MorseKOB Interface Techniques

Contents

Interface options

Key only

Sounder driver

Loop interface

USB-to-serial adapters

Snubbers

Serial port pins

Interface options

While it’s possible to send and receive with MorseKOB using just the keyboard and speakers, sooner or later you’ll probably want to hook up a key and maybe a sounder. There are three options to choose from:

Key only

The simplest approach is to connect a key to the computer and use the program’s simulated sounder for receiving. This method involves just a simple modification to an off-the-shelf serial cable—no soldering or electronic circuitry required. It allows you to send Morse manually with a straight key or bug, and if you close your eyes you can imagine that you’re hearing an actual sounder.

Sounder driver

If you’d like to connect a sounder to your computer, the most straightforward approach is to use a simple sounder driver. This circuit requires only a few commonly available components, and assembly is not difficult for someone who knows how to use a soldering iron.

Loop interface

A loop interface circuit provides the most realistic and flexible solution. It allows the computer to be placed in series with a telegraph loop consisting of a power source and one or more keys, sounders, and relays. Although this design is more complex than the simple sounder driver, circuit boards are available to make assembly less of a challenge.

Each of these interface options connects to the computer via an RS232 serial port. Since few computers come with a serial port nowadays, most likely you’ll need a USB adapter cable to make the necessary connection. See the USB-to-serial adapters section for some recommendations.

Warning: Switching the current on and off in a circuit containing inductive components like the coils of a sounder or relay can generate high voltage transients which are likely to interfere with and possibly damage sensitive electronic devices. It’s important to guard against this by placing a “snubber” across all sounder or relay terminals. See the Snubbers section for more information. Static electricity is another source of high voltage that can damage electronic components, for example if you touch the metal part of your key while your body is charged with static electricity. Although unlikely, it has happened, so take appropriate precautions. Using a USB-to-serial adapter cable instead of connecting directly to the computer’s serial port can limit the extent of possible damage.

Key only

Serial cable and USB adapter for connecting a key

The easiest way to connect a key is to take a serial cable with a female DB9 connector on one end and simply cut off the connector at the other end. You can buy a Belkin F2N209-06 serial extension cable for $5 or less from many online suppliers.

You’ll need to find which wires correspond to pins 4 and 6 on the connector, and connect those two wires to your key. Unfortunately, there’s no standard color code for serial cables. For the Belkin F2N209-06 cable the correct wires are orange and green, but if you have a different cable then you’ll need to use an ohmmeter or continuity tester to determine the right wires.

Sounder driver

Note: RTS = pin 7, SG = pin 5
Maximum supply voltage: 60 volts

In order to drive an external sounder from the serial port, you’ll need an interface circuit like the one shown above. The circuit can be built from parts available at Radio Shack for about $6. The components can even be made to fit inside a DB9 serial port connector, although you may prefer to build it in a project box.

Serial port connector with sounder interface circuit inside

Loop interface

The loop interface circuit allows the computer to function as part of a normal telegraph loop. In its simplest configuration, that telegraph loop would consist of a key, sounder, and power supply, but it could contain other instruments as well. A circuit diagram and explanation of its operation can be found in the MorseKOB Loop Interface document.

Loop interface circuit mounted on a wood base

Circuit boards for the loop interface are available from the Morse Telegraph Club. You’ll have to buy the circuit components yourself, mount and solder the components on the board, and provide your own enclosure. Write to interface@morsetelegraphclub.org for information on availability and cost of the circuit boards.

USB-to-serial adapters

Most likely you’ll be using a USB adapter cable to provide the necessary serial port for your interface. It’s important to understand that MorseKOB uses the RS232 control lines of the serial interface, not the data lines. This means that an adapter that may perform very well in a more conventional application could be unsuitable for MorseKOB. To play it safe, use one of the following adapters that are known to work. And remember, you may have to install driver software that comes with your particular adapter.

Cables to Go “Port Authority” USB Serial DB9 Adapter Cable (product #26886)

I like this adapter because it’s compact and inexpensive (less than $20 at www.tigerdirect.com). However, some users have run into performance problems, and it’s not compatible with the Mac.

Radio Shack USB-to-Serial Port Cable (catalog #26-183)

This adapter is more expensive than the Cables to Go model and it’s a bit bulkier, but it does have the potential advantage of being readily available at local Radio Shack stores.

Tripp Lite “Keyspan” High-Speed USB to Serial Adapter (model USA-19HS)

This is the one to use if you have a Mac. It works with Windows and Linux as well.

Dynex USB/PDA Serial Adapter Cable (model DX-UBDB9)

I don’t have personal experience with this adapter, but it’s reported to perform well in situations where the Cables to Go model fails.

Snubbers

When a key in a telegraph loop is opened, the collapsing magnetic field in the inductors in the circuit (sounder and relay coils) caused by the sudden cessation of current flow creates a large transient voltage across the key contacts. A snubber, also known as a spark suppressor or spark killer, is a device used to damp this transient spike. In traditional telegraph circuits, the snubber was used to prevent pitting of the key contacts. With MorseKOB, however, the more important reason for using a snubber is to prevent interference or possible damage to sensitive components in the computer or USB-to-serial adapter.

A snubber in place across the sounder terminals

The best place to put the snubber is directly across the terminals of every sounder or relay in the circuit, as this minimizes the amount of radiated electromagnetic interference. If that’s not practical, then snubbers should be placed across all keys in the circuit. (Note: a snubber is not required for the “key only” interface. It’s only an issue when a sounder or relay is in the circuit.)

The following snubber designs have all been used successfully:

RC network

The traditional Western Union snubber consisted of a 22 Ω resistor in series with a 0.1 μFd capacitor. It’s important that the capacitor be nonpolarized (not electrolytic), and its voltage rating should be at least 600 volts.  You can make your own, or buy an off-the-shelf snubber such as the Cornell Dubilier model 104M06QC22 arc suppressor.

Diode

A simple diode, such as a 1N4005, makes a very effective snubber. The polarity of the diode needs to be in the reverse direction of the normal current flow. This approach has the possible disadvantage of making the sounder slow to release, resulting in an undesirable sluggish sound.

Back-to-back Zeners

A pair of Zener diodes, connected in series with each other and with opposite polarities, works well as a snubber. Unlike the simple diode approach, you don’t have to worry about the direction of the current in the circuit, and it’s less likely to result in sluggish response. Being more compact than an RC network, it’s easier to conceal this type of snubber under the base of a sounder. The voltage rating of the Zeners needs to be greater than the normal voltage drop across the sounder.

Transorb

A bidirectional transient-voltage-suppression (TVS) diode, also known as a transorb, is a device similar in function to the back-to-back Zeners just described, but specifically engineered for high power operation and integrated into a single package. As in the case of Zeners, the voltage rating of the transorb needs to be greater than the normal voltage drop across the sounder.

Neon bulb

I haven’t used this approach myself, but a standard NE-2 bulb is reported to be an effective snubber. It has the unique appeal of flashing in sync with the circuit transitions, so you always know if it’s working. The lifetime of the bulb is a possible issue in this application.

Serial port pins

The following table identifies the RS232 lines used by the program to interface with an external key or sounder.

*The color code shown is for the Belkin model F2N209-06 serial extension cable. Some other cables follow the same color code, some do not.

Revised 2012-11-19
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