set the slice mode (SmartSDR tunes the spot's freq but not its mode)
 	spotByCall      map[string]int // callsign → live spot index, so re-spotting a call replaces its old spot (WSJT decodes re-fire every cycle)
-	sentCmds     map[int]string // seq → command text, so an R<seq> error names the command
+	sentCmds        map[int]string // seq → command text, so an R<seq> error names the command
 
 	// OnSpotClick is called (off the reader goroutine's hot path) when the user
 	// clicks one of our spots on the panadapter, with the spot's callsign and

internal/cat/kenwood.go

@@ -513,6 +513,43 @@ func isKenwoodDataMode(mode string) bool {
 	return true
 }
 
+// SetSplit arms or clears split, and when arming puts txHz on the transmit VFO.
+//
+// Both halves in one call on purpose. WSJT-X sends "split on, VFO B" and "VFO B
+// to 14075300" as two commands, and honouring only the first is worse than
+// honouring neither: split would arm on whatever VFO B happened to hold, so the
+// operator transmits somewhere they never chose while the software reports
+// exactly what they asked for. Nothing is armed here until the frequency is on
+// the dial.
+//
+// FR selects the receive VFO, FT the transmit one — the same pair the poll loop
+// already reads to detect split, so this writes what State() knows how to read.
+func (k *Kenwood) SetSplit(on bool, txHz int64) error {
+	k.mu.Lock()
+	defer k.mu.Unlock()
+	if k.port == nil {
+		return fmt.Errorf("kenwood: not connected")
+	}
+	if !on {
+		// Transmit follows receive again. FR is left alone: which VFO the operator
+		// listens on is theirs to choose, and clearing split should not move them.
+		return k.write("FT0;")
+	}
+	if txHz <= 0 || txHz > 99_999_999_999 {
+		return fmt.Errorf("kenwood: split TX frequency %d out of the 11-digit CAT range", txHz)
+	}
+	// The transmit dial FIRST, then arm. Arming first would transmit on the old
+	// contents of VFO B for however long the next command takes to arrive — brief,
+	// but on the wrong frequency, and this runs the instant before a transmission.
+	if err := k.write(fmt.Sprintf("FB%011d;", txHz)); err != nil {
+		return err
+	}
+	if err := k.write("FR0;"); err != nil { // receive on A
+		return err
+	}
+	return k.write("FT1;") // transmit on B
+}
+
 func (k *Kenwood) SetPTT(on bool) error {
 	k.mu.Lock()
 	defer k.mu.Unlock()

internal/cat/kenwood_test.go

@@ -102,7 +102,7 @@ func TestKenwoodModeDigit(t *testing.T) {
 		{"FM", 145000000, '4'},
 		{"FT8", 7074000, '2'},  // data is ALWAYS USB, even below 10 MHz (K3 "DATA REV" otherwise)
 		{"FT8", 14074000, '2'}, // …and above
-		{"", 14074000, 0}, // nothing to set
+		{"", 14074000, 0},      // nothing to set
 	}
 	for _, c := range cases {
 		if got := kenwoodModeDigit(c.mode, c.hz); got != c.want {

internal/rigctld/rigctld.go

@@ -49,6 +49,10 @@ type Rig interface {
 	SetFreq(hz int64) error
 	SetMode(mode string) error
 	SetPTT(on bool) error
+	// SetSplit arms or clears split and places the transmit frequency. Returns an
+	// error on a rig that cannot: a refusal the client can report is worth far
+	// more than a success it has no way to check.
+	SetSplit(on bool, txHz int64) error
 }
 
 type Server struct {
@@ -76,6 +80,9 @@ type Server struct {
 	// be told from the very first call — where the radio's state is unknown and
 	// the command must go through.
 	pttKnown atomic.Bool
+	// splitWanted remembers a set_split_vfo that arrived before the frequency it
+	// needs, so the pair can be honoured in the order the client sends them.
+	splitWanted atomic.Bool
 }
 
 func New(port int, rig Rig, logf func(string, ...any)) *Server {
@@ -383,7 +390,31 @@ func (s *Server) handle(line string) (resp string, quit bool) {
 		}
 		return fmt.Sprintf("%d\nVFOB\n", n), false
 	case "S", "\\set_split_vfo":
-		return rprt(0), false // see set_vfo — split is driven from the rig
+		// "S <0|1> <VFO>". The VFO argument is ignored: which dial transmits is the
+		// rig's own business, and every backend here puts it on the second one.
+		//
+		// This used to answer RPRT 0 and do NOTHING. WSJT-X in "Split Operating:
+		// Rig" sends this and set_split_freq, believed both, and transmitted on the
+		// RECEIVE frequency — on a pileup, straight onto the DX, while the software
+		// showed exactly what the operator had asked for. A lie that leaves no
+		// trace anywhere is the worst kind of bug, so it now works or says so.
+		if len(args) < 1 {
+			return rprt(-1), false
+		}
+		if args[0] != "0" {
+			// Arming needs a frequency, and WSJT-X sends set_split_freq AFTER this.
+			// Remember the request and let that command do the work: alone, this
+			// would arm split on whatever the transmit VFO happens to hold.
+			s.splitWanted.Store(true)
+			return rprt(0), false
+		}
+		s.splitWanted.Store(false)
+		if err := s.rig.SetSplit(false, 0); err != nil {
+			s.log("rigctld: split off failed: %v", err)
+			return rprt(-9), false
+		}
+		s.log("rigctld: split off")
+		return rprt(0), false
 	case "i", "\\get_split_freq":
 		_, tx := s.rig.Split()
 		if tx <= 0 {
@@ -391,6 +422,20 @@ func (s *Server) handle(line string) (resp string, quit bool) {
 		}
 		return fmt.Sprintf("%d\n", tx), false
 	case "I", "\\set_split_freq":
+		if len(args) < 1 {
+			return rprt(-1), false
+		}
+		// Hamlib sends a float ("14075300.000000"), so parse as one.
+		hz, err := strconv.ParseFloat(args[0], 64)
+		if err != nil || hz <= 0 {
+			return rprt(-1), false
+		}
+		if err := s.rig.SetSplit(true, int64(hz)); err != nil {
+			s.log("rigctld: split TX %.0f Hz failed: %v", hz, err)
+			return rprt(-9), false
+		}
+		s.splitWanted.Store(true)
+		s.log("rigctld: split ON, TX %.0f Hz", hz)
 		return rprt(0), false
 
