cSerialPort Properties Reference

📋 Properties List

cSerialPort Properties

Property	Type	Access	Description
`IsOpen`	Boolean	Read-only	Whether the serial port is open
`PortName`	String	Read/Write	Port name, cannot be modified after opening
`Handle`	Long	Read-only	Serial port handle
`Config`	cSerialConfig	Read/Write	Configuration object
`LastError`	Long	Read-only	Last error code
`LastErrorMsg`	String	Read-only	Last error description
`ReceivedCount`	Long	Read-only	Cumulative received bytes
`SentCount`	Long	Read-only	Cumulative sent bytes
`InBufferCount`	Long	Read-only	Current bytes in input buffer
`OutBufferCount`	Long	Read-only	Current bytes in output buffer
`CtsHolding`	Boolean	Read-only	CTS signal status
`DsrHolding`	Boolean	Read-only	DSR signal status
`RingHolding`	Boolean	Read-only	RING signal status
`CdHolding`	Boolean	Read-only	CD/RLSD signal status
`IsWaitingOnCts`	Boolean	Read-only	Whether waiting on CTS
`IsWaitingOnDsr`	Boolean	Read-only	Whether waiting on DSR
`IsWaitingOnRlsd`	Boolean	Read-only	Whether waiting on RLSD
`IsXoffHold`	Boolean	Read-only	Whether paused by XOFF
`IsXoffSent`	Boolean	Read-only	Whether XOFF has been sent
`Monitoring`	Boolean	Read-only	Whether monitoring is active
`PollInterval`	Long	Read/Write	Polling interval(ms), 10~60000

cSerialConfig Properties

Property	Type	Default	Description
`BaudRate`	Long	9600	Baud rate
`DataBits`	Long	8	Data bits, 4~8
`StopBits`	Long	0	Stop bits
`Parity`	Long	0	Parity
`FlowControl`	Long	0	Flow control
`DtrControl`	Long	1	DTR control mode
`RtsControl`	Long	1	RTS control mode
`ReadIntervalTimeout`	Long	50	Read interval timeout(ms)
`ReadTotalTimeoutMultiplier`	Long	0	Read per-byte timeout multiplier
`ReadTotalTimeoutConstant`	Long	100	Read fixed timeout(ms)
`WriteTotalTimeoutMultiplier`	Long	0	Write per-byte timeout multiplier
`WriteTotalTimeoutConstant`	Long	100	Write fixed timeout(ms)
`InBufferSize`	Long	4096	Input buffer size(bytes)
`OutBufferSize`	Long	4096	Output buffer size(bytes)
`XonChar`	Byte	&H11	XON character
`XoffChar`	Byte	&H13	XOFF character
`ErrorChar`	Byte	0	Parity error replacement character
`EofChar`	Byte	0	EOF character
`EvtChar`	Byte	0	Event trigger character
`NullDiscard`	Boolean	False	Whether to discard NULL bytes
`AbortOnError`	Boolean	False	Whether to abort read/write on error
`TXContinueOnXoff`	Boolean	False	Whether to continue sending after XOFF
`ParityReplace`	Boolean	True	Whether to replace characters on parity error

🔧 cSerialPort Properties Details

IsOpen

Whether the serial port is open.

If sp.IsOpen Then
    Debug.Print "Serial port is open"
End If

PortName

Port name. Cannot be modified after opening, must close first.

sp.PortName = "COM3"
sp.OpenPort
' sp.PortName = "COM4"  ' Error! Cannot change after opening
sp.ClosePort
sp.PortName = "COM4"     ' Can change after closing

Config

Configuration object, type cSerialConfig. Changes while port is open require calling ApplyConfig to take effect.

' Read configuration
Debug.Print sp.Config.BaudRate

' Modify configuration
sp.Config.BaudRate = br115200
If sp.IsOpen Then sp.ApplyConfig   ' Must manually apply if already open

' Replace entire configuration object
Dim cfg As New cSerialConfig
cfg.BaudRate = br9600
Set sp.Config = cfg                ' Automatically applies

LastError / LastErrorMsg

Last Win32 error code and description. OpenPort returns False and sets LastError on failure.

