[na-user] update on Icom 706MkIIG interface
Tue, 08 Aug 2000 20:32:33 -0400
At 11:28 AM 8/8/00 -0400, Dan Kovatch wrote:
>Many thanks to Dave K8CC and Chuck N6OJ for their right on response about
>the radio address change needed. This did not effect a total cure. I
>operated for a few hours in NAQP and quit because condx were bad hr in OH.
>I spent time trouble shooting the homebrew interface and found several
>problems on the board but it STILL doesn't work. The MFJ5383I cable that I
>ordered didn't wrk either. Yesterday, I drove into Cleveland for a meeting
>and stopped at AES and bought the CT17
I've had some experience with the MFJ5383 family of radio
interfaces. (They make one for ICOM, KENWOOD, and YAESU.) These
interfaces get their power from the serial port itself. The power supply
in the interface connects to one of the COM port handshaking lines, which
are not used with most programs and so are asserted high. This voltage is
then regulated at 5 VDC to run the chip in the interface.
The problem with this scheme is that the handshaking lines are designed to
be signal lines, not power supplies. The chips typically used in these
interfaces don't draw a lot of current, however problems occur when the
line cannot supply enough current for the load, so the voltage level sags,
and the interface does not work properly.
There are ways to make the circuit more robust. Using Schottky diodes
rather than regular silicon diodes can provide another 0.5 VDC of headroom,
and low drop-out 5 VDC regulators which require less input voltage helps
even more. Unfortunately, the MFJ design is a low $$ approach which
incorporates none of this.
My experience has been that the MFJ5383 interfaces work OK on my desktop
computers, but don't even come close on my Dell P-200 laptop. For laptop
use I've had to build my own interfaces running off 13.8 VDC available on
the back of the radio.
Dave Pruett, K8CC
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