Why $event.messageData from serial port event is empty?

Question

I'm trying to receive serial data from Arduino asynchronously via "DataReceived" event, but it $event.messageData is always empty.

Here's my code so far:

$port = New-Object System.IO.Ports.SerialPort COM7,9600,None,8,one
$port.Open()
$port | Format-List -Property *
Register-ObjectEvent -InputObject $port -EventName "DataReceived" -Action {
  $event | Format-List -Property * | Out-Host
}

Here's the output:

PORT

BaseStream             : System.IO.Ports.SerialStream
BaudRate               : 9600
BreakState             : False
BytesToWrite           : 0
BytesToRead            : 0
CDHolding              : False
CtsHolding             : False
DataBits               : 8
DiscardNull            : False
DsrHolding             : False
DtrEnable              : False
Encoding               : System.Text.ASCIIEncoding
Handshake              : None
IsOpen                 : True
NewLine                :

Parity                 : None
ParityReplace          : 63
PortName               : COM7
ReadBufferSize         : 4096
ReadTimeout            : -1
ReceivedBytesThreshold : 1
RtsEnable              : False
StopBits               : One
WriteBufferSize        : 2048
WriteTimeout           : -1
Site                   :
Container              :

EVENT

ComputerName     :
RunspaceId       : 67c40d50-5a9b-4022-974e-0a6a6518c2af
EventIdentifier  : 11
Sender           : System.IO.Ports.SerialPort
SourceEventArgs  : System.IO.Ports.SerialDataReceivedEventArgs
SourceArgs       : {COM7, System.IO.Ports.SerialDataReceivedEventArgs}
SourceIdentifier : 26c24bdb-8085-4e88-828e-44cb7897e416
TimeGenerated    : 6/16/2019 12:52:39 AM
MessageData      :

On the arduino side the code is:

Serial.printlf("ready")

When I use syncronous approach everything works

$port = New-Object System.IO.Ports.SerialPort COM7,9600,None,8,one
$port.Open()
$port.ReadLine()

Output

ready
ready
ready
...

Please, help!

mklement0 · Accepted Answer · 2023-05-16 06:44:02Z

4

$event.MessageData is not the right property to query - it contains custom data passed to the
-MessageData parameter when raising a custom event via the New-Event cmdlet.

For .NET-originated events, it is generally the automatic $EventArgs variable that contains the event-specific data.

In the specific case of the [System.IO.Ports.SerialPort] type's DataReceived event, however, it seems that the data isn't actually passed as an event argument, and you must instead use the $Sender object to gain access to the originating [System.IO.Ports.SerialPort] instance in order to call its .ReadExisting() method, judging by the sample code in the official docs:

$eventJob = 
  Register-ObjectEvent -InputObject $port -EventName "DataReceived" -Action {
    $Sender.ReadExisting() | Out-Host
  }

edited May 16, 2023 at 6:44

answered Jun 15, 2019 at 20:57

mklement0

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3 Comments

not2qubit Over a year ago

How do you write this in powershell? I don't get it. All the docs referring to this, are all about C#, nothing useful for powershell. I would love to see a functioning powersehll implementation for writing and reading on the COM port. E.g. connecting to a serial modem and sending AT commands and being able to receive the response.

mklement0 Over a year ago

@not2qubit, I see that you've since posted a new question, which was the right thing to do. (I personally won't be of much help in this subject matter.)

not2qubit Over a year ago

I solved it there now. And you should always add a name to the handler via -SourceIdentifier SOME_NAME. Thanks.

Joachim Otahal · Accepted Answer · 2024-04-04 14:26:29Z

Register-ObjectEvent has quite some flaws. Works in Powershell ISE, copy pasted into powershell, but not as normal script. My result was as strange as yours. Possibly related: https://github.com/PowerShell/PowerShell/issues/13999 . Solution: Don't use Register-ObjectEvent for this usage scenario. Here my example for reading Growatt MIC series "current Watt" via RS485 without Register-ObjectEvent - albeit sending and getting binary data, not string.

$Timeout = 4

# Modbus command for "give me your data"
[byte[]]$GetGrowattStatus=(0x01,0x04,0x0b,0xb8,0x00,0x7d,0xb2,0x2a)

$port = New-Object System.IO.Ports.SerialPort "COM3",9600,None,8,one
$port.Open()
$port.Write($GetGrowattStatus,0,$GetGrowattStatus.Count)
# Response is not fast, but wait no more than $Timeout seconds
for ($i=0;$i -lt $Timeout*4;$i++) {
    Start-Sleep -Milliseconds 250
    if ($port.BytesToRead -gt 0) {
        #Write-Verbose $i -Verbose
        $Data=@()
        while ($port.BytesToRead -gt 0) {
           $Data += [byte]$port.ReadByte()
        }
        $GrowattWatt = ($Data[7] * 256 + $Data[8])/10
        $i=[int]::MaxValue
    }
}
$port.Close()
$GrowattWatt

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