Recommended Free Tools
A passive RS-232 tap lets you observe traffic traveling in both directions between two devices without sending signals of its own. Connect it inline, capture the exchange on a computer, then compare the recorded bytes and timing with the device’s expected behavior. The right tap depends on the serial standard, speed, timestamp precision, and whether you need to see handshake lines as well as data.
What a passive RS-232 tap does
A tap sits in the serial connection and listens to the transmit signals in both directions. Stratus Engineering describes its EZ-Tap as capturing both directions “without injecting signals or disturbing the connection.” It is USB-powered; after its driver is installed, it appears as a USB COM port. You can capture with EZ-View or a third-party dual-COM serial monitor such as Docklight. The basic EZ-Tap supports capture up to 230,400 bps and provides host-PC timestamps. Stratus Engineering’s EZ-Tap specifications
This approach is useful when a device’s configuration should remain untouched or repeatedly unplugging the serial cable would disrupt the fault you are trying to observe. A saved log lets you inspect the live exchange and compare message order, bytes, and timing with the symptoms.
Choose the tool for the diagnostic question
Check the interface standard and the signals you need to observe before comparing models. A tap that supports only basic RS-232 data may be insufficient if you need precise event timing, hardware flow-control visibility, or support for another serial standard.
Quick wins for a faster PC:
Repair Windows errors before they cause bigger problemsFix Now →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Clear out junk files and repair common Windows errorsFree Scan →#1 Best Overall
- Monitor and record serial data streams
- Easy to install, may be left in place
- Transparent connections
- No external power required
| Tool | Useful capabilities | Best fit |
|---|---|---|
| EZ-Tap | RS-232, both directions, host-PC timestamps; up to 230,400 bps. | Basic two-way RS-232 capture. Stratus Engineering |
| EZ-Tap Pro | Microsecond hardware time-tagging; all six RS-232 handshake lines; automatic parameter detection up to 921,600 bps; operation up to 1 Mbps. | When you need finer timestamps, handshake-line visibility, or higher-speed operation. Stratus Engineering |
| Versa-Tap | RS-232, RS-422, RS-485, and 3.3/5 V TTL; microsecond timestamps; six handshake lines; up to 2,000,000 bps; automatic detection up to 921,600 bps. | When your work spans multiple serial electrical standards or requires the higher stated rate. Stratus Engineering |
| Serial breakout box | Lets you inspect pinout, DTE/DCE type, and signal voltages; it is a diagnostic tool, not a two-direction logging tap. | When you need to identify or verify wiring before connecting a tap. Cisco recommends a breakout box or digital voltmeter for asynchronous-interface troubleshooting. Cisco troubleshooting guidance |
The figures in the tap rows are manufacturer specifications, not independent comparative test results. Maximum operating speed, automatic parameter-detection range, and capture-rate claims describe different capabilities; confirm the exact requirement against the device and tap documentation.
Verify the interface and wiring before connecting
Do not assume a connector shape identifies the electrical standard. RS-232, TTL UART, and RS-485 are different interfaces; a DB9 connector alone does not establish that a device uses RS-232. IO Ninja likewise cautions that similar-looking connectors can have different electrical standards and pinouts, and advises using breakout cables or adapters matching the required characteristics. Do not connect an RS-485 device directly to an RS-232 DB9 tap port. IO Ninja manual
Rank #2
- Expansion Interface: Easily add 2 RS232 DB9 serial ports to desktop PCs or industrial devices via the motherboard's PCI Express slot, meeting a variety of device connection needs
- Wide Compatibility: Seamlessly connect to DB9 serial devices such as POS systems, industrial testing and control equipment, security systems, logistics management devices, scanners, and printers, suitable for various application scenarios
- Multi-Platform Support: Driver compatible with Windows XP, Vista, 7, 8.x, 10, 11 (32/64-bit) and Linux kernels 2.6.x, 3.x, 4.x, 5.x, ensuring smooth installation and use across different systems
- Efficient Data Transmission: Utilizes the ASIX AX99100 solution, supporting 256-byte deep FIFO transmission, compatible with Intel, AMD, and ARM hardware platforms, providing stable serial data transmission performance
- Flexible Installation: Comes with a standard full-size bracket for standard PCs, and includes 2 low-profile brackets for perfect compatibility with slim PCs. Based on the PCIe X1 interface, compatible with X1, X4, X8, and X16 slots
For a common DB9 RS-232 assignment, Cisco lists TXD on pin 3, RXD on pin 2, RTS on pin 7, CTS on pin 8, DSR on pin 6, DCD on pin 1, and DTR on pin 4. These are common assignments, not a guarantee that a particular device follows them. Confirm the device documentation and connector orientation, and establish whether each endpoint is DTE or DCE before wiring. Cisco also describes RS-232 mark and space voltage ranges in its asynchronous serial troubleshooting guidance. Cisco’s DB9 assignments and voltage guidance
- Confirm the interface is RS-232, rather than TTL UART or RS-485.
