[Elecraft] sharing K3 RS232 to CAT and Remoterig

[Joe Subich, W4TV]

On 11/14/2017 9:34 PM, Don Wilhelm wrote:
 >
 > Let me give you a concrete example of why 2 RS-232 ports cannot share
 > the same port.

Not necessarily.  One can successfully share a device like the K3 CAT
port between two computers (or a computer and SteppIR controller,
computer and Remote Rig box, etc.) by using isolation/steering diodes.
The circuit is shown in the B&B Electronics 9PMDS/232MDS:
   http://www.bb-elec.com/Products/Datasheets/9PMDS-232MDS_2917ds.pdf

The "modem sharing" device is not fool proof - it does not resolve the
issues of both devices polling at the same time nor does it resolve
the problem with "unexpected data" received by the inactive device but
it does allow two devices to "share" a third under controlled
conditions.

Note: both "computers" will need serial ports that source at least
+10V in the active state.  Many newer USB to serial converters are
3.3V devices and the diode drop will reduce the active level at the
shared device into the "undefined" range which can result in
unreliable operation.

73,

    ... Joe, W4TV


On 11/14/2017 9:34 PM, Don Wilhelm wrote:
> Hi all,
> 
> Let me give you a concrete example of why 2 RS-232 ports cannot share 
> the same port.
> 
> Look at the RS-232 Specifications.
> 
> The normal voltage for an RS-232 signal line is held at the "Mark level" 
> of anywhere between -3 and -15 volts  in its idle state (as measured at 
> the receiver).
> A "Space" voltage is anywhere between +3 volts and +15 volts (again as 
> measured at the receiver).
> 
> The driver is a voltage source.  So imagine 2 drivers trying to put a 
> signal on a single receiver (or multiple receivers).
> The idle driver will be putting a negative voltage on the line, while 
> the active driver trying to send data will be putting on a positive 
> voltage.  The resulting voltage will be unpredictable and depends on the 
> impedance of each driver and the resistance of the signal line.
> 
> What will happen if you connect two voltage sources (say 2 power 
> supplies) together, one having a negative voltage and the other having a 
> positive voltage.  They will cancel one another (besides drawing a lot 
> of current from both supplies).
> The net effect is that 2 RS-232 drivers on the same line will "fight" 
> and proper communications will not be possible.
> 
> RS-232 is a point to point operation.  A single driver sending data to a 
> single device.  Other receivers may 'listen in' to the communications, 
> but if 2 drivers are present on the same signal line, the result is chaos.
> 
> Virtual ports can take care of that situation, but one cannot simply 
> parallel live hardware RS-232 signals (with a "Y" cable or other means) 
> and expect success, so hardware solutions are non-existent.  This is a 
> characteristic of RS-232 communications, and not a problem if you follow 
> the rules.
> 
> 73,
> Don W3FPR
> 
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