feat: amplifier band-follow — answer the amp's frequency polls

On its CAT/AUX connector an ACOM is the MASTER: it polls a transceiver and
changes band from the reply, so nothing can be pushed to it. internal/catemu
answers those polls on a second serial port, in ACOM command set 5 (Kenwood /
Elecraft): FA;, FB;, IF; and ID;, and nothing else — a wrong-length reply is
worse than none, it desynchronises the amp's parser for the following poll.

Also offered for SPE. Some amps do not poll at all but read the radio↔PC CAT
line in parallel; those never hear a responder, so an optional unprompted send
(500/1000 ms) covers them.

The TX frequency is what is sent: in split the amp must be tuned where we
transmit. Frame lengths are pinned by tests — the failure they guard against is
silent and only shows up as an amp that mistunes.

Untested on hardware.

app.go

@@ -30,6 +30,7 @@ import (
 	"hamlog/internal/backup"
 	"hamlog/internal/cabrillo"
 	"hamlog/internal/cat"
+	"hamlog/internal/catemu"
 	"hamlog/internal/clublog"
 	"hamlog/internal/cluster"
 	"hamlog/internal/contest"
@@ -917,6 +918,10 @@ func (a *App) startup(ctx context.Context) {
 			wruntime.EventsEmit(a.ctx, "cat:state", s)
 		}
 		a.emitRadioUDP(s)
+		// Feed the frequency to any amplifier we are pretending to be a radio
+		// for. Just two atomic stores per amp — the reply itself is built when
+		// the amp polls, so a fast-tuning VFO costs nothing here.
+		a.feedAmpBandFollow(s)
 		// Drive station relays by the current frequency/band (PstRotator-style
 		// automatic control). Cheap cached-flag check keeps this a no-op when the
 		// feature is off; when on, run off this callback so a slow relay board never
@@ -13343,6 +13348,18 @@ type AmpConfig struct {
 	Port      int    `json:"port"`
 	ComPort   string `json:"com_port"`
 	Baud      int    `json:"baud"`
+
+	// Band-follow: an ACOM is the MASTER on its CAT/AUX connector — it polls a
+	// transceiver and changes band from the reply, so following OpsLog means
+	// answering those polls on a SECOND serial port (independent of the one used
+	// above for metering; both run at once). See internal/catemu.
+	FreqOut     bool   `json:"freq_out"`
+	FreqComPort string `json:"freq_com_port"`
+	FreqBaud    int    `json:"freq_baud"`
+	// FreqBroadcastMs > 0 also SENDS the frequency unprompted at that interval,
+	// for an amplifier that listens to a transceiver's CAT stream instead of
+	// polling it. 0 = answer polls only.
+	FreqBroadcastMs int `json:"freq_broadcast_ms"`
 }
 
 type ampInst struct {
@@ -13350,6 +13367,7 @@ type ampInst struct {
 	pgxl   *powergenius.Client
 	spe    *spe.Client
 	acom   *acom.Client
+	catemu *catemu.Server // Kenwood-format responder for band-follow (ACOM)
 }
 
 func (i *ampInst) stopAll() {
@@ -13362,6 +13380,9 @@ func (i *ampInst) stopAll() {
 	if i.acom != nil {
 		i.acom.Stop()
 	}
+	if i.catemu != nil {
+		i.catemu.Stop()
+	}
 }
 
 // ampTypeLabel is the default display name for an amp type.
@@ -13491,12 +13512,61 @@ func (a *App) startAmps() {
 				a.spe = inst.spe
 			}
 		}
+		// Band-follow on a SECOND serial port, for any amp that takes its band from
+		// a transceiver CAT link (ACOM and SPE both do). Independent of the control
+		// transport above, so metering and band-follow run at the same time.
+		if c.FreqOut && strings.TrimSpace(c.FreqComPort) != "" {
+			inst.catemu = catemu.New(catemu.Config{
+				ComPort: c.FreqComPort, Baud: c.FreqBaud,
+				BroadcastMs: c.FreqBroadcastMs,
+			}, applog.Printf)
+			if st := a.cat.State(); st.Connected {
+				inst.catemu.SetFrequency(st.FreqHz)
+				inst.catemu.SetMode(st.Mode)
+			}
+			inst.catemu.Start()
+			applog.Printf("amp %s: band-follow on %s at %d baud (Kenwood format, broadcast=%dms)",
+				c.Name, c.FreqComPort, c.FreqBaud, c.FreqBroadcastMs)
+		}
 		a.ampsMu.Lock()
 		a.ampInsts[c.ID] = inst
 		a.ampsMu.Unlock()
 	}
 }
 
