How Can I Send Serial COM Port Commands from a Command Line Batch File?

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For a simple ASCII command, configure the port with mode and redirect the command to the COM device:

@echo off
mode COM3:9600,n,8,1
echo STATUS>COM3

Replace COM3, the serial settings, command text, and line ending with the values specified by your device manual. This works for basic text protocols, but native batch becomes unreliable when you need exact bytes, binary packets, response parsing, timeouts, retries, or precise flow-control handling.

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What a “serial COM port command” actually is

mode and echo are Windows commands. STATUS, ATZ, VER?, and *IDN? are examples of commands understood by the attached device—not by Windows itself.

Before writing the batch file, obtain these details from the device documentation:

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• Baud rate
• Data bits
• Parity
• Stop bits
• Hardware or software flow control
• Required terminator: carriage return (CR), line feed (LF), CRLF, or none
• Any startup or inter-command delay
• Whether the device echoes commands
• Expected response and timeout

Do not assume every COM device accepts readable text. Instruments, PLCs, controllers, and embedded devices may require binary frames, checksums, escape bytes, acknowledgements, or fixed-length packets.

Before you start: identify and free the port

1. Open Device Manager.
2. Expand Ports (COM & LPT).
3. Find the device or USB-to-serial adapter, such as USB Serial Port (COM3).
4. Close PuTTY, Tera Term, Arduino IDE serial monitors, vendor tools, and other programs that may have the port open.

Only one application can normally open a COM port for exclusive serial communication. You can display the current status with:

mode COM3

For a reusable script, keep the port and settings in variables:

@echo off
set "PORT=COM3"
set "BAUD=9600"

mode %PORT%: baud=%BAUD% parity=n data=8 stop=1
echo STATUS>%PORT%

Batch has no convenient, dependable built-in port-discovery workflow. PowerShell can enumerate available ports through System.IO.Ports.SerialPort.GetPortNames(). See Microsoft’s SerialPort documentation.

Configure the COM port with mode

The general syntax is:

mode COM<m>: baud=<rate> parity=<p> data=<bits> stop=<bits>

A common, but not universal, configuration is 9600 baud, 8 data bits, no parity, and one stop bit:

mode COM3:9600,n,8,1

mode also supports serial handshaking and signaling options, including XON/XOFF, DSR/DTR, and CTS/RTS. Use the syntax documented for your Windows version and match the device manual rather than relying on 9600 8-N-1. Microsoft’s mode reference lists the available parameters.

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Method 1: send a simple text command with echo

For a device that accepts one plain-text command and the line ending produced by the command path, use:

@echo off
mode COM3:9600,n,8,1
echo ATZ>COM3

You can send several lines as one redirected block:

@echo off
mode COM3:9600,n,8,1
(
  echo LOGIN
  echo STATUS
  echo EXIT
)>COM3

This is convenient, but echo is text-oriented and appends the command interpreter’s line ending. Do not treat that as a protocol guarantee. A device requiring CR only may reject a command when the transmitted data includes LF as well, while another device may require LF or CRLF.

Batch also has special-character parsing rules. Characters such as &, |, <, >, ^, and parentheses can be interpreted by cmd.exe instead of being sent literally. The Microsoft cmd reference explains command-interpreter parsing.

Method 2: send exact bytes with copy /b

Use a prepared file when the payload must contain an exact terminator, non-printable bytes, a checksum, or a binary packet:

@echo off
mode COM3:9600,n,8,1
copy /b command.bin COM3

The file might contain:

• ATZ followed by byte 0x0D for CR-only termination
• STATUS followed by byte 0x0A for LF-only termination
• A binary header, length field, payload, checksum, and terminator

copy /b is appropriate for binary-oriented transmission, but it can only send the bytes actually present in command.bin. A text editor may add a UTF-8 BOM, convert newlines to CRLF, change encoding, or append an unintended final newline.

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For a repeatable CR-terminated ASCII file, generate it explicitly:

powershell -NoProfile -Command ^
  "[IO.File]::WriteAllBytes('command.bin',[Text.Encoding]::ASCII.GetBytes('ATZ' + [char]13))"

copy /b command.bin COM3

Keep payload generation separate from transmission and document the intended byte sequence. For checksums or dynamic binary packets, PowerShell or a dedicated program is usually safer than hand-editing a file.

COM10 and higher-numbered ports

Legacy command contexts may handle high-numbered ports differently. Some redirection and copy operations may require the Windows device path:

echo STATUS>.COM10
copy /b command.bin .COM10

In publication HTML, the literal Windows form is:

echo STATUS>.COM10
copy /b command.bin .COM10

More precisely, the intended device-path spelling is \.COM10 at the Windows command line. Test the exact command you use; do not assume every legacy command treats COM10 identically. PowerShell’s SerialPort API avoids this particular naming issue:

$port = [System.IO.Ports.SerialPort]::new(
    'COM10', 9600, 'None', 8, 'One'
)
$port.Open()
$port.Write("STATUS`r")
$port.Close()

Method 3: call PowerShell from the batch file

Use PowerShell when the workflow needs explicit line endings, response reads, timeouts, exceptions, or guaranteed cleanup. Save this as send-serial.ps1:

param(
    [string]$PortName = 'COM3',
    [int]$BaudRate = 9600,
    [string]$Command = 'STATUS',
    [string]$Terminator = "`r",
    [int]$WaitMilliseconds = 500
)

