What’s the best way to test if my serial port is working?

Networking
1

I’m trying to figure out if a serial port on my PC is actually working or not. I’m not sure which tools or methods I should use to test it properly, or how to tell whether issues I’m seeing are from the cable, the device, or the port itself. Can anyone walk me through some reliable steps or tools to test and troubleshoot a serial port?

2

Best way to test a serial port is to separate things: port, cable, device, and software. Here’s a simple way to do it without overthinking.

  1. Do a loopback test on the PC
    This checks if the port itself works.

    • Power off or unplug first.
    • On a DB9 connector, short pin 2 to pin 3 with a paperclip or short wire.
    • Power up.
    • Open a terminal app:

    • On Windows: PuTTY, Tera Term, RealTerm.
    • Set the COM port, baud, data bits, parity, stop bits to something simple like 9600 8N1, no flow control.
      • Type on the keyboard.
      If every character you type shows up in the terminal, the UART is sending and receiving through that loop.
      If nothing appears, or you only see some characters, the port, driver, or settings have an issue.

  2. Check the port in the OS
    Windows:
    • Device Manager → Ports (COM & LPT).
    • See if the COM port shows without a yellow mark.
    • Right click, Properties → check “Device status”.
    Linux:
    • dmesg | grep tty
    • Look for /dev/ttyS*, /dev/ttyUSB*, etc.
    • Use: stty -F /dev/ttyS0 9600 cs8 -cstopb -parenb

    If the OS does not see the port, no cable or device test will help until that is fixed.

  3. Eliminate the cable as a problem
    Once loopback on the DB9 works, test with a known good cable.
    • If you have another machine with a serial port, connect them with a null modem cable.
    • Run a terminal on both PCs, same settings, disable flow control.
    • Type on PC A, see if it appears on PC B, then the other way.
    If only one direction works, suspect wrong wiring or RTS/CTS or DTR/DSR issues.

  4. Use a proper serial monitoring tool
    When you are not sure if the problem comes from the PC or the device, monitoring traffic helps a lot.
    A tool like “Serial Port Monitor” lets you watch every byte and every control signal going in and out of a COM port in realtime. That is much easier than guessing in a dumb terminal.
    Take a look at advanced serial port testing software.
    It lets you:
    • Open a port that another app uses.
    • See data, control lines, errors, and timings.
    • Compare two sessions when you tweak wiring or settings.

    For example, if your device expects 115200 7E1 and you send 9600 8N1, you will see junk or nothing. A monitor tool shows exactly what your PC sends and what comes back.

  5. Check control signals if needed
    Some devices need RTS, CTS, DTR, DSR high in order to talk.
    • Many terminal apps let you toggle RTS and DTR.
    • Use a multimeter or LED tester on the DB9 to verify those pins change when you toggle them.
    • If they never change, driver or hardware might be broken.

  6. Simple test order
    Follow this order to avoid confusion.
    • Confirm the OS sees the port.
    • Do a local loopback on TX/RX.
    • Test handshaking lines if the device needs them.
    • Test with another PC or known working device.
    • Use Serial Port Monitor to inspect traffic when loopback works but device communication fails.

If you stick to that sequence you will know if the fault sits in the PC port, the cable, or the external device, instead of guessing and swapping random settings forever.

What’s the best way to test if my serial port is working?
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6 Likes
3

You’re on the right track separating “port vs cable vs device,” and @jeff already covered the classic loopback + device manager route. I’d tweak the approach a bit and add a few things that catch the non-obvious failures.

1. Don’t start with loopback, start with electrical sanity

Loopback is great, but it won’t tell you if your TX line is physically dead or if levels are wrong. Quick electrical checks:

  • Use a cheap USB‑to‑serial adapter as a reference tool.
  • With a multimeter:
    • Check pin 5 (GND) to chassis ground.
    • Check pin 3 (TX on DB9) against GND:
      • For a real RS‑232 port, you should see around −5 to −12 V when idle.
      • If you see +5 V or +3.3 V only, it might actually be a TTL‑level port and will never talk to a proper RS‑232 device correctly.
  • If TX sits at 0 V and never moves, that’s already suspicious before you even open a terminal.

This catches those “BIOS‑disabled / fried line driver / fake adapter” issues that a basic loopback can sometimes confuse you on.

2. Use a logic probe or USB scope if you have one

Not required, but very helpful:

  • Connect a simple logic analyzer or USB scope to TX and RX:
    • When you send characters from your terminal, you should see bursts on TX.
    • If you see data on TX but absolutely nothing on RX during loopback or device communication, that’s more than just “wrong baud” most of the time.