+// feedAmpBandFollow pushes the rig's frequency/mode to every amplifier we are
+// emulating a transceiver for. Called on each CAT state change.
+//
+// The TX frequency is what the amp must follow: in split it has to be tuned
+// for where we transmit, not where we listen.
+func (a *App) feedAmpBandFollow(s cat.RigState) {
+	if !s.Connected || s.FreqHz <= 0 {
+		return
+	}
+	a.ampsMu.Lock()
+	defer a.ampsMu.Unlock()
+	for _, inst := range a.ampInsts {
+		if inst.catemu != nil {
+			inst.catemu.SetFrequency(s.FreqHz)
+			inst.catemu.SetMode(s.Mode)
+		}
+	}
+}
+
+// GetAmpBandFollowStatus returns the band-follow link state per amplifier id,
+// so the settings panel can show whether the amp is actually polling us.
+func (a *App) GetAmpBandFollowStatus() map[string]catemu.Status {
+	out := map[string]catemu.Status{}
+	a.ampsMu.Lock()
+	defer a.ampsMu.Unlock()
+	for id, inst := range a.ampInsts {
+		if inst.catemu != nil {
+			out[id] = inst.catemu.GetStatus()
+		}
+	}
+	return out
+}
+
 // AmpStatus is one amp's live state for the UI poll — exactly one of the
 // per-family payloads is set, per the amp's type.
 type AmpStatus struct {

frontend/src/components/SettingsModal.tsx

@@ -627,7 +627,10 @@ function ADIFMonitorPanel() {
 }
 
 // AmpUI mirrors the backend AmpConfig — one configured amplifier.
-type AmpUI = { id: string; name: string; enabled: boolean; type: string; transport: string; host: string; port: number; com_port: string; baud: number };
+type AmpUI = { id: string; name: string; enabled: boolean; type: string; transport: string; host: string; port: number; com_port: string; baud: number;
+  // Band-follow (ACOM): a SECOND serial port on which OpsLog answers the amp's
+  // frequency polls, pretending to be a Kenwood-format transceiver.
+  freq_out?: boolean; freq_com_port?: string; freq_baud?: number; freq_broadcast_ms?: number };
 
 // RelayAutoPanel configures automatic control of the Station Control relay boards
 // from the rig's frequency / band (PstRotator-style). Each relay carries one rule:
@@ -2811,6 +2814,7 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
     });
     const addAmp = () => setAmps((l) => [...l, {
       id: '', name: '', enabled: true, type: 'spe13', transport: 'tcp', host: '', port: 9008, com_port: '', baud: 115200,
+      freq_out: false, freq_com_port: '', freq_baud: 9600, freq_broadcast_ms: 0,
     }]);
     return (
       <>
@@ -2927,6 +2931,62 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
                   </div>
                 )}
 
