FT-990 and non-Yaesu radio interfaces

[s57ad@s55tcp.ampr.org]

To whom it may concern.

One of my friends bought FT-990 few months ago and decided to solder up
radio interface by shematic diag in ARRL Handbook 1995 (MAX-232 and 7417),
rather than pay $ 200 for Yaesu FIF-232 interface. ($ 200 is a bit too much 
for simple circuit with four transistors and one double optocoupler, indeed).

After first testings, TR LOG controled radio functions, but didn't followed
band or mode changes made on radio, nor read frequency. 

He started TR RADIODEBUG, but get only couple of "timeout" messages in
RADIO.DBG file. The same thing happened with CT instead of TR.

Radio interface converts TTL (logical 0 = 0 volts and logical 1 is 5 volts)
to RS-232 levels (+ 15 volts for logical 0 and - 15 volts for logical 1):

Radio TXD     Logical         CAT     ->        RS-232       (pin 3 DB-25)  

                0             (0 V)              + 15 V      
                1             (5 V)              - 15 V 


Note: + or - 15 V is actually + or - 9 volts on MAX-232, and must be min.
+ or - 4 V!

When he measured voltages on Radio RXD and Radio TXD lines, he found there
is 1,4 V on Radio TXD line ( = - 9 V on CPU RxD line) = logical 0!
He got + 9 V only, when he connected Radio TxD to ground!

After studying shematic diagram of FT-990 (and Yaesu uses same circuit in
their another radios), he found that CAT output uses "open emitor" circuit.
Input impedance of 7417 buffer, used in radio interface, is high, and emitor
"hangs in air"... So he put 1 kOhm resistor from 7417 pin to ground, and
CAT works now fine indeed!

It is interesting, how simple solutions solve great (at first sight) problems.

How about Icom (and Ten-Tec) radios (using single line for data transfer)? 
I believe (hadn't time to test it yet) that 4k7 resistor between I/O bus and 
ground couldn't hurt! (I have almost the same problems with my IC-751: he
doesn't follow band or mode changes, made on radio; doesn't read frequency
or read it incorrectly <I got ocassionally something like 900003.5 instead
of real frequency>, etc.)...

73 & enjoy TR!

Mirko, S57AD
s57ad@s55tcp.ampr.orga