$port = [System.IO.Ports.SerialPort]::new(
    $PortName,
    $BaudRate,
    [System.IO.Ports.Parity]::None,
    8,
    [System.IO.Ports.StopBits]::One
)

$port.Handshake = [System.IO.Ports.Handshake]::None
$port.ReadTimeout = 1000
$port.WriteTimeout = 1000
$port.NewLine = $Terminator

try {
    $port.Open()
    Start-Sleep -Milliseconds 200
    $port.Write($Command + $Terminator)

    Start-Sleep -Milliseconds $WaitMilliseconds

    if ($port.BytesToRead -gt 0) {
        $response = $port.ReadExisting()
        $response
    }
}
finally {
    if ($port.IsOpen) {
        $port.Close()
    }
    $port.Dispose()
}

Call it from send.bat:

@echo off
powershell.exe -NoProfile -File "%~dp0send-serial.ps1" ^
  -PortName COM3 ^
  -BaudRate 9600 ^
  -Command STATUS ^
  -Terminator "`r"

The Terminator value above is a carriage return. Use "`n" for LF or "`r`n" for CRLF when required. Prefer a separate .ps1 file over an inline command because quoting, error handling, settings, and reuse are clearer.

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For a quick one-line invocation:

powershell.exe -NoProfile -Command ^
  "$p=[IO.Ports.SerialPort]::new('COM3',9600,'None',8,'One'); ^
   $p.ReadTimeout=1000; ^
   $p.WriteTimeout=1000; ^
   $p.Open(); ^
   $p.Write('STATUS' + [char]13); ^
   Start-Sleep -Milliseconds 500; ^
   if($p.BytesToRead -gt 0){$p.ReadExisting()}; ^
   $p.Close()"

The .NET SerialPort API supports port opening, text and byte writes, reads, encoding, timeouts, and handshaking. The available PowerShell and .NET versions depend on the Windows installation and PowerShell edition.

Why explicit terminators matter

WriteLine() appends the value of SerialPort.NewLine. Microsoft documents the default NewLine value as LF, not automatically CRLF. Set it explicitly or write the terminator yourself:

$port.NewLine = "`r"
$port.WriteLine('STATUS')

# Equivalent explicit write:
$port.Write('STATUS' + [char]13)

See Microsoft’s documentation for WriteLine() and NewLine.

Reading and saving the response

A quick diagnostic is:

copy COM3 CON

However, a COM port is a stream, not a finite file. This command can wait indefinitely and provides no protocol-level timeout or response validation.

Choose the read method based on the protocol:

• Immediate available data: $port.ReadExisting(). It returns currently available data and does not itself impose a timeout.
• Line-oriented response: $port.ReadLine(). It waits for the configured newline and can throw a timeout exception when ReadTimeout is set.
• Fixed-length binary response: use Read() into a byte buffer and continue until the required length arrives.
• Terminator-based response: use ReadTo() with the protocol’s terminator.
• Unknown-length response: read until a deadline, terminator, or protocol-specific acknowledgement.

For a text response, capture and log it from PowerShell:

$response = $port.ReadExisting()
$response | Out-File -FilePath '.serial-response.log' -Encoding utf8

For binary data, do not decode it as ordinary text. Read bytes and save bytes so encoding cannot corrupt the response. Microsoft’s ReadExisting() documentation also warns that mixing text and binary reads requires clearly defined protocol boundaries.

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Timing, readiness, and flow control

Opening a port can change control lines such as DTR, and some devices reset or need boot time when that happens. Other devices need a pause before accepting a command or between commands:

$port.Open()
Start-Sleep -Milliseconds 200
$port.Write("STATUS`r")
Start-Sleep -Milliseconds 500

These delays are examples, not universal requirements. Use the device’s documented timing and acknowledgement rules.

A script using Handshake=None may appear to write successfully but fail with a device requiring hardware handshaking. Conversely, enabling hardware flow control on a cable that lacks the required signals can prevent transmission. DTR and RTS states may also need explicit control.

Batch error handling: what success does and does not mean

At minimum, check whether Windows accepted the configuration and write operation:

@echo off
set "PORT=COM3"

mode %PORT%:9600,n,8,1
if errorlevel 1 (
    echo Could not configure %PORT%.
    exit /b 1
)

echo STATUS>%PORT%
if errorlevel 1 (
    echo Failed to send command.
    exit /b 1
)

A successful redirection generally means Windows accepted the write operation. It does not prove that the cable is wired correctly, that the device understood the framing, or that the device acted on the command. Application-level success requires checking the response or acknowledgement.

Troubleshooting guide

When batch is not the right tool

Use native batch for a one-shot ASCII write or a small, non-interactive sequence. Use copy /b when the payload is already prepared and exact bytes matter.

Use PowerShell and SerialPort when you need explicit settings, terminators, byte handling, bounded reads, exceptions, response validation, logging, retries, or cleanup.

A terminal utility can be better for interactive diagnosis, session logging, or macro playback. Tera Term documents command-line options such as /C= for selecting a serial port, /WAITCOM, and /M= for starting a macro. PuTTY’s documentation covers its terminal utilities and workflows. Exact options and behavior depend on the selected program and version.

For binary protocols with checksums, retries, multiple devices, long-running monitoring, or strict acknowledgement rules, a dedicated PowerShell, Python, or compiled application is usually easier to test and maintain than a growing batch file.

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