This is where I slightly disagree with just relying on software tests: software happily lies; hardware pins do not.

3. Make sure your “serial” port is actually what you think it is

I’ve seen:

  • “COM” ports that are:
    • RS‑485
    • TTL only
    • Shared with some embedded controller
  • Laptops where BIOS lets you:
    • Disable the port
    • Change its mode (e.g., “debug only”)

So:

  • Check BIOS / UEFI for serial settings.
  • If it is a USB‑serial dongle:
    • Confirm the VID/PID in Device Manager or lsusb.
    • Install the proper driver, not the default one Windows grabs sometimes.

If the OS keeps randomly reassigning COM numbers, that’s another red flag.

4. Don’t trust just a dumb terminal

Dumb terminals hide timing, control signals, and error counters. @jeff mentioned monitoring traffic, and I’d double down on that: use a dedicated tool like Serial Port Monitor rather than just PuTTY.

A proper serial analyzer lets you:

  • See each byte, including control characters.
  • Watch RTS/CTS, DTR/DSR toggling live.
  • View parity / framing errors.
  • Compare two working vs non‑working runs.

If your issue is “works with vendor tool, fails with my software,” this is almost mandatory.

For getting the tool, check out
get Serial Port Monitor for advanced COM port diagnostics
so you can actually see what’s going over the wire instead of guessing.

5. Use a “known‑good device” instead of only another PC

Null‑modem tests between two PCs are fine, but they can mask mismatches because both ends are “too forgiving.” If you can, grab:

  • Any simple, known‑good RS‑232 device:
    • Old serial GPS
    • Serial console port on a router/switch
    • A device with a documented, simple protocol (like sending “?” returns some text)

Then:

  1. Connect PC to that device.
  2. Match the device’s exact serial settings (baud, data bits, parity, stop bits, flow control).
  3. Send its documented command and see if you get the expected response.

If it works with that device, your port and cable are probably fine and your problem is config or wiring with the other device.

6. Abuse the port a bit

To really convince yourself the port is solid:

  • Change baud rates wildly (9600, 19200, 115200) during loopback:
    • Port should be stable and echo everything, no random disconnects.
  • Toggle hardware flow control:
    • Even if you are not using it, flipping it should not hang or crash anything.
  • Open/close the COM port quickly with your test software:
    • Flaky drivers often reveal themselves with repeated open/close cycles.

If it only fails at one specific speed or with hardware flow control on, that points directly to misconfig or a marginal driver, not a dead port.

7. When it’s probably the cable or wiring

You asked how to tell if the issue is cable vs device vs PC:

  • Port passes:
    • OS sees it cleanly.
    • Electrical levels OK.
    • Local loopback passes.
    • Works with a second known device.
  • But fails with one particular setup:

Then it’s almost always one of:

  1. Wrong wiring:
    • Straight cable where null modem is needed or vice versa.
    • Missing GND.
  2. Wrong gender / adapters:
    • Too many adapters that “flip” signals without you noticing.
  3. Device expects:
    • Hardware flow control but you turned it off (or don’t have the lines wired).
    • Rare frame config like 7E1 at 115200.

That’s when Serial Port Monitor really shines: you can confirm what’s being sent and what, if anything, is coming back, instead of assuming.

TL;DR:
Use OS checks + multimeter + maybe a logic probe to verify the physical port, then terminal + loopback to confirm basic TX/RX, then Serial Port Monitor plus a known device to nail down config and wiring issues. Don’t just stare at a blank PuTTY window and hope the cable is fine.

4

You’ve already got solid advice from @jeff and the follow‑up about loopback, electrical checks, and BIOS. I’ll focus on a slightly different angle: testing the whole chain as a system, not just the port pins.

1. Start with “known‑good configuration snapshots”

Instead of only asking “is the port alive,” treat it like “can I reproduce a known working scenario on this machine?”

  1. Grab a setup that definitely works somewhere else:
    • Same device
    • Same cable
    • Same serial settings (baud, bits, parity, stop, flow)
  2. Replicate that config on the suspect PC exactly.
  3. Document all of it:
    • COM number
    • Settings
    • Any special wiring (null modem, gender changers, etc.)

If you can’t make a known‑good setup work on the suspect PC, then you’re down to:

  • Bad port
  • Bad driver
  • Some OS / security / virtualization weirdness

This avoids getting lost in “maybe the device is weird” while you’re still unsure about the port itself.

