--- In "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.
The standard Centronics style printer interface uses parallel IO with
clock signals for shifting the data into registers, all on TTL logic
levels. Typically, there is a low-pass filter on each output, composed
of a low value resistor (about 100 Ohms) and a small capacitor (10 -
100 pf) on the load side of the resistor. This filters the high
harmonics that are part of the signal edges, and prevents them from
cross-coupling to unwanted other signals. I have seen working cables
in the tens of metres in length, although this exceeds any
conservative published specs. Simply changing your cable type to use
twisted pairs, with one conductor of each pair connected to ground on
one end may help greatly.
Your signal timing will probably not play much of a role on affecting
cross-talk on the cable, as it is the edges on you clock signals that
are propagating to other channels.
--- rod.
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