Computer Automation Technology

USB to RS-232 Converter Cables

Computers today do not have a RS-232 port. Since CAT products communicate using RS-232 this presents a problem. The solution is to purchase a USB to RS-232 Converter Cable and install its accompanying software on your computer. When the cable is plugged in, a “VIRTUAL COM PORT” will be created. All of the CAT editor programs can only be assigned to COM1, through COM4. Sometimes the software will create a COM5 or higher. In this case you must reassign the COM port’s number to work with the CAT editors.


To reassign the COM port follow the procedure described below:

1. Click START.
3. Click SYSTEM.
4. Click the HARDWARE tab.
6. From the list of devices click the [+] box next to Ports (COM & LPT) to display: “Your Converter Cable Name” USB-to-Serial Comm Port (COM?).
If (COM?) is COM5 or above it must be changed to a COM port between COM1 and COM4 to operate with the CAT Editor programs.

7. RIGHT CLICK on “Your Converter Cable Name” USB-to-Serial Comm Port (COM?).
8. From the drop down box click PROPERTIES.
9. CLICK on the PORT SETTINGS tab.
11. CLICK on the COM PORT NUMBER cell.
12. From the drop down box select an unused COM port below COM5 CLICK OK and return to the DESKTOP.
13 Repeat steps 1 through 6 to verify “Your Converter Cable Name” USB-to-Serial Comm Port (COM?) has been changed to the new COM port setting.


CAT products only require three connections to the RS-232 port. They are pins 2, 3 and 5 of the standard 9 pin “D” connector. When connected to a CAT product, pin 7 is connected to pin 8 and pins 1, 4 and 6 are connected to each other. This is a standard connection when these pins are not being used. This does not cause a problem with a standard RS-232 port. However two of the cables tested show excessive jitter and the CAT products were unable to decode the signal properly.

It was determined that when pin 6 was disconnected from pins 1 and 4 the signal from the computer was stable, jitter free and easily decoded by the various CAT products.

In summary make sure the virtual COM port that is created can be found by the CAT software and if you encounter jitter of the RS-232 signal from the USB to Serial Converter cable remove pin 6 from the 9 pin “D” connector.

Computer Automation Technology recommends the EasySink Model ES-U-1001-C100 Premier USB-RS232 Adapter cable. This cable can be purchased from the EasySink USA web store at

Computer Automation Technology tested the Digitus Model PL-2303 Adapter cable. This cable can be purchased from the Digi-Key web store at The Digi-Key part number is AE1488-ND. This adapter exhibited the jitter problem. It was necessary to remove pin 6 from the 9 pin “D” connector.