A Replacement Soundcard Interface

2 minute read

February 2008

It had to happen, sooner or later, and although G4BMK’s DOS software, BMK-Multy, is superb, it was written in the days before Windows 95, when a powerful PC was a 386SX running at 25MHz. BARTG produced a equally superb design for an interface allowing the transceiver to interface with a PC serial port. This included all the necessary tone generation for RTTY, AMTOR and PacTor as well as the necessary audio filters required for decoding these modes plus CW. Nowadays with increasingly more powerful processors available, all this can be done in software, and all that is needed is a simple ‘transceiver to soundcard’ interface.
BARTG MultyTerm
The new Soundcard Interface
As can be seen from the photographs above, the new Soundcard Interface based on a design by VU2FD is significantly less complex than the MultyTerm. A pair of 600 ohm 1:1 isolating transformers provide the interface for the audio I/O and a pair of Opto-Isolators serve to provide PTT control and CW keying whilst maintaining isolation between the PC and transceiver grounds. The new interface would be even simpler if it weren’t for the fact that my board caters for two separate transceivers, my IC706MK2G and IC725. The device at the bottom edge of the board is a reed relay which is activated by the adjacent opto-isolator and is necessary for provinding the PTT ‘pull down’ for Icoms.

Looking not unlike a well known proprietary interface, the new box sits on top of the stack of little boxes where the MultyTerm used to sit. I have included a ‘quarter Inch’ jack socket for plugging in a straight or Bug Key. The switch on the left serves three functions. Apart from switching the 13.8V for the reed relay, it also isolates the PTT and Key outputs. This is provided as a precaution since when the PC boots, both the RTS (PTT) and DTR (Key) lines on the serial port toggle and would likely cause the selected transceiver to transmit. Ironically, after building this unit, I discovered that the PC to which it was to be connected, did not have a free serial port. The single port being already taken up by my CI-V Interface. However there were several USB ports available and a USB to Serial adapter which was known to work with BMK-Multy and the MultyTerm resolved the issue.