[time-nuts] RS 232
Bob Camp
lists at rtty.us
Fri Jul 26 20:05:13 UTC 2013
Hi
I can't say if they were more than 10 years old or not, but I have seen some ports that do not work reliably with a 0-5V signal in the last few years.
Bob
On Jul 26, 2013, at 3:58 PM, Didier Juges <shalimr9 at gmail.com> wrote:
> 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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> --
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