Re: [NTLK] FDTI usb

From: Johannes Wolf (
Date: Mon Mar 21 2005 - 02:00:02 PST

Indeed it looks like that,
but Tx+ and Tx- are SYMMETRIC differential output signals of the RS 422
Tx+ ranges from 0 to + 12V and Tx- from 0 to - 12V.
When you apply an unipolar logic high signal (i.e. from the Cirrus chip),
you get +12V at the Tx+ and -12V at the Tx- output at the same time, but the
converting the symmetric signals back to unipolar logic using a differential
input, you recover the original signal in the first place.
But dependent on the levels, a differential input connected to 0V and Tx-
might interprete the signal correctly and indeed then you have the inverted
signal of the original input.


> -----Original Message-----
> From:
> []On Behalf Of Adriano
> Sent: Monday, March 21, 2005 10:36 AM
> To:
> Subject: Re: [NTLK] FDTI usb

> What does it mean: " Also, some of the signals are inverted."

> 'TX+' have to me inverted to 'TX-' ?

> Il giorno 07/dic/04, alle 14:30, Humphreys, David ha scritto:

> In order to make the board as user-install-friendly as possible it was
> necessary to use the internal connector.
> Unfortunately, this connector doesn't carry the buffered output from
> the AppleTalk chip but rather the raw inputs from
> the Cirrus asic. These are logic level signals that I would not want to
> appear on an external connector for someone to
> short out.

> Also, some of the signals are inverted.

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