param(
    [switch]$ListPorts,
    [string]$PortName = 'COM3',
    [int]$Baud = 9600,
    [string]$Parity = 'None',
    [string]$Send,
    [int]$Listen = 0,
    [switch]$Interactive
)

if ($ListPorts) {
    [System.IO.Ports.SerialPort]::GetPortNames() | Sort-Object
    return
}

$parityValue = [Enum]::Parse([System.IO.Ports.Parity], $Parity)
$sp = New-Object System.IO.Ports.SerialPort($PortName, $Baud, $parityValue, 8, [System.IO.Ports.StopBits]::One)
$sp.ReadTimeout = 300
$sp.NewLine = "`n"
$sp.Encoding = [System.Text.Encoding]::ASCII

try {
    $sp.Open()
    Write-Host "opened $PortName @ $Baud 8N1, Ctrl+C to exit"

    if ($Send) {
        $payload = $Send -replace '\\n', "`n" -replace '\\r', "`r"
        $sp.Write($payload)
        Write-Host ("TX: " + ($payload -replace "`n", '\n' -replace "`r", '\r'))
    }

    if ($Interactive) {
        while ($true) {
            $line = Read-Host 'TX'
            if ([string]::IsNullOrEmpty($line)) { continue }
            $sp.Write($line + "`n")
            Start-Sleep -Milliseconds 200
            try {
                while ($true) {
                    $rx = $sp.ReadExisting()
                    if ($rx) { Write-Host ("RX: " + ($rx -replace "`n", '\n' -replace "`r", '\r')) }
                    break
                }
            } catch {}
        }
    }

    if ($Listen -gt 0) {
        $deadline = (Get-Date).AddSeconds($Listen)
        $buffer = ''
        while ((Get-Date) -lt $deadline) {
            try {
                $chunk = $sp.ReadExisting()
                if (-not $chunk) {
                    Start-Sleep -Milliseconds 50
                    continue
                }
                $buffer += $chunk
                $parts = $buffer -split "`n"
                $buffer = $parts[-1]
                $lines = $parts[0..($parts.Count - 2)]
                foreach ($line in $lines) {
                    Write-Host ("RX: " + ($line -replace "`r", '') -replace '`n', '')
                }
            } catch {}
        }
    }
} finally {
    if ($sp.IsOpen) { $sp.Close() }
}