[time-nuts] RS 232

Didier Juges shalimr9 at gmail.com
Fri Jul 26 19:58:10 UTC 2013 

I believe the trend for the last 10 years has been to use a positive threshold. I mess with serial ports all the time professionally and not and I do not remember the last time I saw one that did not work with a positive threshold.

Didier KO4BB

Bob Camp <lists at rtty.us> wrote:
>Hi
>
>The gotcha with "standard" RS-232 is that the range from -3 to +3 volts
>is defined as "undefined". In other words, a negative should be below
>-3V and a positive should be above +3 volts. Indeed most (but not all)
>chips count zero as a negative. A few have hysteresis and others have
>some odd thresholds. 
>
>Bob
>
>On Jul 26, 2013, at 12:14 PM, Angus <not.again at btinternet.com> wrote:
>
>> Hi,
>> 
>> #5 is one of the reasons I preferred not to take the signal negative.
>I suppose it depends on the situation, but for the spitting data into a
>pc I used to find that a 0 to 5v or so signal powered from a control
>line almost always worked. The most common problem was either a faulty
>com port or an optocoupler circuit that did not have enough gain to
>pull the line close to 0v. ( that was a while ago though - not so much
>'legacy' as 'vintage'! )
>> 
>> Angus.
>> 
>> 
>> From: "Bob Camp" 
>> To: "Discussion of precise time and frequency measurement" 
>> Sent: July 26, 2013 1:37 PM
>> Subject: Re: [time-nuts] RS 232
>> 
>> Hi
>> 
>> In order to get voltages from the other pins on the D connector:
>> 
>> 1) You have to program them to be in the "correct" state (either high
>or low)
>> 2) They have to be present on the "driver" side (usually, but not
>always true)
>> 3) They need to be > 3 V (diode drops etc…)
>> 4) They need to supply enough current (as in no isolating resistors,
>usually true)
>> 5) You need to have two of them to spare that meet these criteria
>(one positive and one negative)
>> 
>> Its a little like rolling dice, sometimes it will work, sometimes it
>won't work. 
>> 
>> Bob
>> 
>> On Jul 26, 2013, at 7:01 AM, EWKehren at aol.com wrote:
>> 
>>> Since there is repeated request for the circuit se attachment. My
>question  
>>> is what can be done to get a positive voltage from some of the other
>unused 
>>> pins  of the DB 9 connector. What is needed is any where from 3 to
>15 volt. 
>>> Diodes  work with 1 mA but I set Rx for 2 mA. Any thing else look at
>H11 
>>> data  sheet.
>>> Bert Kehren
>>> 
>>> 
>>> In a message dated 7/26/2013 4:06:37 A.M. Eastern Daylight Time,  
>>> rexa at sonic.net writes:
>>> 
>>> FWIW, it  was clear as mud for me too.
>>> 
>>> Bert began with, "Since joining time   nuts over four years ago I
>have 
>>> not used a  single MAX
>>> 232 chip.  Two  reasons MAX do not give me isolation and do generate
>  noise
>>> in critical  applications."
>>> 
>>> From that I took that he was  doing RS232 using opto isolators. That
>
>>> implies + and - voltages to me.  Where do they come from and where
>are 
>>> they relative to the isolation  boundary? If the goal is really 
>>> isolation, how do these supplies get  isolated? If the noise is
>cured by 
>>> isolation, these details seem important  as the supplies need
>isolation 
>>> too. Maybe the switcher in the MAX232 is  causing the noise. Then
>how do 
>>> we get a negative supply from, say, +5V  without the noise? Then,
>maybe 
>>> he was saying RS232 sucks and this other  way (not RS232 compatible)
>
>>> works better.
>>> 
>>> The word picture of Bert's  solution, which provided more details,
>left 
>>> me less much less than clear  too. Maybe I'm just not up on circuit 
>>> shorthand terminology enough to  follow what sort of current limiter
>is 
>>> limiting what current to what, and  what is being blocked by a diode
>from 
>>> which negative level. Not really  sure if I even got the big picture
>of 