+                {/* Band-follow, for any amp that takes its band from a transceiver
+                    CAT link (ACOM and SPE both do) — never PowerGenius, which is
+                    driven over its network protocol. A SECOND serial port,
+                    separate from the metering one above. */}
+                {!isPGXL && (
+                  <div className="border-t border-border/60 pt-3 space-y-3">
+                    <label className="flex items-center gap-2 text-sm cursor-pointer">
+                      <Checkbox
+                        checked={!!amp.freq_out}
+                        onCheckedChange={(c) => patchAmp(i, { freq_out: !!c })}
+                      />
+                      {t('amp.freqOut')}
+                    </label>
+                    {amp.freq_out && (
+                      <>
+                        <div className="grid grid-cols-3 gap-3">
+                          <div className="space-y-1 col-span-2">
+                            <Label>{t('amp.freqPort')}</Label>
+                            <div className="flex items-center gap-2">
+                              <Select value={amp.freq_com_port || '_'} onValueChange={(v) => patchAmp(i, { freq_com_port: v === '_' ? '' : v })}>
+                                <SelectTrigger className="h-9 flex-1"><SelectValue placeholder="— COM —" /></SelectTrigger>
+                                <SelectContent>
+                                  {wkPorts.length === 0 && <SelectItem value="_" disabled>No ports found</SelectItem>}
+                                  {wkPorts.map((p) => <SelectItem key={p} value={p}>{p}</SelectItem>)}
+                                </SelectContent>
+                              </Select>
+                              <Button size="sm" variant="outline" className="h-9" onClick={() => ListSerialPorts().then((p) => setWkPorts((p ?? []) as string[])).catch(() => {})}>
+                                <ArrowDown className="size-3.5 rotate-90" />
+                              </Button>
+                            </div>
+                          </div>
+                          <div className="space-y-1">
+                            <Label>Baud</Label>
+                            <Input type="number" min={1200} value={amp.freq_baud ?? 9600}
+                              onChange={(e) => patchAmp(i, { freq_baud: parseInt(e.target.value) || 9600 })} className="font-mono" />
+                          </div>
+                        </div>
+                        <div className="grid grid-cols-3 gap-3">
+                          <div className="space-y-1 col-span-2">
+                            <Label>{t('amp.freqBroadcast')}</Label>
+                            <Select value={String(amp.freq_broadcast_ms ?? 0)} onValueChange={(v) => patchAmp(i, { freq_broadcast_ms: parseInt(v) })}>
+                              <SelectTrigger className="h-9"><SelectValue /></SelectTrigger>
+                              <SelectContent>
+                                <SelectItem value="0">{t('amp.freqPollOnly')}</SelectItem>
+                                <SelectItem value="500">{t('amp.freqEvery', { ms: 500 })}</SelectItem>
+                                <SelectItem value="1000">{t('amp.freqEvery', { ms: 1000 })}</SelectItem>
+                              </SelectContent>
+                            </Select>
+                          </div>
+                        </div>
+                        <p className="text-[10px] text-muted-foreground">{t('amp.freqHint')}</p>
+                      </>
+                    )}
+                  </div>
+                )}
+
                 {amp.enabled && amp.id && <AmpStatusCard id={amp.id} />}
                 {!isPGXL && !isACOM && (
                   <p className="text-[10px] text-muted-foreground">

