[time-nuts] RS 232

Bob Camp lists at rtty.us
Fri Jul 26 12:42:34 EDT 2013


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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