While most SO2R boxes connect with a computer via traditional parallel (LPT) ports, such ports these days are obsolete - replaced by USB ports. I decided to design a USB SO2R box that would be simple. My eventual goal is to have a box that's small enough to take along when I travel.
Simple is nice, but an SO2R box should have a full set of features. This design:
- switches headphones, microphone, CW keyer and PTT (from the computer or a footswitch)
- uses relays for headphone and microphone switching
- communicates through, and is powered by, a USB port
- includes override switches for transmit and receive and LEDs to display status
- incorporates a BLEND control that blends left and right channels in stereo mode
- provides four-bit outputs, with band information for each radio. The outputs are compatible with Array Solutions, Top-Ten, Unified Microsystems or similar band decoders.
- includes a "latch" mode. If the computer transmits (i.e., sets PTT), the headphones will be connected in mono to the non-transmitting radio, reverting to normal operation when the radio stops transmitting.
- can be set so it will not go into stereo - useful for people who have a hearing issue in one ear.
- is compatible with N1MM Logger, WriteLog, Win-Test and similar logging programs.
The open two-radio switching protocol (OTRSP) is a simple serial protocol that's supported by the most popular contest loggers. The protocol specification (see www.k1xm.org/OTRSP/ is available under a Creative Commons license, which allows anyone to use it.
The Arduino is a single-board computer designed for teaching purposes. It includes the microprocessor, crystal and USB interface. It appears to the PC as a serial (COM) port. It can be programmed through the USB interface, and the software to do this is free. There are many clones of the Arduino in different shapes and sizes. An Arduino or clone is almost perfect for this project.
OTRSP uses the DSR modem line to control PTT. The Arduino has DSR available on a pin of the USB interface chip, but it is not connected to the microprocessor. This can be fixed by soldering a 1000 ohm resistor between the appropriate pins of two ICs.
The circuitry to make an Arduino into an SO2R box is straightforward. Figure 1 (see PDF) is a block diagram. Two relays are used for the headphones. One relay switches between the two radios; the other switches to stereo. One relay switches the microphone between the two radios, and, to avoid hum, it switches both the hot and ground sides of the microphone.
Two transistors are used for PTT - one for each radio, and two more are used for CW keying. This could have been done using another relay, but transistors are cheaper [and don't make noise - Ed.].
The AUX ports are driven by a shift register IC. The shift register shares two outputs with the headphone relays. The shift register loads within microseconds, so the relays are not affected by the sharing. The front panel has two switches, one for transmit and one for receive. They select Radio 1, Auto, or Radio 2. It also has four LEDs - two for transmit and two for receive. Figure 2 shows the schematic.
For the complete version of this article as published in the NCJ, view the pdf version.