[time-nuts] RS 232
Bob Camp
lists at rtty.us
Fri Jul 26 12:37:18 UTC 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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