2 Port 54mm ExpressCard RS232 Serial Adapter Card - 16550 UART

Add 2 high-speed RS-232 Serial ports to your laptop via a 54mm ExpressCard slot

Product ID: EC2S55254

5 stars (1 reviews) |
  • Two 16C450/550 compatible UART serial ports
  • Supports baud rates up to 128000bps
  • 256 Byte deep transmit/receive FIFO

The EC2S55254 2-Port Serial ExpressCard (54mm) adds 2 RS232 serial ports to your laptop using an available 54mm ExpressCard slot. The adapter features a native chipset that provides reliability and performance improvements, as well as lower CPU loads and power consumption, over conventional bridge-chip serial ExpressCards.

This high-performance RS232 ExpressCard adapter includes a breakout cable that uses the full 54mm ExpressCard form factor, for a perfect fit and additional durability.

Designed for use with laptop computers that are equipped with 54mm ExpressCard slots, this 2-Port serial ExpressCard offers a broad operating system compatibility list that includes Windows®, Mac OS X and Linux, for simple integration into almost any system.

Backed by a StarTech.com Lifetime warranty and free lifetime technical support.


  • Add serial ports to newer systems that don't offer integrated RS232 connections, or expand the number of available RS232 ports
  • Perfect for mobile administrators who are required to connect to and configure routers, switches and industrial serial devices
  • Embedded system coding, building, and diagnostics
  • POS retail applications in grocery and other store locations to control keyboards, cash drawers, receipt printers, card readers / card swipes, scales, and elevated displays
  • Control surveillance / security cameras in parking structures, office buildings, etc

The StarTech.com Advantage

  • High-performance single-chip design optimizes speed, reliability, and power, while reducing CPU load by as much as 48% over conventional (bridge chip) adapters
  • Full width 54mm ExpressCard is the perfect fit for business laptops, ensuring a long life without damaging the card, or your computer
  • The included breakout cable spreads/spaces out the 2 serial ports, to avoid crowded peripheral connections

Partner Numbers

  • UPC 065030843621


  • Windows 8 logo
  • Windows 7 logo
  • Linux logo
  • Mac OS X Yosemite (10.10) logo
  • Windows 10 logo
  • Windows 11 logo
Windows® 11, 10 (32/64), 8 / 8.1 (32/64), 7 (32/64), Vista (32/64), XP (32/64), 2000
Windows Server® 2003, 2000
Mac OS® 10.10, 10.4
Linux 2.6.x+


Warranty Information Warranty Lifetime
Connector(s) 1 - Connector Type(s) ExpressCard (54mm)
2 - External Ports DB-9 (9 pin, D-Sub)
Environmental Operating Temperature -10°C to 80°C (14°F to 176°F)
Storage Temperature -20°C to 100°C (-4°F to 212°F)
Humidity 5~85% RH
Hardware Ports 2
Interface Serial
Bus Type ExpressCard
Card Type 54mm ExpressCard
Port Style Dongle (Included)
Chipset ID ASIX - MCS9922CV-AA
Packaging Information Package Length 6.9 in [17.5 cm]
Package Width 5.6 in [14.3 cm]
Package Height 1.2 in [30 mm]
Shipping (Package) Weight 0.6 lb [0.3 kg]
Performance Serial Protocol RS-232
Max Baud Rate 128 Kbps
Data Bits 5, 6, 7, 8, 9
FIFO 256 byte
Flow Control None , RTS/CTS
Stop Bits 1, 1.5, 2
Physical Characteristics Product Length 4.7 in [12 cm]
Product Width 2.3 in [59 mm]
Product Height 0.8 in [2 cm]
Weight of Product 1.7 oz [49 g]
Software OS Compatibility Windows® 11, 10 (32/64), 8 / 8.1 (32/64), 7 (32/64), Vista (32/64), XP (32/64), 2000
Windows Server® 2003, 2000
Mac OS® 10.10, 10.4
Linux 2.6.x+
Microsoft WHQL Certified Yes
What's in the Box 1 - Included in Package 54mm RS232 ExpressCard
Breakout Cable
Driver CD
User Manual



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Frequently Asked Questions


When you troubleshoot issues with a serial device, there are some quick tests that you can complete to rule out potential problems. You can test to make sure that the following components are working correctly and are not the source of the issue:

  • Serial cables

  • Serial ports

  • Serial devices

To test your setup components, try the following:

  • Use the serial cable, serial port, and serial device in another setup to see if the problem is with the components or the setup.

  • Use a different serial cable, serial port, and serial device in your setup to see if the problem persists. Ideally, you should test a component that you know works in another setup.

When you test your cables, it is recommended that you do the following:

  • Test each cable individually.

  • Use short cables when you are testing.

When you test the serial ports and serial device, it is recommended that you do the following:

  • Press the Windows key + R, type devmgmt.msc, and press Enter to open Device Manager. Check to see if your device is listed under Ports (COM & LPT).

  • Make sure that the COM port number is the correct number for the serial device and that the software being used to connect the computer to the serial device uses the correct COM port number. 

  • If the device is listed with an error, follow the instructions on the website to reinstall the drivers.

Note: Some serial devices work only if the COM port number is between a certain range. For example, COM1 to COM4.

To check whether or not an RS-232 serial port is working, perform an RS-232 loopback test by doing the following:

  1. If your serial port is not female, convert it by taking a female/female cable or gender changer and plugging it into the serial port.
  2. Take a metal paperclip or wire and cross pins 2 and 3. If you look closely at the female end, the pins should be numbered.
  3. Open a telnet session on the COM port number of the device that you are testing. To find out the COM port number, refer to the following FAQ: https://www.startech.com/faq/com-port-listing-windows.

Note: To open a telnet session on the COM port, you need a telnet client like PuTTY or Hyper Terminal. Windows XP comes with Hyper Terminal.

  1. When the session is open, anything you type into it you should see. The loopback test fails when you cannot see what you are typing.

If the loopback test fails, make sure that the serial cable or gender changer that you are using works and that the adapter is in the correct port. 

You can check multiple ports at the same time by opening multiple sessions, putting the loopback adapter on one port, and trying to type into each session.  When you can see what you are typing, you know that the COM port is working and you can see which port number the physical serial port is. Close the window for the port that you just tested to speed up the testing of the remaining serial ports.

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Compliance and safety

Compliance letters

Safety Measures

  • If product has an exposed circuit board, do not touch the product under power.
  • If Class 1 Laser Product. Laser radiation is present when the system is open.
  • Wiring terminations should not be made with the product and/or electric lines under power.
  • Product installation and/or mounting should be completed by a certified professional as per the local safety and building code guidelines.
  • Cables (including power and charging cables) should be placed and routed to avoid creating electric, tripping or safety hazards.
*Product appearance and specifications are subject to change without notice.

Customer Reviews

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(5 Stars)

Great features.. works also as RS485

By: Suraj - 06/16/2015
Works as great as if it is an actual serial port on the Laptop
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