Re: Electronics question

[Kevin Hawkins <yahoogroupskh@xxxxxxxxxxxxxx>]

 I guess that depends on which version of the RS232 spec document
you're
reading - and I don't have a copy of the original 1962 EIA-232 one to
double check so I could be misinformed but  I have always believed +/-
5V was the minimum requirement for an output and the inputs had a
threshold spec of +/- 3V as per the Dallas Semiconductor document, or
the Maxim one, here... This allowed for an a rather generous 2V noise
margin.

http://www.lammertbies.nl/download/dallas-appl-83.pdf

http://www.maxim-ic.com/appnotes.cfm/an_pk/83/

.   RS232 was ratified before TTL and since then there have been loads
of unofficial interpretations/relaxations of the specification to allow
laptops and other low power devices to be more tolerant of the lower
voltages used in  TTL or even CMOS at 5V or 3.3V even - so indeed that
3.5V will likely work with most modern serial ports / devices and some
older kit too -  but it is still outside of the official RS232
specification I believe...

Regardless it's a rather mute issue as it's certainly not going to
work with a 5V regulator ;-)

K



Neil Wrightson wrote:
> Hi,
>
> Actually, 3.5 V is enough for RS232C communication. The voltage levels
must
> be between 3V and 15V.
> That is a logic 0 is between -3V..-15V and a logic 1 is between
+3V..+15V.
> A voltage between -3V..+3V is deemed to be in the nomans land and an
illegal
> value.
>
> RS232 voltages were not intended to be used as power supplies.
> It just happens that older equipment could typically supply ~20mA at
these
> voltages so people/companies took advantage of this.
> This was more the case when the actual driver IC i.e. 1488 had it's
own
> external +/-12V supplies fed to it and it just switched these
supplies.
> IC's such as the MAX232 run directly from the +5V rail and generate
their
> own +/- supply usually in the +8/-8 voltage region.
> Which is still a legal voltage.
>
>
> Regards,
>
> Neil Wrightson.
>
>
>   _____
>
> From: ukha_d@xxxxxxx [mailto:ukha_d@xxxxxxx] On Behalf Of
> Kevin Hawkins
> Sent: Friday, 26 June 2009 1:09 AM
> To: ukha_d@xxxxxxx
> Subject: Re: [ukha_d] Electronics question
>
>
>
>
>
> Could it be anything as simple as some configuration parameters for
the
> specific Avocent port in use that causes the port to assert DTR ? 3.5V
> is not a large enough voltage here to work with most RS232 equipment
> -and obviously isn't enough to power a 5V regulator. The port was
> 'open' when you made this measurement I assume.
>
> Ant's suggestions is neat way around this - albeit requiring a
secondary
> PSU.
>
> K
>
> ant wrote:
>
>> Hi Paul,
>>
>> Yeah you're pretty much there:
>>
>> 1) disconnect the DTR line from your circuit (belt and braces)
>> 2) connect the +ve wire from a 9v DC PSU to pin 1 of the 78L05
>> 3) connect the ground wire from the 9v DC PSU to one of the
>> regulator's ground pins (I can't read the schematic, there might
only
>> be one ground pin, depends on the package)
>> 4) leave the RXD and GROUND connections to the serial port
>>
>> cheers
>>
>> ant
>>
>>
>> ------------------------------------
>>
>> Yahoo! Groups Links
>>
>>
>>
>>
>>
>>
>
>
>
>
>
>
>
> [Non-text portions of this message have been removed]
>
>
>
> ------------------------------------
>
> Yahoo! Groups Links
>
>
>
>
>



------------------------------------