Feature RS485

serial.go

@@ -101,6 +101,7 @@ type Mode struct {
 	Parity            Parity           // Parity (see Parity type for more info)
 	StopBits          StopBits         // Stop bits (see StopBits type for more info)
 	InitialStatusBits *ModemOutputBits // Initial output modem bits status (if nil defaults to DTR=true and RTS=true)
+	RS485             RS485Config      // RS485 configuration
 }
 
 // Parity describes a serial port parity setting
@@ -131,6 +132,22 @@ const (
 	TwoStopBits
 )
 
+// RS485Config -- platform independent RS485 config. Thie structure is ignored unless Enable is true.
+type RS485Config struct {
+	// Enable RS485 support
+	Enabled bool
+	// Delay RTS prior to send
+	DelayRtsBeforeSend time.Duration
+	// Delay RTS after send
+	DelayRtsAfterSend time.Duration
+	// Set RTS high during send
+	RtsHighDuringSend bool
+	// Set RTS high after send
+	RtsHighAfterSend bool
+	// Rx during Tx
+	RxDuringTx bool
+}
+
 // PortError is a platform independent error type for serial ports
 type PortError struct {
 	code     PortErrorCode
@@ -165,6 +182,12 @@ const (
 	PortClosed
 	// FunctionNotImplemented the requested function is not implemented
 	FunctionNotImplemented
+	// ReadFailed indicates the read failed
+	ReadFailed
+	// ConfigureRS485Error indicates an error configuring RS485 on the platform
+	ConfigureRS485Error
+	// NoPlatformSupportForRS485 indicates no platform support for RS485
+	NoPlatformSupportForRS485
 )
 
 // EncodedErrorString returns a string explaining the error code
@@ -194,6 +217,12 @@ func (e PortError) EncodedErrorString() string {
 		return "Port has been closed"
 	case FunctionNotImplemented:
 		return "Function not implemented"
+	case ReadFailed:
+		return "Read failed"
+	case ConfigureRS485Error:
+		return "Error configuring RS485 on the platform"
+	case NoPlatformSupportForRS485:
+		return "Platform does not support RS485"
 	default:
 		return "Other error"
 	}

serial_unix.go

@@ -15,6 +15,7 @@ import (
 	"sync"
 	"sync/atomic"
 	"time"
+	"unsafe"
 
 	"go.bug.st/serial/unixutils"
 	"golang.org/x/sys/unix"
@@ -29,6 +30,22 @@ type unixPort struct {
 	opened      uint32
 }
 
+const (
+	rs485Enabled      = 1 << 0
+	rs485RTSOnSend    = 1 << 1
+	rs485RTSAfterSend = 1 << 2
+	rs485RXDuringTX   = 1 << 4
+	rs485Tiocs        = 0x542f
+)
+
+// rs485_ioctl_opts is used to configure RS485 options in the driver
+type rs485IoctlOpts struct {
+	flags              uint32
+	delayRtsBeforeSend uint32
+	delayRtsAfterSend  uint32
+	padding            [5]uint32
+}
+
 func (port *unixPort) Close() error {
 	if !atomic.CompareAndSwapUint32(&port.opened, 1, 0) {
 		return nil
@@ -279,6 +296,12 @@ func nativeOpen(portName string, mode *Mode) (*unixPort, error) {
 
 	port.acquireExclusiveAccess()
 
+	// Enable RS485, if requested
+	if err = port.enableRS485(&mode.RS485); err != nil {
+		port.Close()
+		return nil, err
+	}
+
 	// This pipe is used as a signal to cancel blocking Read
 	pipe := &unixutils.Pipe{}
 	if err := pipe.Open(); err != nil {
@@ -465,3 +488,28 @@ func (port *unixPort) acquireExclusiveAccess() error {
 func (port *unixPort) releaseExclusiveAccess() error {
 	return unix.IoctlSetInt(port.handle, unix.TIOCNXCL, 0)
 }
+
+// enableRS485 enables RS485 functionality of driver via an ioctl if the config says so
+func (port *unixPort) enableRS485(config *RS485Config) error {
+	if !config.Enabled {
+		return nil
+	}
+	rs485 := rs485IoctlOpts{
+		rs485Enabled,
+		uint32(config.DelayRtsBeforeSend / time.Millisecond),
+		uint32(config.DelayRtsAfterSend / time.Millisecond),
+		[5]uint32{0, 0, 0, 0, 0},
+	}
+
+	if config.RtsHighDuringSend {
+		rs485.flags |= rs485RTSOnSend
+	}
+	if config.RtsHighAfterSend {
+		rs485.flags |= rs485RTSAfterSend
+	}
+	if config.RxDuringTx {
+		rs485.flags |= rs485RXDuringTX
+	}
+
+	return unix.IoctlSetInt(port.handle, rs485Tiocs, int(uintptr(unsafe.Pointer(&rs485))))
+}