fix: bug on steppir control

internal/steppir/steppir.go

@@ -74,6 +74,12 @@ type Client struct {
 	connMu sync.Mutex
 	conn   io.ReadWriteCloser
 
+	// ioMu serialises EVERY exchange on the shared connection — a status query
+	// (write "?A" then read 11 bytes) and a command write must never interleave,
+	// or their bytes mix on the wire and both frames are corrupted. The status
+	// poll runs on one goroutine, tuning on another, so this is essential.
+	ioMu sync.Mutex
+
 	statusMu   sync.RWMutex
 	lastStatus *Status
 	lastSetKHz int
@@ -85,11 +91,6 @@ type Client struct {
 	pendingDirAt  time.Time
 	pendingDirSet bool
 
-	// After a Home/Retract the controller drops out of AUTOTRACK and ignores
-	// frequency sets until it is turned back ON. Set on Retract, cleared by
-	// re-enabling on the next SetFrequency.
-	needAutotrack bool
-
 	stopChan chan struct{}
 	running  bool
 }
@@ -231,6 +232,8 @@ func (c *Client) queryStatus() (*Status, error) {
 	if conn == nil {
 		return nil, fmt.Errorf("steppir: not connected")
 	}
+	c.ioMu.Lock()
+	defer c.ioMu.Unlock()
 	setDeadline(conn, 3*time.Second)
 	if _, err := conn.Write([]byte("?A\r")); err != nil {
 		return nil, fmt.Errorf("write status cmd: %w", err)
@@ -299,6 +302,9 @@ func (c *Client) writeCmd(pkt []byte) error {
 	if conn == nil {
 		return fmt.Errorf("steppir: not connected")
 	}
+	c.ioMu.Lock()
+	defer c.ioMu.Unlock()
+	log.Printf("steppir: → % X", pkt)
 	setDeadline(conn, 3*time.Second)
 	if _, err := conn.Write(pkt); err != nil {
 		c.closeConn()
@@ -309,16 +315,19 @@ func (c *Client) writeCmd(pkt []byte) error {
 	return nil
 }
 
-// SetFrequency tunes the elements to freqKhz with the given direction. If a prior
-// Retract dropped AUTOTRACK, re-enable it first — otherwise the set is ignored.
+// SetFrequency tunes the elements to freqKhz with the given direction.
+//
+// AUTOTRACK is (re-)enabled first, EVERY time: the controller ignores frequency
+// sets unless it is in AUTOTRACK mode ("when not in AUTOTRACK only CALIBRATE and
+// RETRACT work"), and it can be out of AUTOTRACK at power-on, after a Home, or if
+// switched off on the front panel. Sending the 'R' command each tune is cheap and
+// makes tuning work regardless of the controller's current mode — which is what
+// was silently failing before.
 func (c *Client) SetFrequency(freqKhz int, direction int) error {
-	if c.needAutotrack {
-		if err := c.writeCmd(buildSet(freqKhz*1000, direction, 'R')); err != nil {
-			return err
-		}
-		c.needAutotrack = false
+	if err := c.writeCmd(buildSet(freqKhz*1000, direction, 'R')); err != nil { // AUTOTRACK ON
+		return err
 	}
-	if err := c.writeCmd(buildSet(freqKhz*1000, direction, '1')); err != nil {
+	if err := c.writeCmd(buildSet(freqKhz*1000, direction, '1')); err != nil { // set freq + dir
 		return err
 	}
 	c.statusMu.Lock()
@@ -343,8 +352,9 @@ func (c *Client) SetDirection(direction int) error {
 	return c.SetFrequency(khz, direction)
 }
 
-// Retract homes the elements into the hubs (storage). This leaves AUTOTRACK off,
-// so the next SetFrequency re-enables it.
+// Retract homes the elements into the hubs (storage). This drops the controller
+// out of AUTOTRACK, but that is handled transparently: the next SetFrequency
+// re-issues AUTOTRACK ON before tuning.
 func (c *Client) Retract() error {
 	// A valid frequency must accompany the command; reuse the last one.
 	khz := c.LastSetKHz()
@@ -355,9 +365,5 @@ func (c *Client) Retract() error {
 			khz = 14000 // any in-range value; the controller just homes
 		}
 	}
-	if err := c.writeCmd(buildSet(khz*1000, DirNormal, 'S')); err != nil {
-		return err
-	}
-	c.needAutotrack = true
-	return nil
+	return c.writeCmd(buildSet(khz*1000, DirNormal, 'S'))
 }

internal/steppir/steppir_test.go

@@ -50,10 +50,32 @@ func TestBuildSetDirectionAndCommand(t *testing.T) {
 	}
 }
 
+// A REAL status frame captured off an SDA controller (F4BPO's, 2026-07-15):
+// 40 41 00 4C C5 84 00 87 30 38 0D — 50.313 MHz, idle, bidirectional, interface
+// version "08". Using the actual device output (rather than hand-built bytes)
+// pins parseStatus to real hardware, and independently confirms the ÷10 wire
+// scale: 0x4CC584 × 10 = 50 313 000 Hz, a genuine 6 m frequency.
+func TestParseStatusRealFrame(t *testing.T) {
+	frame := []byte{0x40, 0x41, 0x00, 0x4C, 0xC5, 0x84, 0x00, 0x87, 0x30, 0x38, 0x0D}
+	st, err := parseStatus(frame)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if st.Frequency != 50313 {
+		t.Errorf("freq = %d kHz, want 50313 (50.313 MHz)", st.Frequency)
+	}
+	if st.Direction != DirBi { // 0x87 & 0xE0 = 0x80 = bidirectional
+		t.Errorf("direction = %d, want %d (bidirectional)", st.Direction, DirBi)
+	}
+	if st.MotorsMoving != 0 {
+		t.Errorf("moving = %d, want 0 (motors byte 0x00)", st.MotorsMoving)
+	}
+}
+
 // parseStatus decodes what the controller sends back — the inverse of buildSet's
-// frequency field, plus the direction nibble.
+// frequency field, plus the direction nibble and the motors-busy byte.
 func TestParseStatus(t *testing.T) {
-	frame := []byte{0x00, 0x00, 0x00, 0x15, 0x79, 0xA8, 0x01, 0x40, '1', '2', 0x0D}
+	frame := []byte{0x40, 0x41, 0x00, 0x15, 0x79, 0xA8, 0x01, 0x40, '0', '8', 0x0D}
 	st, err := parseStatus(frame)
 	if err != nil {
 		t.Fatal(err)