[Elecraft] KAT500 serial cable

[Jim Brown]

On 1/19/2014 3:46 PM, Don Wilhelm wrote:
> I do not have the schematic for the KAT500. but the circuit for the 
> "RS-232" interface used in the XG3 has become sort of the standard at 
> Elecraft as far as I know.  I initially said it went to +5 volts, but 
> after checking I see it can go to +9 volts. If your Ardrino TTL 
> compatible sink cannot accept +9 volts without damage, you should use 
> the RS-232 connection.  It will work fine with relatively short cables 
> (but not the very long cables supported by real RS-232 interfaces).

Not quite. RS232 is a NON-impedance matched system, and it is 
unbalanced. That is, the source output Z is low and the receiving input 
Z is high, so almost no current flows,  resistance of the "high" 
conductor does not matter. The distance limit on RS232 is the 
capacitance of the interconnecting cable. What DOES matter is the 
resistance of the "return" conductor, as we will see from the next 
paragraphs

The voltage limit is dependent to a great extent on noise in the 
circuit, which, because RS232 is unbalanced,  is essentially equal to 
the noise voltage between the two interconnected pieces of gear. That 
voltage is established by equipment powered from different AC outlets, 
and/or grounded at different points, and is largely harmonics of the AC 
power system. It is not uncommon for the AC-related noise voltage to be 
in the range of several hundred milivolts.

Thus, if we are concerned with carrying RS232 signals a long distance, 
we need cable with very low capacitance between conductors, and we need 
to minimize the voltage difference between the chassis of the equipment 
we are connecting. All cables in the CAT5/6/7 family make excellent 
serial cables, both because their capacitance is quite low, and because 
they are very good twisted pairs. When used in this manner, each 
signaling circuit should be carried on a pair. For example, Orange to 
Pin 2, Blue to Pin 3, Or/Wh and Bl/Wh to ground. The voltage drop on the 
return circuit can also be reduced by wiring all un-used conductors to 
ground at both ends.

Beginning in the 90's, we have been using lots of very complex DSP 
equipment in large pro sound systems, and at least in those days, we 
configured and controlled them on RS232 circuits that were often in the 
range of 200 ft.  We did this using nothing more sophisticated than low 
capacitance cable between ordinary RS232 interfaces and carefully 
avoiding large voltage differences between the interconnected equipment.

73, Jim K9YC