CW Keying Interface Guide: WinKeyer, DTR and RTS Compared
Sending Morse by hand is a pleasure, but in a contest or during a long pileup the computer does a better job. A logging program can send your callsign, the exchange and a series of repeated calls with perfect spacing while you concentrate on listening. To do that it needs a way to key the transmitter, and that connection is the CW keying interface. The choice looks trivial at first and turns out to matter: a poorly chosen interface gives irregular timing, keys the radio at start-up, injects noise into the receiver or simply refuses to work when the computer is busy. This guide compares the common approaches, explains what each is good and bad at, shows how to wire a simple isolated interface and lists the faults that appear most often, so that your computer sends clean code every time.
What the Interface Has to Do
A CW transmitter is keyed by closing a contact, normally to ground. The key jack on the radio provides a line with a small voltage and current, and shorting it to ground makes the radio transmit. Everything else, the speed, the dot-to-dash ratio, the sidetone and the spacing, is generated before that contact closes. The interface has to translate a signal from the computer into that contact closure, and it has to do so with consistent timing. Because Morse timing is measured in tens of milliseconds, small delays in the computer produce obviously uneven code, which is audible to the other operator and annoying to listen to.
Two separate jobs are often combined. One is generating the timing of the characters, the other is closing the contact. Some interfaces do only the second job and leave the timing to the computer; others generate the timing in a small processor of their own and only take commands from the computer. That difference is the key to understanding the options below.
Option 1: Serial Port Lines (DTR and RTS)
The simplest approach uses a control line of a serial port. A classic serial interface, described in the standard summary of RS-232, provides handshake lines such as Data Terminal Ready (DTR) and Request To Send (RTS). Logging programs can toggle one of them in time with the Morse code, and a small circuit converts the line into a contact closure to ground. Modern computers use USB-to-serial adapters, which provide the same lines through a driver.
The advantages are real. It costs almost nothing, it is supported by many logging programs, and it needs no firmware. The disadvantage is timing. The program has to switch the line at exactly the right moments, and the computer is doing many other things. Under a heavy load, such as when the logger is checking a large callsign database or the computer is decoding a digital mode, the keying can become uneven. A USB adapter also adds its own latency. For casual use at moderate speeds the result is usually fine, but at high speed in a busy contest it is the weakest option.
The Start-Up Problem
A frequent annoyance with serial keying is that the control lines can change state when the port is opened. On some systems the line goes active for a moment when the computer starts or a program opens the port, which keys the transmitter, sometimes into an antenna that is not ready. Use a setup that keeps the radio from transmitting before the program has taken control: set the correct line as the key line, disable the line you do not use and test with the transmitter connected to a dummy load first.
Option 2: A Hardware Keyer Such as WinKeyer
A keyer in the radiotelegraph sense is an electronic device that turns key input into properly timed Morse. The WinKeyer family of hardware keyers goes further: it contains a small processor that accepts characters and commands from the computer over a serial or USB connection and generates the Morse timing itself. The computer only sends text and settings; the keyer produces perfect code regardless of what the computer is doing. A paddle can also be connected to the same unit, so one device serves both manual and computer-sent code.
That makes a hardware keyer the most robust choice for contesting. Timing is independent of the computer load and of the USB driver, speed can be changed from a knob or from the logging program, and the unit usually provides outputs for both the key line and push-to-talk. The costs are the price of the device and the need for the program to support it; most major logging programs do. If you use a paddle, the article on iambic keying modes A and B explains the settings that matter, and our guide to CW keys helps you choose the paddle itself.
Option 3: Keying Through the Radio’s Own Commands
Many modern transceivers can send Morse themselves. The computer sends text over the same CAT control connection that it uses to read the frequency, and the radio generates the code with its internal keyer. This requires no extra interface hardware, so it is attractive if you are already connected to the radio over USB. The quality of the timing depends on the radio, and the features, such as the ability to change speed during a message or to interrupt sending quickly, vary between models and between programs. If you already follow the approach described in the article on rig control with Hamlib, check whether your radio and software support CW sending over CAT before buying anything else.
Option 4: Audio Tone Through an SSB Transmitter
A fourth approach produces a Morse tone with the computer’s sound card and feeds it to the transmitter in SSB mode. It needs no keying line at all. It is not a real CW signal: the shape of the envelope is produced by software and audio processing, and the transmitted signal can be wider and less clean than proper CW if the audio levels and filters are not carefully controlled. It also gives up break-in features and normally will not satisfy a contest sponsor’s expectations of a CW emission. Treat it as a temporary workaround, not as a standing solution.
