[Elecraft] K3 - remote control head

[Jim Brown]

:
> The K3/0 should allow you to control a K3 over a RS232 connection,
> which could be a fairly long cable (up to 50 feet, perhaps?),

Another very useful fact -- RS232 uses an unbalanced connection, and the 
connection is NOT impedance-matched.  That is, the source has a low 
source impedance and the receiver has a high input impedance, so the 
interconnecting cable appears as a capacitive load to the driving 
source. A decade or so ago, we often used RS232 for the connection 
between a computer in the audience in a large performance space to 
signal processing equipment buried in the bowels of the building. We 
were able to reliably use runs on the order of 200 ft at decent speeds 
by using CAT5 cable for the connection, using a pair for each signalling 
circuit, by dedicating all un-used conductors to the return, and 
connecting the returns to the SHELL of the DB9 connectors at both ends. 
It works because CAT5 cable has good bandwidth AND low capacitance 
between the conductors of a pair, the twisted pair rejects noise, 
connecting to the shells at both ends gets around Pin One Problems that 
may be present, and doubling up on the return conductors minimizes the 
IR drop from AC power leakage currents that may be present because the 
equipment is powered (and perhaps grounded) at widely separated points.

Another method to consider for longer runs is RS422, the BALANCED 
configuration of the serial interface.  A very good company in Illinois 
called B&B makes very nice, fairly inexpensive RS232 to RS422 
converters. 
http://www.bb-elec.com/Learning-Center/All-White-Papers/White-Papers-Category1/232_422-Converter-Connections-for-Extending-RS-232.aspx

73, Jim K9YC