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A simple USB digi-modes radio interface

Work in progress I've been wanting to try out digital modes on the HF and VHF bands for some time, receiving is easy - you just need a connection between the radio and the computer's soundcard line-in port. Transmission is a little more complex, depending on both the radio used and the software on the computer. You often have the option of either feeding data from the soundcard to the radio's mic socket and using an RS232 serial port to control the PTT line, or doing all the data transmission via the serial port and only using the soundcard for reception. There are various DIY and commercial designs for both types of interface, this page documents my version built for as little money as possible using parts from eBay and the poundshop.

I want an entirely self contained interface, one USB connection to the computer and the necessary connections to the radio. To achieve this I need at least three things, a USB hub, a USB soundcard and a USB to RS232 converter. A quick trawl though eBay turned up the soundcard and the RS232 adaptor.

Cheap USB soundcard
USB to RS232 interface

The USB-serial converter was the first item to arrive, it uses a Prolific chipset instead of the more common FTDI parts. "Carefully" removing the soft plastic enclosure revels a small PCB about 10x15mm. The connections are shown in the next two images. I will de-solder the D-connector and the USB cable to leave a small board that can be integrated into the design with minimal wasted space. When I have all of the parts, I'll probably design a PCB to mount them all on.

The underside of the USB-RS232 interface showing power and data lines and 12MHz resonator
The topside of the USB-RS232 interface with the epoxy encapsulated Prolific interface chip