[nos-bbs] More on these RTS based PTT circuits everywhere

M Langelaar maiko at pcsinternet.ca
Tue Feb 2 08:29:12 EST 2021

Hi Bob,

I won't plague the list with this anymore, but here are links from 
others with similar issues :


This one below is quite interesting, and exactly illustrates and 
discusses the issue I am facing


Interesting to say the least ...

the diode is just transient protection, I checked it to make sure it's 
'okay', same diff.


On 02/02/2021 12:52 a.m., Boudewijn (Bob) Tenty wrote:
> I have an Dell and I believe it is a Optiplex. It was previously in use as my Gateway
> but I had no problem with it driving a hw tnc. I will measure what the RTS rest state is. Of course
> a software crash with RTS/RS-232 stuck in a certain state wouldn't be stopped by that FET.  I may add
> that I have a diode in parallel with the input of my Optocoupler to clamp negative RTS pulses to ground
> or at least 0.6V, just in case.
> Bob VE3TOK
> On 2021-02-01 05:10, M Langelaar wrote:
>> I did some searching Bob and it would appear that some PC manufacturers
>> and some USB to RS232 controllers actually invert the logic of their serial
>> ports for the default rest state. I am using a Dell Optiplex with Windows 7,
>> and quite honestly it's usually a Dell even on my linux systems. That's how
>> I burned out my Tekk Data Radio a few years ago. JNOS crashed and I had
>> not had the watchdog component of my circuit built at the time (I did after
>> the incident).
>> I even read a post where a guy has requested a 'invert RTS' option. I often
>> wonder why nobody else has said anything about these posts. I'm not making
>> this stuff up, it's very consistent, and that's why I'm going through all this trouble.
>> I mean who needs to come home and find their radio smoked cause a program
>> crashed. In my case anyways, and it would appear also for the guy who came
>> up with the circuit in the link I provided where he put in the BS170 inverter.
>> Maiko
>> On 01/02/2021 3:41 a.m., M Langelaar wrote:
>>> Well there in lies the problem. my rest RTS is always high, interesting.
>>> What computer model do you have, and version of O/S.
>>> On 31/01/2021 10:07 p.m., Boudewijn (Bob) Tenty wrote:
>>>> I have the RTS line connected via an resistor to the anode of the diode inside the Optocoupler, with the cathode in there
>>>> connected to ground. In rest RTS is negative.  When I activate PTT in FLdigi, WSJTX, etc. RTS goes positive and Optocoupler
>>>> pulls PTT to ground and operates TX as you said. If I quite my programs like the above, RTS just remains negative, no tx is triggered here
>>>> after that.  No problem here.
>>>>> converts negative logic to positive ...
>>>> That is the only thing I shouldn't  have said as RTS is using NRZ.
>>>> Bob VE3TOK
>>>> On 2021-01-31 21:25, M Langelaar wrote:
>>>>> Hey Bob,
>>>>>> In those other 100s of circuits the Optocoupler already
>>>>>> converts negative logic to positive ...
>>>>> How ? why then when I quite most of my programs like winmor,
>>>>> or WinRPR, optocoupler is energized and PTT is taken to ground
>>>>> and TX is triggered, unless I'm missing something really obvious :(
>>>>> Maiko

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