2. Check for “invisible” troublemakers: drivers & OS features

A part a lot of people skip:

  • Windows power management
    • Some chipsets aggressively power down unused ports.
    • In Device Manager, turn off “Allow the computer to turn off this device to save power” for the COM device.
  • Virtualization / USB redirection
    • If you are in a VM, confirm who owns the port:
      • Host vs guest
    • Serial in VMs is a constant source of “my loopback works on the host but not in the VM.”
  • Multiple serial drivers fighting
    • Old driver remnants can make the port flaky.
    • Uninstall ghost COM ports and restart, then test again.

These won’t be caught by a basic loopback or multimeter check and can masquerade as a “dead port.”

3. Use software that shows everything, not just text

Here is where I partially disagree with relying mostly on terminal + manual scope checks. For many real‑world bugs, what you want is visibility into the protocol, control signals, and timing, without reinventing tools.

A dedicated tool such as Serial Port Monitor is practical here because it:

Pros:

  • Shows every byte including non‑printable control characters.
  • Lets you watch RTS / CTS / DTR / DSR line changes in real time.
  • Logs sessions so you can compare “working setup” vs “broken setup.”
  • Often supports sniffing another app’s traffic without rewriting code.

Cons:

  • It is overkill if all you need is a 5‑minute “does it echo” check.
  • Paid tools can be unnecessary for ultra‑occasional use.
  • You still need some serial knowledge to interpret parity or framing errors.

I like using Serial Port Monitor to record a baseline on a system where everything is working. Then on the suspect PC, I record the same interaction and diff the captures:

  • Same data out, no data back? Cable/wiring/target.
  • Different baud or parity visible? Config issue.
  • Repeated framing errors? Electrical / signal integrity problem.

This is more systematic than just staring at PuTTY and guessing.

4. Pay attention to control lines, not just TX/RX

@jeff’s loopback test covers TX/RX, but plenty of devices depend on handshake lines and quietly ignore you if they never see the right signals.

So explicitly test:

  1. DTR to DSR
  2. RTS to CTS

Quick method:

  • Use a breakout box or simple jumper wires.
  • In your serial tool, toggle DTR/RTS and verify that the corresponding input line changes.
  • Watch it in Serial Port Monitor so you see if your software is actually toggling them like you think.

A surprising number of “dead” ports are just “device refuses to talk unless DSR is asserted” cases.

5. Look for partial failures, not just “works / doesn’t work”

Ports can be partly bad:

  • Works at 9600 but not at 115200:
    • Marginal driver, noisy line, or long cable.
  • Transmit OK, receive corrupted:
    • Bad RX input, wrong ground reference, or crosstalk.
  • Only fails when hardware flow control is enabled:
    • Handshake lines not connected or miswired.

To expose this:

  1. Run a loopback test at multiple baud rates and with different flow control settings.
  2. Blast a large amount of data (e.g., continuous test pattern, not just a few characters).
  3. Watch for:
    • Random characters changing
    • Missed bytes
    • Errors in Serial Port Monitor (parity/framing/overrun)

If only heavy traffic or high speeds fail, it is almost never “BIOS disabled” and almost always signal / driver quality.

6. Compare behavior with another machine rather than just another port

Instead of only checking PC‑to‑PC null modem like @jeff suggested, also:

  1. Take the same device + same cable to a second known‑good computer.
  2. Verify it works there first.
  3. Then:
    • Swap only the computer, keep device and cable.
    • If it breaks only on one PC, that PC is the suspect.
    • If it breaks on both, suspect cable or device.

This “A/B” test sounds trivial but saves a lot of time hunting imaginary COM problems that are really bad adapters or weird device requirements.

7. When Serial Port Monitor is worth the effort

Use a terminal when:

  • You just want to confirm “something is alive” on TX/RX.

Use Serial Port Monitor or a similar analyzer when:

  • You have a protocol to debug.
  • You suspect flow control or timing issues.
  • The device works with vendor software but not with your program, and you need to spy on what the vendor tool is actually doing.

For completeness in a comparison context:

  • @jeff’s advice is great for quick, low‑level electrical and BIOS checks.
  • A monitor tool complements that by giving you an application‑level view of what is going wrong.

In practice, a good sequence is:

  1. Confirm OS sees the port and driver is clean.
  2. Do a short electrical / loopback check.
  3. Verify a known‑good device + cable works on another PC.
  4. Reproduce that setup on the suspect PC.
  5. If there is still trouble, capture traffic and control lines with Serial Port Monitor and compare to a working capture.

By the time you finish those steps, you can usually say with confidence whether the serial port itself is fine and whether the real culprit is cable, wiring, configuration, or the attached device.