frontend/src/lib/i18n.tsx

@@ -170,7 +170,7 @@ const en: Dict = {
   'gen.showBeam': 'Show the antenna beam heading on the Main map',
   'gen.startEqEnd': 'QSO start time = end time', 'gen.startEqEndHint': '(matches LoTW when you call a while)',
   'gen.showQsoRate': 'Show QSO rate in the header', 'gen.showQsoRateHint': '(QSOs/hour, projected from the last 10 / 60 min)',
-  'gen.lookupOnBlur': 'Look up the callsign only after leaving the field', 'gen.lookupOnBlurHint': '(not while typing)', 'amp.hint': 'Configure one or several amplifiers — each panel card has a dropdown to pick which one it shows.', 'amp.none': 'No amplifier configured yet.', 'amp.namePh': 'Name (e.g. SPE left)', 'amp.remove': 'Remove this amplifier', 'amp.add': 'Add amplifier',
+  'gen.lookupOnBlur': 'Look up the callsign only after leaving the field', 'gen.lookupOnBlurHint': '(not while typing)', 'amp.hint': 'Configure one or several amplifiers — each panel card has a dropdown to pick which one it shows.', 'amp.none': 'No amplifier configured yet.', 'amp.namePh': 'Name (e.g. SPE left)', 'amp.remove': 'Remove this amplifier', 'amp.add': 'Add amplifier', 'amp.freqOut': 'Send the frequency to the amplifier (band follow)', 'amp.freqPort': 'CAT/AUX COM port', 'amp.freqBroadcast': 'Also send unprompted', 'amp.freqPollOnly': 'No — answer the amplifier only', 'amp.freqEvery': 'Yes, every {ms} ms', 'amp.freqHint': 'OpsLog pretends to be a transceiver on this second port, in Kenwood format: set the amplifier to that command set (set 5 on an ACOM) at the same baud rate, and put it in OPERATE — in standby it acknowledges but does not switch band. An amplifier that POLLS (ACOM) needs nothing more; one that only LISTENS to the CAT line of the radio hears nothing unless you also turn on the unprompted send.',
   'gen.groupDigital': 'Group digital modes as one (DXCC-style)', 'gen.groupDigitalHint': '(matrix badges + cluster: FT8/FT4/RTTY… count as a single Digital mode; off = each digital mode is its own slot)',
   // Password encryption
   'gen.pwEnc': 'Password encryption',
@@ -563,7 +563,7 @@ const fr: Dict = {
   'gen.showBeam': 'Afficher le cap de l\'antenne sur la carte principale',
   'gen.startEqEnd': 'Heure de début du QSO = heure de fin', 'gen.startEqEndHint': '(correspond à LoTW quand tu appelles un moment)',
   'gen.showQsoRate': 'Afficher le rythme QSO dans la barre du haut', 'gen.showQsoRateHint': '(QSO/heure, projeté sur les 10 / 60 dernières min)',
-  'gen.lookupOnBlur': 'Rechercher l\'indicatif seulement après avoir quitté le champ', 'gen.lookupOnBlurHint': '(pas pendant la saisie)', 'amp.hint': 'Configurez un ou plusieurs amplificateurs — chaque carte de panneau a une liste déroulante pour choisir lequel afficher.', 'amp.none': 'Aucun amplificateur configuré.', 'amp.namePh': 'Nom (p. ex. SPE gauche)', 'amp.remove': 'Supprimer cet amplificateur', 'amp.add': 'Ajouter un amplificateur',
+  'gen.lookupOnBlur': 'Rechercher l\'indicatif seulement après avoir quitté le champ', 'gen.lookupOnBlurHint': '(pas pendant la saisie)', 'amp.hint': 'Configurez un ou plusieurs amplificateurs — chaque carte de panneau a une liste déroulante pour choisir lequel afficher.', 'amp.none': 'Aucun amplificateur configuré.', 'amp.namePh': 'Nom (p. ex. SPE gauche)', 'amp.remove': 'Supprimer cet amplificateur', 'amp.add': 'Ajouter un amplificateur', 'amp.freqOut': "Envoyer la fréquence à l'amplificateur (suivi de bande)", 'amp.freqPort': 'Port COM CAT/AUX', 'amp.freqBroadcast': 'Envoyer aussi sans être interrogé', 'amp.freqPollOnly': "Non — répondre seulement à l'amplificateur", 'amp.freqEvery': 'Oui, toutes les {ms} ms', 'amp.freqHint': "OpsLog se fait passer pour un transceiver sur ce second port, au format Kenwood : réglez l'amplificateur sur ce jeu de commandes (le jeu 5 sur un ACOM) à la même vitesse, et mettez-le en OPERATE — en veille il acquitte mais ne change pas de bande. Un amplificateur qui INTERROGE (ACOM) n'a besoin de rien de plus ; un amplificateur qui se contente d'ÉCOUTER la liaison CAT de la radio n'entendra rien tant que l'envoi spontané n'est pas activé.",
   'gen.groupDigital': 'Regrouper les modes digitaux en un seul (style DXCC)', 'gen.groupDigitalHint': '(badges de la matrice + cluster : FT8/FT4/RTTY… comptent comme un seul mode Digital ; décoché = chaque mode digital est un slot distinct)',
   // Chiffrement des mots de passe
   'gen.pwEnc': 'Chiffrement des mots de passe',

frontend/wailsjs/go/main/App.d.ts

@@ -6,6 +6,7 @@ import {main} from '../models';
 import {cat} from '../models';
 import {profile} from '../models';
 import {acom} from '../models';
+import {catemu} from '../models';
 import {antgenius} from '../models';
 import {award} from '../models';
 import {awardref} from '../models';
@@ -344,6 +345,8 @@ export function GetActiveProfile():Promise<profile.Profile>;
 
 export function GetAlertEmailTo():Promise<string>;
 
