[time-nuts] RS 232
Bob Camp
lists at rtty.us
Fri Jul 26 16:42:34 UTC 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
>>> _______________________________________________
>>>
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to
>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>>
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at febo.com
>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> and follow the instructions there.
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
More information about the Time-nuts_lists.febo.com
mailing list