Author: Igor NN1I - nn1i@arrl.net.MODIFICATION.NET
Computer RS-232 port requires voltages between +5 and +15 volts
for logical 0, and voltages between -5 and -15 volts for logical 1.
However, FT-840 CAT output voltages are TTL standard between 0 and
+5 volts only. This radio requires FIF-232C interface that cost
about $100. In addition, FIF-232C requires AC power.
You can make your own interace under $25, plus you can add
computer controlled PTT option for PSK31, RTTY, SSTV and other
digital modes sharing the same RS-232 port.
The heart of this interface is MAX232A. You can use any similar
chip MAX203, MAX232, MAX233, but check documentation for proper
connections. Power supply +5 volts is coming from voltage regulator
78L05 that takes +13.5 volts from the back of FT-840 radio (RCA
connector). PTT controlled via optocoupler 4N33 by the RTS signal
from RS-232. I used DB9 serial port. If you use DB25, then you have
to change pin numbers according DB25 assignment. CAT plug is 6 pin
Mini DIN.
PTT circuit is optional. You don't need it, if you just want to
read radio frequencies and modes into you logging software. However,
it is a good idea to share one serial port for CAT and PTT, if you
use computer soundcard for digital modes. Make sure that you
configured your software to share PTT and radio port, and set PTT
control by RTS. Of cause, you will also need Rx audio cable to your
soundcard, and Tx microphone cable with attenuator or isolation
transformer. More detailed information can be found on the web (www.qsl.net/soundcardpacket/cables.htm)
This interface was tested with Logger 6.11 (freeware by K4CY, www.qsl.net/kc4elo/logger.htm).
It did not work with Zakanaka 1.24 stand-alone, because this
software had limited number of radio types in configuration, but it
could read frequency from the Logger (www.qsl.net/kc4elo/). If your
software does not have FT-840 listed in configutration of radio
type, then try to use FT-900 settings. It may also work with FT-890
settings, but slow polling rate. Besides radio type, you must set
baud rate 4800, 8 data bits, 2 stop bits, no flow control, no
parity, echo off.
My design is FREE. If you are not completely satisfied, I will
give you a full refund. No extra cost for having fun! If you have
some questions, e-mail me at nn1i@arrl.net
73 Igor NN1I
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