diff --git a/main.go b/main.go
index 392b0fc..2fa9b2a 100644
--- a/main.go
+++ b/main.go
@@ -74,7 +74,7 @@ func statusHandler(w http.ResponseWriter, r *http.Request) {
 }
 
 func readRegs(client modbus.Client, start, qty uint16) {
-	data, err := client.ReadInputRegisters(5000, 50)
+	data, err := client.ReadInputRegisters(start, qty)
 	if err != nil {
 		die("Couldn't read input registers %d-%d: %v", start+1, start+qty, err)
 	}
diff --git a/sungrow.go b/sungrow.go
index e02f6f9..7f0f060 100644
--- a/sungrow.go
+++ b/sungrow.go
@@ -44,7 +44,7 @@ type sungrowConn struct {
 	block cipher.Block
 	fifo  *bytes.Buffer
 
-	mu   sync.Mutex
+	mu   sync.RWMutex
 	txid uint16
 }
 
@@ -54,8 +54,8 @@ func (c *sungrowConn) Write(msg []byte) (int, error) {
 	}
 
 	c.mu.Lock()
+	defer c.mu.Unlock()
 	c.txid = binary.BigEndian.Uint16(msg[:2])
-	c.mu.Unlock()
 
 	c.fifo.Truncate(0)
 
@@ -83,6 +83,8 @@ func (c *sungrowConn) Read(out []byte) (int, error) {
 	}
 
 	if c.fifo.Len() > 0 {
+		c.mu.RLock()
+		defer c.mu.RUnlock()
 		return c.fifo.Read(out)
 	}
 	// 4-byte header describes how much to read
@@ -101,9 +103,9 @@ func (c *sungrowConn) Read(out []byte) (int, error) {
 	for cp := pkt; len(cp) > 0; cp = cp[bs:] {
 		c.block.Decrypt(cp, cp)
 	}
-	c.mu.Lock()
+	c.mu.RLock()
+	defer c.mu.RUnlock()
 	binary.BigEndian.PutUint16(pkt, c.txid)
-	c.mu.Unlock()
 	c.fifo.Write(pkt[:pktlen])
 	return c.fifo.Read(out)
 }