+export function GetAmpBandFollowStatus():Promise<Record<string, catemu.Status>>;
+
 export function GetAmpStatuses():Promise<Array<main.AmpStatus>>;
 
 export function GetAmplifiers():Promise<Array<main.AmpConfig>>;

frontend/wailsjs/go/main/App.js

@@ -638,6 +638,10 @@ export function GetAlertEmailTo() {
   return window['go']['main']['App']['GetAlertEmailTo']();
 }
 
+export function GetAmpBandFollowStatus() {
+  return window['go']['main']['App']['GetAmpBandFollowStatus']();
+}
+
 export function GetAmpStatuses() {
   return window['go']['main']['App']['GetAmpStatuses']();
 }

frontend/wailsjs/go/models.ts

@@ -1442,6 +1442,10 @@ export namespace main {
 	    port: number;
 	    com_port: string;
 	    baud: number;
+	    freq_out: boolean;
+	    freq_com_port: string;
+	    freq_baud: number;
+	    freq_broadcast_ms: number;
 	
 	    static createFrom(source: any = {}) {
 	        return new AmpConfig(source);
@@ -1458,6 +1462,10 @@ export namespace main {
 	        this.port = source["port"];
 	        this.com_port = source["com_port"];
 	        this.baud = source["baud"];
+	        this.freq_out = source["freq_out"];
+	        this.freq_com_port = source["freq_com_port"];
+	        this.freq_baud = source["freq_baud"];
+	        this.freq_broadcast_ms = source["freq_broadcast_ms"];
 	    }
 	}
 	export class AmpStatus {

