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OT- RS232 CARD input clamped to 2.5V

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alsopb

OT- RS232 CARD input clamped to 2.5V

Guys,

Looking for some insight.

I was doing some checkout of a device. The device output a +10/-10V
RS232 signal swing from a MAX232 device.

When it was attached to a PCI serial port card, the output dropped to
+2.5/-10 V. In other words, the PCI card was clamping the input
positive value. It does work OK even clamped like this.

I'm guessing the internal chips are low voltage devices.

I though one of the redeeming values (for hams at least) of the +15/-15
volt RS232 spec was noise immunity. Clamping it to 2.5 Volts means
considerably less noise (RFI) immunity?

True we do get along with lots of 5V TTL stuff.

73 de Brian/K3KO

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Pete Smith N4ZR

Re: OT- RS232 CARD input clamped to 2.5V

Sure. I've been dealing for years with 3.3 volt parallel port cards. I
think that the noise immunity is much more a function of good design
than the voltage level.

73, Pete N4ZR
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On 3/15/2013 3:40 PM, Brian Alsop wrote:

> Guys,
>
> Looking for some insight.
>
> I was doing some checkout of a device. The device output a +10/-10V
> RS232 signal swing from a MAX232 device.
>
> When it was attached to a PCI serial port card, the output dropped to
> +2.5/-10 V. In other words, the PCI card was clamping the input
> positive value. It does work OK even clamped like this.
>
> I'm guessing the internal chips are low voltage devices.
>
> I though one of the redeeming values (for hams at least) of the
> +15/-15 volt RS232 spec was noise immunity. Clamping it to 2.5 Volts
> means considerably less noise (RFI) immunity?
>
> True we do get along with lots of 5V TTL stuff.
>
> 73 de Brian/K3KO
>
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 2012.0.2240 / Virus Database: 2641/5677 - Release Date: 03/15/13
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

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Don Wilhelm-4

Re: OT- RS232 CARD input clamped to 2.5V

In reply to this post by alsopb

Brian,

Many PC related "RS-232" ports and devices do not conform to the
switching levels of RS-232 standard.
Yes, that means they will work, but only over short distance cables
(which is the norm in the PC world).
If the recieving end is clamping the voltage, it should not reduce the
noise immunity unless it is taking significant current from the driver.
There are factors involved other than the voltage - like the slope of
the positive going pulses.

73,
Don W3FPR

On 3/15/2013 3:40 PM, Brian Alsop wrote: