USB CAT Cable Chipsets: FTDI, CP210x, CH340 and COM Port Fixes

A USB CAT cable is the small link between your computer and your radio that makes logging programs, digital mode software and remote control work. When it works, you forget about it. When it does not, the symptoms are baffling: the radio is not found, the COM port number changes after a reboot, the frequency display freezes, or the radio keys itself the moment you open a program. Most of these problems come from the chip inside the cable, its driver and the way the operating system assigns port numbers, and not from the radio. This guide explains the common chipsets, which drivers they need, how to recognise a counterfeit or a faulty cable and how to solve the problems that you are likely to meet.

What a CAT Cable Really Is

CAT stands for computer aided transceiver, and the interface is a serial link. Many radios still speak a serial protocol at TTL or RS-232 levels, and the USB cable contains a small chip that creates a virtual serial port on the computer. The FTDI article describes this class of chips: they convert USB signals to and from protocols such as RS-232 or TTL serial, to provide support for legacy devices on modern computers that lack a serial port. The same job is done by chips from other makers. Inside the cable there may also be a level converter, for example from the 5 V logic of the chip to the 3.3 V or other levels of the radio, and, in some cables, an opto-isolator that separates the radio from the computer.

Some radios have a USB connector and need no external cable. They contain a chip of the same kind inside the radio, and you see a virtual COM port in the same way. The problems and the solutions below apply to both, and our guide to rig control with Hamlib covers the software side once the port is working.

The difference between the signal levels matters when you build or buy a cable. A TTL signal swings between zero and a few volts, while RS-232 uses higher positive and negative voltages with inverted logic, and connecting one to the other without a converter gives no communication or can damage a port. For this reason a cable is made for a particular radio, and the same plug on two models of one brand does not guarantee the same wiring. Icom radios use a single-wire bus called CI-V, and cables for them include a small circuit that combines the transmit and receive lines. Other brands use separate lines. Before you buy a cable, find out what the radio’s connector provides.

The Common Chipsets Compared

The table is general, and radio makers may use their own choices. In practice, the best chipset is the one that the manual of your radio recommends, which is usually the one with a reliable driver and a record in the community. A well-made FTDI or CP210x cable costs a little more than an unbranded one, but it saves time.

The FTDI Counterfeit Story and Why It Matters

The Wikipedia article on FTDI describes a controversy that every amateur with an old cable should know. On 29 September 2014 FTDI released an updated Windows driver that, according to reports, could soft-brick counterfeit and software-compatible clones of its chips by changing their USB product ID to 0000. After the criticism, the company withdrew the driver on 24 October 2014. In February 2016, a further driver was reported that replaced the data from non-genuine devices with a looped text string. The practical message is that a cheap cable sold as “FTDI” may contain a clone, that the behaviour of a clone depends on the driver version, and that a cable that worked yesterday can behave differently after a Windows update. If a cable shows a strange name or sends repeated text, the chip is the likely cause.

• Buy from a vendor who names the chip and has a record in the amateur community.
• Do not update drivers in a hurry if your system works. Keep a copy of the working driver installer and note the version.
• Test a new cable before an important event, such as a contest or a Field Day, and keep a spare cable of a different type.

Installing the Driver Step by Step

1. Connect the cable to the computer first, with the radio switched off, and watch the operating system. Windows may install a driver by itself, and Linux and macOS usually recognise the common chips immediately.
2. Open the device manager in Windows and look under “Ports (COM and LPT)”. A working cable appears with a name and a number, such as “USB Serial Port (COM5)”. If the device is under “Other devices” with a yellow mark, the driver is missing.
3. Install the driver for the chip from the maker’s website or from the radio manufacturer’s page, and follow the instructions. Restart the computer if it asks.
4. Note the COM port number and set it in your software. If the number is higher than 9 or in a range that your older program cannot use, change it in the device properties, under advanced port settings.
5. Match the settings in the radio and the software: baud rate, data bits, stop bits and handshake must be the same. Radios have a menu item for the CAT or CI-V speed, and a mismatch is the most frequent reason for a port that opens but does not talk.

Solving the Common Problems

The COM port number keeps changing

Windows assigns a COM number to a device based on its identity and, for chips without a serial number, on the USB socket where it is plugged in. A cable with a CH340 chip and no serial number may get a new port each time you change the socket. Use the same socket every time, or set a fixed number in the device manager. A chip with a unique serial number, such as many FTDI types, keeps its number wherever you plug it in. On Linux, the device name such as /dev/ttyUSB0 can also change, and a stable name is available under /dev/serial/by-id for devices that report a serial number.

The port is “in use” or access is denied

Only one program can open a serial port at a time. Close the other logging or digital program, and check for a background program that reads the radio, such as a rig control server, a spotting program or a second copy of your logger. On Linux, your user must be in the group that owns serial devices, often called dialout, to open the port.

The radio keys itself or goes into transmit when I start the program

Some interfaces use the control lines of the serial port, DTR and RTS, for push-to-talk or CW keying. When a program opens the port, these lines change state, and a cable that is wired to them may key the radio. Set the lines to be off in the software, or disable them in the interface settings, and check the cable’s wiring. Our guide to the CW keying interface and DTR/RTS explains how this works and how to prevent an accidental carrier.

The frequency freezes or the program loses the radio at random

This is often caused by RF getting into the cable. The USB cable and the computer are antennas, and a strong signal on the HF bands can upset the serial chip. Add ferrite cores at both ends of the cable and keep it away from the feedline, as explained in our guide to common mode chokes. Use a short, good quality cable with a shield. A USB hub that is not powered correctly can also drop the connection, so try the cable directly in a socket of the computer.

The cable is recognised but nothing works

Check the baud rate, the radio’s CAT setting and the type of cable. A cable made for one brand of radio may use a different plug and a different level, and the same chip can be wired in different ways. Test with a simple terminal program, or the radio’s own utility, before you try your logger.

Isolation and Ground Loops

A serial cable connects the computer and the radio, and a connection of the grounds between them can bring noise into the receiver or hum into the audio. Some cables include an opto-isolator, and they are worth the price if you have a noisy shack. A separate audio interface for digital modes has its own isolation needs, discussed in our article on the digital interface, sound card isolation and ground loops. If the radio’s USB port is built in, the same ideas apply, and a USB isolator placed between the computer and the radio can help when a noise problem remains after the chokes.

Keeping the Setup Reliable

• Label the cables with the chipset and the date, and keep the driver installers in a folder with the notes of the settings.
• Use a good cable length. USB works up to 5 metres without a hub, but a long cable near an antenna picks up more RF. A shorter cable with ferrites is better.
• Disable USB power saving in the power settings of Windows for the port, if the radio drops out after a period of inactivity. Laptops sometimes switch off the USB port to save power.
• Test before an event. Run the program for an hour, key the radio at full power into a dummy load and check that the control stays.
• Keep a spare. A second cable with a different chip is the cheapest insurance against a failure on the day.

The Bottom Line

A USB CAT cable is a USB-to-serial converter with a chip such as an FTDI, CP210x, CH340 or PL2303. Choose a cable from a reliable source, install the correct driver, note the COM port, match the baud rate and keep the same USB socket if the chip has no serial number. Take care with counterfeit FTDI clones and with driver updates that change the behaviour of a cable. Add ferrites to stop RF from upsetting the link, set the DTR and RTS lines properly to avoid accidental keying and test the whole setup before you depend on it. With those habits, rig control stays a background service, which is what it should be.

Frequently Asked Questions

What chip is best in a USB CAT cable?

There is no single best chip. FTDI and Silicon Labs CP210x are widely used in quality cables and radios, and CH340 is common in low-cost ones. Follow the radio manufacturer’s recommendation and buy from a reliable seller.

Why does my COM port number change?

Some chips, such as many CH340 types, have no unique serial number, so Windows assigns a number according to the USB socket. Use the same socket or set a fixed number in the device manager.

Why did my FTDI cable stop working after a driver update?

It may contain a counterfeit chip. FTDI drivers released in 2014 and 2016 were reported to affect non-genuine devices. Keep a working driver version and buy cables with genuine chips.

Why does my radio transmit when I open the logging program?

The program may be setting the DTR or RTS line, which the cable uses for push-to-talk or keying. Turn these lines off in the software or change the interface setting.

Do I need ferrite beads on a USB CAT cable?

They often help. RF on the cable can cause dropouts and frozen frequency displays, and ferrite cores at both ends reduce the common-mode current.

Can a USB CAT cable cause noise in my receiver?

Yes. A computer and its cables can radiate noise, and a ground connection through the cable can bring hum. Use ferrites, a short shielded cable and, if needed, an isolated cable or a USB isolator.