internal/catemu/catemu.go

@@ -0,0 +1,376 @@
+// Package catemu emulates a transceiver on a serial port so a device that
+// POLLS a radio for its frequency can follow OpsLog instead.
+//
+// This exists for the ACOM amplifiers: on their CAT/AUX connector the amp is
+// the master — it polls the transceiver every few hundred milliseconds and
+// changes band only when it gets a valid reply. There is no way to push a
+// frequency to it, so following OpsLog means answering its polls.
+//
+// The dialect is ACOM "command set 5" (Kenwood / Elecraft RS-232, also what
+// Flex and SunSDR users select): plain ASCII commands terminated by ';'. It is
+// the simplest of the five sets by a wide margin, which is why the SDC utility
+// uses it to steer an ACOM with no physical radio attached.
+//
+// Only the handful of commands an amp actually asks for are implemented:
+//
+//	FA;  → FA00014025000;   TX frequency, 11 digits, Hz
+//	FB;  → same (sub VFO — amps poll it on some firmware)
+//	IF;  → the 38-character TS-2000 status frame
+//	ID;  → ID019;  (TS-2000 — a known model keeps the amp from timing out)
+//
+// Anything else is ignored rather than answered: a wrong-length reply is worse
+// than none, because it desynchronises the amp's parser for the next poll.
+package catemu
+
+import (
+	"fmt"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"go.bug.st/serial"
+)
+
+// Config is the serial port the amplifier's CAT/AUX cable is wired to. This is
+// a SECOND port, independent of the one used for the amp's own remote/metering
+// protocol — both run at the same time on an ACOM.
+type Config struct {
+	ComPort string
+	Baud    int
+
+	// BroadcastMs > 0 also sends the frequency UNPROMPTED every that many
+	// milliseconds. Some amplifiers do not poll at all: they sit in parallel on
+	// the radio↔PC CAT line and read whatever goes past. Such an amp would never
+	// hear us, since answering polls means speaking only when spoken to.
+	// 0 = answer polls only.
+	BroadcastMs int
+}
+
+// Status is what the settings panel shows about the link.
+type Status struct {
+	Enabled   bool   `json:"enabled"`
+	Connected bool   `json:"connected"`
+	Port      string `json:"port"`
+	Polls     int64  `json:"polls"`    // replies sent since start
+	LastCmd   string `json:"last_cmd"` // last command received, e.g. "FA;"
+	LastAt    string `json:"last_at"`  // RFC3339 of the last poll, "" if none
+	FreqHz    int64  `json:"freq_hz"`  // what we are currently answering
+	Error     string `json:"error"`    // last open/IO failure
+}
+
+// Server answers a polling amplifier on one serial port.
+type Server struct {
+	cfg Config
+
+	mu     sync.Mutex
+	port   serial.Port
+	status Status
+
+	freqHz atomic.Int64
+	mode   atomic.Value // string, ADIF-ish ("CW", "USB"…)
+
+	stop chan struct{}
+	done chan struct{}
+
+	logf func(string, ...any)
+}
+
+// New builds a server. Nothing is opened until Start.
+func New(cfg Config, logf func(string, ...any)) *Server {
+	if cfg.Baud <= 0 {
+		cfg.Baud = 9600
+	}
+	s := &Server{cfg: cfg, logf: logf}
+	s.mode.Store("")
+	s.status.Port = cfg.ComPort
+	return s
+}
+
+func (s *Server) log(format string, args ...any) {
+	if s.logf != nil {
+		s.logf(format, args...)
+	}
+}
+
+// SetFrequency updates the frequency reported to the amplifier. Called from the
+// CAT state callback; safe from any goroutine and never blocks — the serve loop
+// reads the value when a poll arrives, so a fast-tuning VFO costs nothing.
+func (s *Server) SetFrequency(hz int64) { s.freqHz.Store(hz) }
+
+// SetMode updates the mode digit in the IF frame. Optional: the amp only cares
+// about the frequency, but a coherent frame avoids odd firmware behaviour.
+func (s *Server) SetMode(mode string) { s.mode.Store(strings.ToUpper(strings.TrimSpace(mode))) }
+
+// Start opens the port and serves polls until Stop. It returns immediately;
+// a port that is missing or busy is retried every 5 s, because the amplifier is
+// often powered on after the software.
+func (s *Server) Start() {
+	s.stop = make(chan struct{})
+	s.done = make(chan struct{})
+	go s.run()
+}
+
+// Stop closes the port and waits for the loop to end.
+func (s *Server) Stop() {
+	if s.stop == nil {
+		return
+	}
+	close(s.stop)
+	s.mu.Lock()
+	if s.port != nil {
+		_ = s.port.Close()
+		s.port = nil
+	}
+	s.mu.Unlock()
+	<-s.done
+	s.stop = nil
+}
+
+// GetStatus returns a snapshot for the UI.
+func (s *Server) GetStatus() Status {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	st := s.status
+	st.Enabled = true
+	st.FreqHz = s.freqHz.Load()
+	return st
+}
+
+func (s *Server) setErr(msg string) {
+	s.mu.Lock()
+	s.status.Error = msg
+	s.status.Connected = false
+	s.mu.Unlock()
+}
+
+func (s *Server) run() {
+	defer close(s.done)
+	for {
+		select {
+		case <-s.stop:
+			return
+		default:
+		}
+		if err := s.open(); err != nil {
+			s.setErr(err.Error())
+			s.log("catemu: %s open failed: %v (retry in 5s)", s.cfg.ComPort, err)
+			select {
+			case <-s.stop:
+				return
+			case <-time.After(5 * time.Second):
+			}
+			continue
+		}
+		s.serve()
+	}
+}
+
+func (s *Server) open() error {
+	if strings.TrimSpace(s.cfg.ComPort) == "" {
+		return fmt.Errorf("no COM port configured")
+	}
+	p, err := serial.Open(s.cfg.ComPort, &serial.Mode{
+		BaudRate: s.cfg.Baud,
+		DataBits: 8,
+		Parity:   serial.NoParity,
