Back in '95 I asked how to build an interface for my Icom 737 on
one (don't remember which one) of the reflectors. Most replies
recommended the Feb. '93 QST circuit. Amoung the replies was
the note I am attaching below. I used the pull-up resistor then and
it's still in the circuit I use with my Icom 765. The only problem I
have ever had with TRLog is with the split function and most of that
was resolved by increasing the icom command pause (went to 500ms on the
765 but only had to go to 400ms on the 737). I say most because when
I set split and am in the cw narrow mode, the narrow mode is not set
on the split xmit freq.
Anyway, here is the note on the mod to the QST circuit....Perhaps an EE
could comment on its validity.
Wayne AJ5M ehayes@vnet.ibm.com
=======================================================================
Date: Wed, 19 Apr 1995 14:25:53 +0000
To: ehayes@vnet.ibm.com
Subject: Re: RS232 INTERFACE MODIFICATION
In the QST Feb 93 circuit you will not get +5 volts on the ICOM radio bus
per the spec.
You can check it out by measuring the resting voltage on the ICOM bus. I
measured
+1.8 volts instead of +5 volts.
See the circuit diagram on page 39. I connected a 470 ohm resistor
from pin 4 of U1B tp pin 14 of U1A (Vcc + 5 volts). The circuit shows a
pull-up resistor
on U1A pin 11 to pin 14. It left out the equivalent resistor for U1B. If
this resistor were
within the ICOM, it would not be required. The circuit did work without the
resistor, but the voltage is out of spec. I exchanged e-mail with the author
and he
assumed the ICOM rig provided the pull-up. However even if ICOM provided
the pull-up,
the circuit would not pass the test called for in Table 2 on page 41 when -9
volts is
applied to PC-2 and the bus should = + 5 volts.
I would be interested in what you find. I doubt the IC-737 provides the pull-up.
I am using a 765.
73,
Paul
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