[Elecraft] K3S RS232 "Y" Connector causing port conflict

[Don Wilhelm]

Charles,

I don't know the specifics of your OM2500A amplifier, but I do know 
about RS-232.
First, RS-232 is a point to point system.  The use of "Y" connectors is 
not the norm and are applicable only under certain specific conditions.
Those conditions are that the device added by the "Y" connection has no 
active drivers on the RS-232 signals - only line receivers are permitted.

Think of it this way - a driver on an RS-232 line can have a negative 
voltage of up to -25 volts, and a positive voltage of up to +25 volts.
Imagine that one driver is trying to produce -25 volts while the other 
driver is trying to produce +25 volts.  In other words, they 'fight' 
with one another, and the result is 'confusion' because the receiver 
listening on that RS-232 signal line will not respond to a voltage that 
is less than 3 volts positive.

In other words, use a "Y" connector only when the added device is 
capable of being put into a "listen only" mode (with its drivers disabled).
Look at your amplifier information carefully to see if it can be placed 
in a "listen only" mode.  If not, you cannot use it with a "Y" connector.

The SteppIR is one example of a device that can be placed in listen only 
mode, so the use of a "Y" connector is possible, but that is not the 
general case.

Bottom line, you can connect only *one* RS-232 device to another in the 
general case.  Introducing a 3rd RS-232 device (with a "Y" connector) 
requires special conditions be present in that 3rd device - it must only 
listen to the data traffic and not attempt to transmit data.

73,
Don W3FPR

On 12/13/2015 10:52 AM, charles at k5ua.com wrote:
>   
>
> I have an OM2500A amp that works perfectly with my K3S when it is the
> only thing connected to the RS232 of my K3S.
>
> I am using LP-Bridge and Na3P to create a panadapter and this works
> perfectly when the computer is the only thing connected to the RS232 of
> my K3S.
>
> The manual shows how to use a RS232 "Y" splitter so that two devices can
> share the RS232 port of the K3S, but when I use the "Y" splitter (one
> side to the OM2500A amp and one side to the computer) the following
> malfunctions occur:
>
> 1. The OM2500A display rapidly changes from the correct VFO frequency to
> an out-of-band frequency over 2 Mhz away.
> 2. The pandapter frequency readout ceases to function.
>
> I assume there is a port conflict, and I wonder if anyone knows the
> correct pin-out of the RS232 line to the OM amp so that it will be a
> passive listener on the DB9 cable?
>
>