+		StopBits: serial.OneStopBit,
+	})
+	if err != nil {
+		return err
+	}
+	// A short read timeout keeps the loop responsive to Stop while idle: the amp
+	// may poll only every few hundred ms, and a blocking read would hold the
+	// port open past shutdown.
+	_ = p.SetReadTimeout(200 * time.Millisecond)
+	s.mu.Lock()
+	s.port = p
+	s.status.Connected = true
+	s.status.Error = ""
+	s.mu.Unlock()
+	s.log("catemu: serving Kenwood-format polls on %s at %d baud", s.cfg.ComPort, s.cfg.Baud)
+	return nil
+}
+
+// broadcast sends an unsolicited FA frame at the configured interval, for an
+// amplifier that listens to the CAT line rather than polling it. It stops when
+// the port is closed or Stop is called.
+func (s *Server) broadcast(stopServe <-chan struct{}) {
+	if s.cfg.BroadcastMs <= 0 {
+		return
+	}
+	// Below ~100 ms this is pure noise on the wire; the band only ever changes
+	// at human speed.
+	every := time.Duration(s.cfg.BroadcastMs) * time.Millisecond
+	if every < 100*time.Millisecond {
+		every = 100 * time.Millisecond
+	}
+	t := time.NewTicker(every)
+	defer t.Stop()
+	for {
+		select {
+		case <-s.stop:
+			return
+		case <-stopServe:
+			return
+		case <-t.C:
+			hz := s.freqHz.Load()
+			if hz <= 0 {
+				continue // nothing known yet — say nothing rather than "0 Hz"
+			}
+			s.mu.Lock()
+			p := s.port
+			s.mu.Unlock()
+			if p == nil {
+				return
+			}
+			if _, err := p.Write([]byte(fmt.Sprintf("FA%011d;", hz))); err != nil {
+				s.setErr(err.Error())
+				return
+			}
+		}
+	}
+}
+
+// serve reads commands until the port fails or Stop is called.
+func (s *Server) serve() {
+	// The broadcaster shares this port and must die with it, or it would write
+	// into a closed handle after a reopen.
+	stopServe := make(chan struct{})
+	defer close(stopServe)
+	go s.broadcast(stopServe)
+
+	buf := make([]byte, 64)
+	var acc []byte
+	for {
+		select {
+		case <-s.stop:
+			return
+		default:
+		}
+		s.mu.Lock()
+		p := s.port
+		s.mu.Unlock()
+		if p == nil {
+			return
+		}
+		n, err := p.Read(buf)
+		if err != nil {
+			s.setErr(err.Error())
+			s.log("catemu: %s read failed: %v — reopening", s.cfg.ComPort, err)
+			s.mu.Lock()
+			if s.port != nil {
+				_ = s.port.Close()
+				s.port = nil
+			}
+			s.mu.Unlock()
+			return
+		}
+		if n == 0 {
+			continue
+		}
+		acc = append(acc, buf[:n]...)
+		// Commands are ';'-terminated; handle every complete one in the buffer.
+		for {
+			i := indexByte(acc, ';')
+			if i < 0 {
+				break
+			}
+			cmd := strings.ToUpper(strings.TrimSpace(string(acc[:i])))
+			acc = acc[i+1:]
+			s.handle(p, cmd)
+		}
+		// A runaway buffer means we are seeing something that is not this
+		// protocol (wrong baud, or the amp's other port); drop it rather than
+		// grow without bound.
+		if len(acc) > 512 {
+			acc = acc[:0]
+		}
+	}
+}
+
+func indexByte(b []byte, c byte) int {
+	for i := range b {
+		if b[i] == c {
+			return i
+		}
+	}
+	return -1
+}
+
+// handle answers one command. cmd has no trailing ';'.
+func (s *Server) handle(p serial.Port, cmd string) {
+	hz := s.freqHz.Load()
+	var reply string
+	switch {
+	case cmd == "FA" || cmd == "FB":
+		reply = fmt.Sprintf("%s%011d;", cmd, hz)
+	case cmd == "IF":
+		reply = s.ifFrame(hz)
+	case cmd == "ID":
+		reply = "ID019;" // TS-2000
+	default:
+		// Unknown or a SET command (FA00014025000;) — silently ignored: an amp
+		// never sets our frequency, and answering the wrong length would break
+		// its parser for the following poll.
+		return
+	}
+	if _, err := p.Write([]byte(reply)); err != nil {
+		s.setErr(err.Error())
+		return
+	}
+	s.mu.Lock()
+	s.status.Polls++
+	s.status.LastCmd = cmd + ";"
+	s.status.LastAt = time.Now().Format(time.RFC3339)
+	s.mu.Unlock()
+}
+
+// modeDigit maps our mode name to the Kenwood mode digit used in IF.
+func (s *Server) modeDigit() byte {
+	m, _ := s.mode.Load().(string)
+	switch {
+	case strings.HasPrefix(m, "CW"):
+		return '3'
+	case strings.HasPrefix(m, "LSB"):
+		return '1'
+	case strings.HasPrefix(m, "USB"), strings.HasPrefix(m, "SSB"):
+		return '2'
+	case strings.HasPrefix(m, "FM"):
+		return '4'
+	case strings.HasPrefix(m, "AM"):
+		return '5'
+	case m == "RTTY", strings.HasPrefix(m, "FSK"):
+		return '6'
+	case m == "":
+		return '2'
+	default:
+		// Data modes (FT8, PSK…) ride on SSB as far as an amplifier cares.
+		return '2'
+	}
+}
+
+// ifFrame builds the 38-character TS-2000 IF status frame:
+//
+//	IF | freq(11) | step(4) | RIT(6) | RIT/XIT/…(3) | memch(2) | rx/tx | mode |
+//	FR | scan | split | tone | tone#(2) | shift | ;
+//
+// Only the frequency and mode carry meaning here; the rest is a valid, inert
+// state (no RIT, receiving, simplex) so the amp's parser is satisfied.
+func (s *Server) ifFrame(hz int64) string {
+	return fmt.Sprintf("IF%011d%04d%+06d%03d%02d%01d%c%01d%01d%01d%01d%02d%01d;",
+		hz,            // P1 frequency, Hz
+		0,             // P2 frequency step
+		0,             // P3 RIT/XIT offset, signed 5 digits
+		0,             // P4-P6 RIT off, XIT off, channel-bank
+		0,             // P7 memory channel
+		0,             // P8 0 = RX
+		s.modeDigit(), // P9 mode
+		0,             // P10 VFO A
+		0,             // P11 scan off
+		0,             // P12 split off
+		0,             // P13 tone off
+		0,             // P14 tone number
+		0,             // P15 shift
+	)
+}

