Readling Data from Serial Port Causes Powershell to Hang

Question

I don't have any experience in serial port communications, but I need to come up with a PowerShell tool to query a device (display) connected through my USB port via RS232 (connected using Trendnet /Prolific USB-to-serial adapter)

I use PowerShell v 5.0

I wouldn't expect it to be a very complicated script.

Here is what I currently have

$p = Get-ItemProperty -Path "hklm:\hardware\devicemap\serialcomm\" -name "*prolific*" | Select-Object -ExpandProperty \Device\ProlificSerial*
$PowerStatus = "query power status"
$port= new-Object System.IO.Ports.SerialPort $p,9600,None,8,one
$port.DTREnable =  "true"
$port.RtsEnable = "true"
$port.open()
$port.WriteLine($powerstatus)

# reading operation
$line = $port.ReadLine()
Write-Host $line    

When I run this, Powershell hangs. Sometimes I get either "access to the COM port is denied" or

Exception calling "ReadLine" with "0" argument(s): "The port is closed."

Either way, I don't get any response and have to restart PowerShell every time :)

Apparently I am able to write to the port. I am able to power the display on and off from PowerShell.

When I check the port status after I write the "query power status" command, it looks like this

PS C:\WINDOWS\system32> $port


BaseStream             : System.IO.Ports.SerialStream
BaudRate               : 9600
BreakState             : False
BytesToWrite           : 0
BytesToRead            : 10
CDHolding              : False
CtsHolding             : False
DataBits               : 8
DiscardNull            : False
DsrHolding             : False
DtrEnable              : True
Encoding               : System.Text.ASCIIEncoding
Handshake              : None
IsOpen                 : True
NewLine                : 

Parity                 : None
ParityReplace          : 63
PortName               : COM3
ReadBufferSize         : 4096
ReadTimeout            : -1
ReceivedBytesThreshold : 1
RtsEnable              : True
StopBits               : One
WriteBufferSize        : 2048
WriteTimeout           : -1
Site                   : 
Container              : 

I see there are bytes sitting in the buffer but cannot get a response.

It looks like the port is still being used by the writing operation? so when I try to read, the port is busy

Please let me know what I am doing wrong. Thank you

Answer 1

This will probably fix it:

$port.NewLine                   = "`r"      # [`r]
$port.Handshake                 = 2         # [0]   <-- MOST IMPORTANT!
$port.ReadTimeout               = 3000      # [-1]
$port.WriteTimeout              = 2000      # [-1]
$port.DiscardNull               = $True     # [F] 

(Defaults are shown in brackets.)