If Not sp.OpenPort("COM99") Then
    Debug.Print "Error code: " & sp.LastError
    Debug.Print "Error description: " & sp.LastErrorMsg
End If

ReceivedCount / SentCount

Cumulative send/receive byte count. Can be reset with ResetCounters.

Debug.Print "Received: " & sp.ReceivedCount & " bytes"
Debug.Print "Sent: " & sp.SentCount & " bytes"
sp.ResetCounters

InBufferCount / OutBufferCount

Current byte count in input/output buffer.

If sp.InBufferCount > 0 Then
    Debug.Print "Buffer has " & sp.InBufferCount & " bytes available"
End If

Signal Line Status Properties

Property	Signal	Description
`CtsHolding`	CTS	Clear To Send
`DsrHolding`	DSR	Data Set Ready
`RingHolding`	RING	Ring Indicator
`CdHolding`	CD/RLSD	Carrier Detect

Debug.Print "CTS=" & sp.CtsHolding & " DSR=" & sp.DsrHolding
Debug.Print "RING=" & sp.RingHolding & " CD=" & sp.CdHolding

PollInterval

Monitoring polling interval in milliseconds, range 10~60000. Can be specified when calling StartMonitoring, or modified via property.

sp.PollInterval = 100   ' 100ms polling

⚙️ cSerialConfig Properties Details

BaudRate

Baud rate, use eBaudRate enum constants. Supports up to 256000, far exceeding the MSComm control's limit of 115200.

Constant	Value	Description
`br110`	110
`br300`	300
`br600`	600
`br1200`	1200
`br2400`	2400
`br4800`	4800
`br9600`	9600	Default
`br14400`	14400
`br19200`	19200
`br38400`	38400
`br56000`	56000
`br57600`	57600
`br115200`	115200	Common high-speed
`br128000`	128000
`br256000`	256000	Maximum supported

cfg.BaudRate = br9600      ' Or br115200, br57600, br256000, etc.

DataBits

Data bits, range 4~8, throws error if out of range.

cfg.DataBits = 8

StopBits

Stop bits, use eStopBits enum: sb1(0), sb1_5(1), sb2(2).

Parity

Parity, use eParity enum: ptNone(0), ptOdd(1), ptEven(2), ptMark(3), ptSpace(4).

FlowControl

Flow control, use eFlowControl enum. Setting this automatically synchronizes DTR/RTS control modes.

cfg.FlowControl = fcRtsCts   ' Hardware flow control RTS/CTS

DtrControl / RtsControl

DTR/RTS control modes. Use eDtrControl/eRtsControl enum: dcDisable(0), dcEnable(1), dcHandshake(2), rcToggle(3, RTS only).

Timeout Parameters

Parameter	Description
`ReadIntervalTimeout`	Maximum interval between two reads, return on timeout
`ReadTotalTimeoutMultiplier`	Per-byte timeout multiplier
`ReadTotalTimeoutConstant`	Read fixed timeout
`WriteTotalTimeoutMultiplier`	Write per-byte timeout multiplier
`WriteTotalTimeoutConstant`	Write fixed timeout

Total timeout = Multiplier × byte count + Constant. See Advanced - Timeout Strategies

InBufferSize / OutBufferSize

Buffer sizes, must be greater than 0.

XonChar / XoffChar

XON/XOFF flow control characters, default &H11(DC1) and &H13(DC3).

