[Elecraft] [K3] splitting RS232 output for 2 serial ports

[Don Wilhelm]

John (and information for all)

You need to think about the fact that you are connecting two drivers 
together.
Then think about the fact that RS-232 drivers can provide output that 
ranges from +25 volts to -25 volts - OK most PC based ports do not have 
that much voltage, +12 and -5 volts is more common in the PC world.

Those drivers are voltage sources (generators).
If both drivers are in an idle (space) state, there may not be a 
problem, they are both trying to produce a positive voltage. However 
when one of them wants to send data, its output tries to go negative.  
So we have one driver trying to source a positive voltage and the other 
driver trying to source a negative voltage.  The best case result is 
that the voltage will "settle in" at some level between the two voltages 
- but that will be close to zero volts (absolute) and below the 
threshold where the receiver recognizes a valid signal voltage, but the 
more probable will be that the drivers will draw current from one 
another damaging either one of the semiconductor junctions or its 
positive or negative pullup resistor - whichever is weakest.

In other words - never connect 2 RS-232 drivers together.  Multiple 
receivers are OK, but multiple drivers are a "No-No"

73,
Don W3FPR


On 8/7/2012 5:54 PM, John Shadle wrote:
> Do you think there would be a problem in letting both signals pass
> through to both COM ports? Wouldn't COM 1 only "look" for the
> appropriate rig control signals and COM 2 only "look" for the
> appropriate CW keying signals? Or not...?
>
> Thanks, Iain!
> -john W4PAH
>
> On Tue, Aug 7, 2012 at 5:51 PM, iain macdonnell - N6ML <ar at dseven.org> wrote:
>> On Tue, Aug 7, 2012 at 2:38 PM, John Shadle <shadle at katzenfisch.com> wrote:
>>> Hi all,
>>> I'm trying to accomplish keying with my K3 using the RS232 port and a
>>> COM port on my desktop computer.
>>>
>>> I currently use the RS232 for station logging to provide frequency,
>>> mode, etc. information and for rig control (e.g., click on a spot and
>>> then the radio changes frequency/mode). Unfortunately, the program I
>>> use (AC Log by N3FJP) does not allow you to choose the same COM port
>>> for both rig control and CW keying.
>>>
>>> My question is whether or not I could use a splitter of some type (at
>>> the K3) to go to 2 separate ports (e.g., COM 1 and COM 2), then use
>>> COM 1 for rig control and COM 2 for keying. Has anyone else done this?
>>> Is this acceptable practice?
>> Sure! It'd probably be safest to make the splitter yourself - the
>> "rig-control" side needs TXD, RXD and ground, and the keying side
>> needs RTS, DTR and ground.
>>
>> 73,
>>
>>      ~iain / N6ML
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