Comparison of the Common Interfaces
| Interface | Timing quality | Cost and effort | Typical weakness |
|---|---|---|---|
| Serial DTR or RTS line with a small circuit | Good at moderate speed, can suffer under computer load | Very low, a few parts and a USB serial adapter | Start-up keying, latency, ground loops |
| Hardware keyer such as WinKeyer | Excellent, independent of the computer | Moderate, needs a purchased unit or kit | Cost, extra box and cable |
| CW over the radio’s CAT interface | Good, set by the radio’s keyer | None beyond the CAT connection | Feature support varies by radio and program |
| Audio tone through SSB | Depends on audio processing, not true CW | None | Wider signal, no proper break-in |
Wiring a Simple Isolated Serial Interface
If you choose serial keying, build the interface with isolation. A common design uses an optocoupler between the serial line and the key jack. The computer side drives the light-emitting diode through a resistor, and the output transistor of the optocoupler is connected across the key line of the radio, so there is no direct electrical connection between the computer and the radio. This avoids the ground loops that cause hum and noise in the receiver and protects the serial port from the key line voltage of the radio, which varies between models. Always check the manual of your radio for the keying voltage and current before choosing the output device.
- Choose which control line keys CW and which, if any, controls push-to-talk. Do not use the same line for both.
- Connect the chosen line through a series resistor to the optocoupler’s input diode, sized for the current of the diode and the voltage swing of the line.
- Connect the optocoupler output between the key line and ground of the radio’s key jack, observing polarity.
- Put a ferrite choke on the keying cable and on the USB cable close to the interface to keep RF out of the computer.
- Test first with a dummy load, a low power setting and a monitor receiver, to confirm that the code is clean and that nothing keys at start-up.
The same grounding and RFI lessons apply as for audio interfaces; our article on sound card isolation and ground loops explains why isolation matters for any computer connection to the radio.
Configuring the Logging Program
Most contest loggers have a dialog for selecting the CW output. Common choices include a serial port with a chosen control line, a hardware keyer and the radio’s own keyer. The documentation of N1MM Logger+ at the official documentation site is a good example of how the settings are explained. Whatever program you use, the process is similar: select the port, choose the keying line, set the speed range and the weighting, define function-key messages and test each one while listening on a second receiver.
Set lead-in and tail times with care when your radio uses semi break-in. A slight delay before the first element avoids clipped characters, and a short hang time after the last element prevents the radio from switching between receive and transmit within a word. If you are new to contest logging, the guide to setting up N1MM Logger for your first contest shows the other settings that go along with them.
Speed, Weighting and Good Habits
A reliable interface also makes it easy to send code that is pleasant to copy. Set the speed a little below the speed at which you can comfortably copy replies, because callers tend to answer at your speed and you do not want to be overwhelmed. Keep the weighting at the default unless you have a reason to change it; heavy or light weighting changes how the signal sounds on a crowded band. Record short messages for the exchange and keep the callsign message separate, so you can repeat only the part the other station missed. Finally, always listen to your own transmitted code on a monitor receiver or a sidetone from the radio. If the code sounds ragged to you, it sounds ragged to everyone else.
Troubleshooting Common Faults
- Uneven spacing or dropped characters. The computer is too busy or the USB adapter is slow. Close other programs or switch to a hardware keyer.
- The radio transmits when the program starts. A control line changes state when the port opens. Disable the unused line, use a different adapter or add a pull-up or pull-down as appropriate.
- Hum or noise on receive when keying. A ground loop between computer and radio. Add isolation and chokes.
- Clipped first dot. Increase the lead-in time or the radio’s break-in delay.
- Code is sent but sidetone is wrong. The sidetone comes from the radio or the keyer, not from the logging program. Adjust it at the source.
- Radio stays keyed. The key line is held low by a stuck line or a wrong polarity. Disconnect the interface and test the radio alone.
The Bottom Line
For occasional computer-sent CW, a simple isolated DTR or RTS interface or the radio’s own CAT keyer is enough, provided you test it for start-up keying and ground loops. For serious contesting at speed, a hardware keyer such as WinKeyer is the most dependable choice, because the timing no longer depends on the computer. Whichever you choose, isolate the connection, choke the cables, test with a dummy load and listen to your own signal on a second receiver. Clean, even code is the reward, and your callers will copy you the first time.
Frequently Asked Questions
What is the simplest way to key CW from a computer?
Use a control line of a serial port, normally DTR or RTS, through a small isolating circuit connected to the key jack. It costs very little but can show timing problems under heavy computer load.
What is a WinKeyer?
It is a hardware keyer that accepts text and commands from the computer and generates the Morse timing itself, so the code is correct regardless of what the computer is doing. A paddle can usually be connected to the same unit.
Why does my radio transmit when I start the logging program?
A serial control line can change state when the port is opened, which can key the radio. Use the correct line, disable the unused one and test with a dummy load.
Do I need isolation between computer and radio?
It is strongly recommended. An optocoupler or similar isolation prevents ground loops, protects the serial port and reduces noise on receive.
Can my radio send CW without an extra interface?
Many radios can, using text commands over the CAT connection. Support and features depend on the radio and the logging program, so check both.
Is sending an audio tone through SSB a good alternative?
Not as a standing solution. The result is not a true CW signal, it can be wider than proper keying and it removes break-in operation.