📚 Enum Reference

All enums are defined in class modules (not standard modules), so they are exported with the DLL type library and available to external projects. See Overview - Architecture for details.

eBaudRate (cSerialConfig)

Baud rate constants. Maximum: 256000. See BaudRate for full table.

eParity (cSerialConfig)

Constant	Value	Win32 Constant	Description
`ptNone`	0	NOPARITY	No parity
`ptOdd`	1	ODDPARITY	Odd parity
`ptEven`	2	EVENPARITY	Even parity
`ptMark`	3	MARKPARITY	Mark parity
`ptSpace`	4	SPACEPARITY	Space parity

eStopBits (cSerialConfig)

Constant	Value	Win32 Constant	Description
`sb1`	0	ONESTOPBIT	1 stop bit
`sb1_5`	1	ONE5STOPBITS	1.5 stop bits
`sb2`	2	TWOSTOPBITS	2 stop bits

eDtrControl (cSerialConfig)

Constant	Value	Win32 Constant	Description
`dcDisable`	0	DTR_CONTROL_DISABLE	DTR line disabled
`dcEnable`	1	DTR_CONTROL_ENABLE	DTR line enabled
`dcHandshake`	2	DTR_CONTROL_HANDSHAKE	DTR flow control handshake

eRtsControl (cSerialConfig)

Constant	Value	Win32 Constant	Description
`rcDisable`	0	RTS_CONTROL_DISABLE	RTS line disabled
`rcEnable`	1	RTS_CONTROL_ENABLE	RTS line enabled
`rcHandshake`	2	RTS_CONTROL_HANDSHAKE	RTS flow control handshake
`rcToggle`	3	RTS_CONTROL_TOGGLE	RTS toggle mode

eFlowControl (cSerialConfig)

Constant	Value	Description
`fcNone`	0	No flow control
`fcXonXoff`	1	Software flow control (XON/XOFF)
`fcRtsCts`	2	Hardware flow control (RTS/CTS)
`fcDtrDsr`	3	Hardware flow control (DTR/DSR)
`fcRtsCtsAndXonXoff`	4	Mixed flow control (RTS/CTS + XON/XOFF)

eCommEvent (cSerialPort)

Communication events for SetCommMask / WaitForEvent / PinChanged.

Constant	Value	Description
`EV_RXCHAR`	&H1	Character received
`EV_RXFLAG`	&H2	Event character received
`EV_TXEMPTY`	&H4	Transmit buffer empty
`EV_CTS`	&H8	CTS signal changed
`EV_DSR`	&H10	DSR signal changed
`EV_RLSD`	&H20	RLSD(CD) signal changed
`EV_BREAK`	&H40	Break detected
`EV_ERR`	&H80	Line status error
`EV_RING`	&H100	Ring detected
`EV_PERR`	&H200	Printer error
`EV_RX80FULL`	&H400	Receive buffer 80% full
`EV_EVENT1`	&H800	Device event 1
`EV_EVENT2`	&H1000	Device event 2

eCommError (cSerialPort)

Communication errors from ClearCommError / ErrorOccurred / ReadCommErrors.

Constant	Value	Description
`CE_RXOVER`	&H1	Receive buffer overflow
`CE_OVERRUN`	&H2	Character overrun
`CE_RXPARITY`	&H4	Parity error
`CE_FRAME`	&H8	Frame error
`CE_BREAK`	&H10	Break detected
`CE_TXFULL`	&H100	Transmit buffer full
`CE_PTO`	&H200	Parallel port timeout
`CE_IOE`	&H400	I/O error
`CE_DNS`	&H800	Device not selected
`CE_OOP`	&H1000	Out of paper
`CE_MODE`	&H8000	Mode error

eModemStatus (cSerialPort)

Modem status from GetCommModemStatus / ReadModemStatus.

Constant	Value	Description
`MS_CTS_ON`	&H10	CTS (Clear To Send)
`MS_DSR_ON`	&H20	DSR (Data Set Ready)
`MS_RING_ON`	&H40	RING (Ring Indicator)
`MS_RLSD_ON`	&H80	RLSD/CD (Carrier Detect)

Last Updated: 2026-07-05

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cSerialPort Properties Reference

📋 Properties List