serial.go

package modbus

import	(
	"time"

	"github.com/goburrow/serial"
)

// serialPortWrapper wraps a serial.Port (i.e. physical port) to
// 1) satisfy the rtuLink interface and
// 2) add Read() deadline/timeout support.
type serialPortWrapper struct {
	conf		*serialPortConfig
	port		serial.Port
	deadline	time.Time
}

type serialPortConfig struct {
	Device		string
	Speed		uint
	DataBits	uint
	Parity		uint
	StopBits	uint
}

func newSerialPortWrapper(conf *serialPortConfig) (spw *serialPortWrapper) {
	spw = &serialPortWrapper{
		conf:	conf,
	}

	return
}

func (spw *serialPortWrapper) Open() (err error) {
	var parity	string

	switch spw.conf.Parity {
	case PARITY_NONE:	parity	= "N"
	case PARITY_EVEN:	parity	= "E"
	case PARITY_ODD:	parity	= "O"
	}

	spw.port, err = serial.Open(&serial.Config{
		Address:	spw.conf.Device,
		BaudRate:	int(spw.conf.Speed),
		DataBits:	int(spw.conf.DataBits),
		Parity:		parity,
		StopBits:	int(spw.conf.StopBits),
		Timeout:	10 * time.Millisecond,
	})

	return
}

// Closes the serial port.
func (spw *serialPortWrapper) Close() (err error) {
	err = spw.port.Close()

	return
}

// Reads bytes from the underlying serial port.
// If Read() is called after the deadline, a timeout error is returned without
// attempting to read from the serial port.
// If Read() is called before the deadline, a read attempt to the serial port
// is made. At this point, one of two things can happen:
// - the serial port's receive buffer has one or more bytes and port.Read()
//   returns immediately (partial or full read),
// - the serial port's receive buffer is empty: port.Read() blocks for
//   up to 10ms and returns serial.ErrTimeout. The serial timeout error is
//   masked and Read() returns with no data.
// As the higher-level methods use io.ReadFull(), Read() will be called
// as many times as necessary until either enough bytes have been read or an
// error is returned (ErrRequestTimedOut or any other i/o error).
func (spw *serialPortWrapper) Read(rxbuf []byte) (cnt int, err error) {
	// return a timeout error if the deadline has passed
	if time.Now().After(spw.deadline) {
		err = ErrRequestTimedOut
		return
	}

	cnt, err = spw.port.Read(rxbuf)
	// mask serial.ErrTimeout errors from the serial port
	if err != nil && err == serial.ErrTimeout {
		err = nil
	}

	return
}

// Sends the bytes over the wire.
func (spw *serialPortWrapper) Write(txbuf []byte) (cnt int, err error) {
	cnt, err = spw.port.Write(txbuf)

	return
}

// Saves the i/o deadline (only used by Read).
func (spw *serialPortWrapper) SetDeadline(deadline time.Time) (err error) {
	spw.deadline = deadline

	return
}