For a long time, I operated digital modes on my FT-450D using a basic USB-to-RS232 adapter for CAT control and a DIGIVOX-2 interface linked to a soundcard for audio. It worked, but the cable clutter was a mess. I eventually decided to build a dedicated custom interface: a single cable connecting the radio’s rear DATA port directly to USB.
Starting Point
The FT-450D provides CAT control via RS-232/TTL along with dedicated audio input and output lines on the rear DATA port. This connector uses a standard 6-pin mini-DIN layout (identical to old PS/2 keyboard and mouse ports).
Looking at the pinout, two pins stand out: DATA IN (audio input to the radio) and DATA OUT (audio output to the computer soundcard).
Initial Tests and Design Considerations
You cannot connect these lines directly to a soundcard. The signal levels mismatch, and even low radio volume will clip the soundcard input. A voltage divider paired with a DC-blocking capacitor is required.
After experimenting with component values on a breadboard, I finalized the following circuit:
| Path | Components |
|---|---|
| Radio -> PC | C3 = 0.22 uF, R3 = 10 kOhm, R4 = 2.2 kOhm |
| PC -> Radio | C4 = 22 uF (electrolytic), R5 = 10 kOhm, R6 = 2.2 kOhm |
| GND Filter | C5 = 220 nF |
The receive path (radio -> PC) acts as an AC filter and voltage divider. The transmit path (PC -> radio) uses a similar attenuation stage, while C4 blocks any DC offset from the soundcard output. For safety, radio ground and PC ground are joined through a 220 nF capacitor (C5).
Once testing confirmed everything worked properly, I moved the components from the breadboard to a perfboard. I soldered the interface outputs directly to the pads of a cheap USB soundcard dongle to avoid extra connectors, then housed the entire assembly in a small plastic enclosure with a mini-DIN pigtail on one end and a standard USB connector on the other.
For CAT control, I used a standard USB-to-RS232 adapter, while the audio interface relies on an inexpensive USB soundcard module.
Setup and Operating Results
On the receive path, the audio signal from the radio remained clean and free from overdrive. During transmit, setting the OS mixer output level to around 35-40% produced an ideal drive level – setting it to 100% caused input overload on the radio side. The setup works flawlessly for FT8 and WinLink contacts.
Conclusions
The voltage dividers (R3/R4 and R5/R6) are essential to prevent overdriving either the radio input or the PC soundcard. Ground separation using C5 (220 nF) noticeably improves the signal-to-noise ratio.
While I did not implement full optoisolators for my home setup, adding optocouplers is worth considering if your PC and radio run on separate power phases or operating in high-RF environments. I might add isolation transformers or optocouplers in a future revision.