[Elecraft] KPA500/K3 AUX bus specification

[Jim Brown]

On 4/29/2018 2:45 PM, Don Wilhelm wrote:
> The AUXBUS protocol is Elecraft proprietary and is not published.
> It is the single wire communications between the K3 and its options, 
> both internal and external. 

It DOES seem reasonable to publish at least an electrical transmission 
spec for it -- i.e., voltages, source and load impedances, whether it is 
intended to be terminated (impedance matched) or un-terminated, distance 
limits for various cable parameters. In fact, it seems unreasonable to 
NOT put it in the manual RS232, for example, is runs UN-terminated, and 
the limit on distance is the capacitance of the cable, not it's physical 
length. Many years ago, the RS232 spec was note that capacitance was the 
limitation.

The way to think about this is that UN-terminated circuits are analyzed 
as "lumped parameter" systems, where bandwidth limits boil down to 
degradation of rise time by RC time constants. The source Z is low, the 
receiving Z is high. TERMINATED systems function as transmission lines, 
and everything we know about traansmission lines applies.

I'm using only the AUX Bus for communication between K3 and KRC2, using 
a short length of mini Belden 75 ohm coax. If I were, for example, 
remoting a KPA500 or KPA1500, it  would be nice to run only coax (or a 
single pair out of CAT5) for that bus, which requires that you know the 
electrical limits of the circuit.

73, Jim K9YC