On Thu, 14 Aug 2008, naturalwatt wrote:
> Hi. I'm using a TS7250.
>
> I've built a display using large 7-segment display, hanging off a 64-bit
> shift register. My
> program sends data to the display using one DIO for data and another for
> clock.
>
> This works fine until the cable to the display is more than about a meter in
> length, then it
> gets clocking errors. It doesn't matter how much delay I put in between
> setting the data
> bit and then the clock bit, there are glitches.
>
> What's the best way of extending the distance to about 10 or 20 metres? I can
> think of
> loads of ways of doing it, but I'm interested to see if anybody can suggest a
> cheap and
> simple way.
>
> The display uses 5V TTL - 74LS series.
>
> Options I am considering:
>
> * Some sort of driver at the 7250 end - but this will require power.
>
> * Capacitors across the DIO lines?
>
> * Using 2 DIO pins opposite in phase to make a differential driver - but will
> this still be
> prone to glitches?
>
> * Twisted pair - but I'll still get crosstalk between clock and data?
>
> * Serial port - with a UART at the far end - most complex solution.
>
> Thoughts, anyone?
Sending digital signals over long distances is always a significant
problem, simply due to the many gigahertz of bandwidth it requires.
Personally, I'd get a pair of RS422/RS485 line drivers and a pair of
receivers, then send data and clock over two twisted pairs. You may find
it's actually easier to send the data via rs422 to a microcontroller, or
even move your ts7250 closer to the display and extend the ethernet or
whatever else you have connected to it.
Also remember that the TS7250's DIO lines are all 3.3v, so connecting them
to 5v logic should have buffers anyway.
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