[na-user] update on Icom 706MkIIG interface

k8cc k8cc <k8cc@mediaone.net>
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
DATOM Engineering

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