- Verify DB9 or DB25 pin assignments, DTE/DCE orientation, TX/RX direction, and signal ground.
- Use a breakout box or digital voltmeter to check pinout and idle-state voltages where the wiring is uncertain.
Capture a two-way serial exchange
- Identify the interface standard and determine whether each endpoint is DTE or DCE from its documentation or by checking the signals.
- Use a breakout box or meter to verify the pinout, idle state, and signal ground. Resolve any mismatch before connecting the tap.
- Insert the passive tap inline while keeping the endpoint cable arrangement unchanged.
- Install the tap driver and confirm that it appears as a USB COM port.
- Open EZ-View or another serial-monitor application that can capture both COM directions, then start a log.
- Compare the capture with the device’s expected protocol: framing, baud rate, parity, stop bits, handshaking, timing, and message order.
- If the diagnosis requires more timestamp precision, handshake-line visibility, speed, or support for mixed standards than the basic tap provides, select a model with those documented capabilities.
What to check when the capture is not useful
- No traffic appears: Recheck that the tap is inline on the correct link, the driver and COM-port selection are correct, and the application is configured to capture both directions.
- Bytes are unreadable or inconsistent: Check baud rate, parity, data framing, stop bits, and whether the line is actually RS-232 rather than TTL UART or another standard.
- One direction is missing: Verify TX/RX assignments at both endpoints and confirm that the selected software captures both COM channels.
- Messages arrive but behavior still fails: Compare handshake activity and event timing where available; a basic data capture may not expose every control-line transition or provide hardware-level timestamps.
When two listening adapters are an alternative
A documented low-cost alternative is to use two RS-232-capable adapters: connect each adapter to listen to one device’s transmit line, and share signal ground between them. Each adapter then observes one direction of the conversation. This method can work, but it depends on the adapters being electrically appropriate and the wiring being correct; it is not a reason to attach an arbitrary USB serial adapter to an unknown pinout. Docklight’s serial-monitor documentation
Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsQuick Recap
Best Value
- USB 2.0 TO RS232 SERIAL - 5ft in length serial adapter. Connect legacy DB-9 serial communications devices to a modern computer with USB
- EASY TEST - Easy test RS-232 host to device link. With a full LED, dual color display, tester output. This will make testing your connections with ease.
- FEATURES - Supports reversible USB type-C connection only, can not be connected to any other USB port. ESD Protection for RS-232 I/O +-15KV RS-232 for DTE Mode. PDA RS-232 Port support 56K Modem support Serial Mouse support+ Serial PC camera support.
- COMPATIBILITY - With support on multiple OS platforms this makes our USB RS- 232 very versatile. Supported OS platforms are Windows 7, 8, 10 and 11 as well as Linux and MacOS 10.X
- WHY Gearmo? Your Trusted Partner based in the USA, Providing Advanced Engineering, Highly Reliable and Superior Built Products to handle the most demanding industries for over 20 years. Reach out to us for CAD, Mechanical Drawings and, Data Sheets.
Rank #4
- 10/100M auto-sensing Ethernet
- 4 serial ports supporting RS-232
- Socket modes: TCP server/TCP client/UDP/Real COM
- SNMP MIB-II for network management
- Configure via Telnet/Web/Windows utility
Rank #3
- [ USB to RS-232 Serial Adapter ] : 5ft Cable Length - Easily connect legacy DB-9 serial devices to modern USB-equipped computers. Uses include industrial, lab, and point-of-sale applications.
- [ Easy Testing ] : Built-in signal tester features full LED indicators with dual-color display for quick and easy testing of RS-232 host-to-device connections.
- [ Wide Compatibility ] : Built with an FTDI Chipset. Works seamlessly with Windows 7, 8, 10, 11, Linux, and macOS 10.X, making it a highly versatile solution across platforms.
- [ Why Gearmo? ] : Your trusted partner based in the USA, providing advanced engineering, highly reliable and superior built products to handle the most demanding industries for over 10 years.
- [ Engineering Support ] : Need specs? Contact us for CAD files, mechanical drawings, or datasheets to support your integration or project needs.
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