internal/catemu/catemu_test.go

@@ -0,0 +1,45 @@
+package catemu
+
+import "testing"
+
+// The reply LENGTHS are the contract: an amplifier parses these frames by
+// fixed offsets, so a frame one character short desynchronises its parser for
+// every following poll. Pin them.
+func TestReplyShapes(t *testing.T) {
+	s := New(Config{ComPort: "COM99", Baud: 9600}, nil)
+	s.SetFrequency(14025000)
+	s.SetMode("CW")
+
+	if got := s.ifFrame(14025000); len(got) != 38 {
+		t.Errorf("IF frame is %d chars, want 38: %q", len(got), got)
+	}
+	// TS-2000 IF: "IF" then the 11-digit frequency in Hz.
+	if got := s.ifFrame(14025000); got[:13] != "IF00014025000" {
+		t.Errorf("IF frame frequency field = %q", got[:13])
+	}
+	if got := s.ifFrame(14025000); got[len(got)-1] != ';' {
+		t.Errorf("IF frame not terminated by ';': %q", got)
+	}
+	// Mode digit sits at P9, right after freq(11)+step(4)+rit(6)+3+2+1 — index 29 in the frame.
+	if got := s.ifFrame(14025000)[29]; got != '3' {
+		t.Errorf("CW mode digit = %q, want '3'", got)
+	}
+	s.SetMode("FT8") // data rides on SSB as far as an amp cares
+	if got := s.ifFrame(14074000)[29]; got != '2' {
+		t.Errorf("FT8 mode digit = %q, want '2'", got)
+	}
+}
+
+func TestModeDigit(t *testing.T) {
+	cases := map[string]byte{
+		"CW": '3', "CW-R": '3', "LSB": '1', "USB": '2', "SSB": '2',
+		"FM": '4', "AM": '5', "RTTY": '6', "": '2', "FT8": '2',
+	}
+	for mode, want := range cases {
+		s := New(Config{}, nil)
+		s.SetMode(mode)
+		if got := s.modeDigit(); got != want {
+			t.Errorf("mode %q → %q, want %q", mode, got, want)
+		}
+	}
+}