[time-nuts] RS 232

Bob Camp lists at rtty.us
Fri Jul 26 08:37:18 EDT 2013


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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