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From: rickman on 26 Jul 2010 22:32
It would help if I used the right abbreviation, DDS for Digital Data
System and older telecom data interface similar to T1, but slower,
with no framing I believe.

Rick

On Jul 26, 10:19 am, rickman <gnu...(a)gmail.com> wrote:
> We happen to be doing an upgrade to an existing card I am selling a
> customer and they are asking about the feasibility of providing a DSS
> interface (56 kbps AMI encoding).  Their thought was to use the audio
> I/O which has very strong differential drivers with an output
> impedance of 50 ohms.  We can boost the sample rate up to 212 ksps,
> but with a fixed sample rate.  To extract the clock would require a
> bit of signal processing, but I feel it is "doable".
>
> However, we also have an EIA-422 interface on board and I realized
> that it might be possible to use this instead.  The AMI encoding
> requires detection of three levels, +V, 0V and -V.  The 422 receivers
> have bias resistors added for other purposes which can be used to set
> a threshold on the input of say, 0.4 volts.  One input can be wired
> direct and the other inverted detecting both polarity of pulses as
> distinct from the zero state.
>
> The output can be done by using one pin from each of two differential
> drivers so that both are low, one is high or the other is high.
>
> My concern with this is the lack of adjustment for signal levels.
> Whatever threshold is picked for the input would limit the sensitivity
> for low signals and also limit the noise discrimination for larger
> signals.  Likewise, the output would have a single, fixed level.  It
> looks like these signals are measured as dBdsx (relative to 6 Vpp),
> but I haven't found any info on what levels are used.  I found test
> equipment input specs of +6 to -35 dBdsx.  I believe my input circuit
> would do ok at a range of +6 to -15 dBdsx or possibly down to -20
> dBdsx.  -35 dBdsx is only 0.1 Vpp which is too low to reliably detect
> using this circuit.
>
> Anyone here worked with DSS interfaces?  Any info on what levels are
> used on the line?
>
> Rick

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