>>> what he is describing. Is it an  isolated equivalent of a MAX232 
>>> interface or something else that wouldn't  talk to an RS232 device?
>>> 
>>> So, more clarification, or possibly that  picture (~= 1k words)
>might 
>>> help. Or maybe I'm just obtuse and everyone  else is getting it.
>(Seems 
>>> Marki may also be in the confused  camp.)
>>> 
>>> 
>>> 
>>> On 7/25/2013 3:34 PM, Mark C. Stephens wrote:
>>>> Although your description,
>>>> " I prefer the use of two H11 opto couplers  which
>>>> work  perfect. On the receiving end the diode along with a  current
>
>>> limiter
>>>> and  blocking diode for the negative level works  perfect. On the
>output 
>>> side
>>>> a  power  source is needed." Is  a perfect circuit description, I'd
>be 
>>> more confident with a schematic  :)
>>>> 
>>>> 
>>>> --marki
>>>> 
>>>> 
>>>> -----Original  Message-----
>>>> From: time-nuts-bounces at febo.com 
>[mailto:time-nuts-bounces at febo.com] On 
>>> Behalf Of EWKehren at aol.com
>>>> Sent: Friday, 26 July 2013 5:32 AM
>>>> To: time-nuts at febo.com
>>>> Subject: Re: [time-nuts] RS 232
>>>> 
>>>> I do not understand your  question, I am referring to low noise 
>>> applications  like counters for  dual mixers or other AV
>measurements, but also Shera 
>>> and even  Tbolt  where external noise should be kept to a minimum.
>When you 
>>> chase 1 E-14,   isolation is key and I always like to err on the
>cautious 
>>> side and as I  stated  we use blue tooth or USB but in the case of
>USB there 
>>> are always  H11 in the  circuit. Some still like to use RS 232 and
>the 
>>> subject came  up and I have on my  boards H11's like on the counter
>Corby uses 
>>> but he  ended up using an external  power source and I like to
>eliminate that  
>>> requirement. David had the right  answer using the power that the RS
>232  
>>> mouse uses out of a DB 9, started looking  but I do not have one any
>more  
>>> and I can not find any data.
>>>> Bert
>>>> 
>>>> 
>>>> In a message dated 7/25/2013 2:48:43 P.M.  Eastern Daylight Time, 
>>> marks at non-stop.com.au writes:
>>>> 
>>>> Hi   Bert,
>>>> 
>>>> I am sure your circuit is clear in your head, but would  you mind  
>>> attaching detail?
>>>> You have perked my interest with the  "low-noise" keyword  ;)
>>>> 
>>>> 
>>>> -marki
>>>> 
>>>> -----Original Message-----
>>>> From:  time-nuts-bounces at febo.com 
>[mailto:time-nuts-bounces at febo.com] On
>>>> Behalf Of   EWKehren at aol.com
>>>> Sent: Thursday, 25 July 2013 11:39 PM
>>>> To:  time-nuts at febo.com
>>>> Subject: [time-nuts] RS  232
>>>> 
>>>> Since joining time  nuts over four years ago I have  not used a 
>single 
>>> MAX
>>>> 232 chip. Two  reasons MAX do not  give me isolation and do
>generate  
>>> noise
>>>> in critical   applications. I prefer the use of two H11 opto
>couplers 
>>> which
>>>> work  perfect. On the receiving end the diode along with a current 
>
>>> limiter
>>>> and  blocking diode for the negative level works perfect.  On the
>output 
>>> side
>>>> a  power  source is needed. If one uses  an USB adapter it does
>have the 
>>> + 5
>>>> volt which  again works  perfect.  How ever many prefer to use RS
>232
>>>> direct and that  is  why I hope to get some comments and
>suggestions from 
>>> the
>>>> list. Corby used on our  counter circuit that he described a 
>separate  
>>> power
>>>> source.
>>>> Present MAX circuits use a + 1.4  volt threshold but   considering
>legacy
>>>> the question is what  should the voltage swing be to make  it 
>compatible 
>>> for
>>>> most  PC's and what options exist to get the necessary  voltage. In
> the  
>>> past
>>>> other pins on the RS 232 port where an ideal power  source.  Still
>an  
>>> option?
>>>> Bert  Kehren
>>>> _______________________________________________
>>>> 
>>> 
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