 	default:

internal/rigctld/rigctld_test.go

@@ -2,6 +2,7 @@ package rigctld
 
 import (
 	"bufio"
+	"errors"
 	"fmt"
 	"net"
 	"strings"
@@ -21,11 +22,26 @@ type fakeRig struct {
 	setFreqs []int64
 	setModes []string
 	failSet  bool
+	// noSplit models a backend that cannot set split — the case that must reach
+	// the client as a refusal instead of a silent success.
+	noSplit    bool
+	splitCalls []string
 }
 
 func (f *fakeRig) Freq() int64          { f.mu.Lock(); defer f.mu.Unlock(); return f.freq }
 func (f *fakeRig) Mode() string         { f.mu.Lock(); defer f.mu.Unlock(); return f.mode }
 func (f *fakeRig) Split() (bool, int64) { f.mu.Lock(); defer f.mu.Unlock(); return f.split, f.txFreq }
+
+func (f *fakeRig) SetSplit(on bool, txHz int64) error {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	if f.noSplit {
+		return errors.New("this radio cannot set split from software")
+	}
+	f.splitCalls = append(f.splitCalls, fmt.Sprintf("%v:%d", on, txHz))
+	f.split, f.txFreq = on, txHz
+	return nil
+}
 func (f *fakeRig) SetFreq(hz int64) error {
 	f.mu.Lock()
 	defer f.mu.Unlock()

internal/rigctld/split_test.go

@@ -0,0 +1,52 @@
+package rigctld
+
+import (
+	"strings"
+	"testing"
+)
+
+// WSJT-X in "Split Operating: Rig" sends set_split_vfo then set_split_freq. Both
+// used to answer RPRT 0 and do NOTHING: the software believed it was
+// transmitting up the band while the radio stayed on the receive frequency —
+// on a pileup, straight onto the DX, with no trace anywhere.
+func TestSetSplitReachesTheRig(t *testing.T) {
+	rig := &fakeRig{freq: 14074000, mode: "FT8"}
+	s := New(0, rig, func(string, ...any) {})
+
+	if got, _ := s.handle("S 1 VFOB"); !strings.HasPrefix(got, "RPRT 0") {
+		t.Fatalf("set_split_vfo answered %q", got)
+	}
+	// Arming alone must NOT touch the rig: without a frequency it would transmit
+	// on whatever the second VFO happened to hold.
+	if len(rig.splitCalls) != 0 {
+		t.Errorf("split was armed before a frequency arrived: %v", rig.splitCalls)
+	}
+
+	if got, _ := s.handle("I 14075300.000000"); !strings.HasPrefix(got, "RPRT 0") {
+		t.Fatalf("set_split_freq answered %q", got)
+	}
+	if len(rig.splitCalls) != 1 || rig.splitCalls[0] != "true:14075300" {
+		t.Fatalf("rig saw %v, want one call arming split on 14075300", rig.splitCalls)
+	}
+
+	if got, _ := s.handle("S 0 VFOA"); !strings.HasPrefix(got, "RPRT 0") {
+		t.Fatalf("split off answered %q", got)
+	}
+	if len(rig.splitCalls) != 2 || !strings.HasPrefix(rig.splitCalls[1], "false:") {
+		t.Errorf("rig saw %v, want split cleared", rig.splitCalls)
+	}
+}
+
+// A backend that cannot do split must produce an ERROR the client can report.
+// Answering success and doing nothing is what caused the original fault, and it
+// is the one outcome that must never come back.
+func TestSetSplitRefusalIsReported(t *testing.T) {
+	rig := &fakeRig{freq: 14074000, noSplit: true}
+	s := New(0, rig, func(string, ...any) {})
+
+	s.handle("S 1 VFOB")
+	got, _ := s.handle("I 14075300.000000")
+	if strings.HasPrefix(got, "RPRT 0") {
+		t.Errorf("a rig that cannot split answered %q — the client will transmit on the wrong frequency", got)
+	}
+}