1 - there is no problem in tying the 4 inputs togather -
    its done all the time

2 - the 74lsxxx should handle the speed OK, but why?
    Use 74HCxxx parts - just as fast and easier to drive

3 - a typical 4Nxx opto chip would be pushed at 250KBps, although
    H/P has some FAST 4N parts which would do OK. I don't have
    a data book handy either.

4 - isolated PS are a PITA - depends on how much current you need
    to supply for each output driver. The easiest way is probably
    a charge pump type circuit, with one heavy-duty power driver
    (pair of FETs?) switching from your power rail and driving
    capacitor/diode sets for each output channel. Lots of parts,
    and probably lots of PCB space. but not as bad as 120V PSs for
    each channel.

Just some initial thoughts,
Roy J. Gromlich
Renaissance Technologies

> Hi All. I'm building a little signal splitter/repeater/isolator for a
> 250KBps signal (no, not DMX sadly, I'd feel more comfortable with that).
>
> For the original, I won't have any isolation between the input and
> output. My idea is to have one receiver chip feeding all four inputs of
> a quad buffer/driver chip, and each output then feeds its own
> transmitter chip. Question is, can I tie all four inputs together like
> that? I don't think there's a problem. Second, would a 74lsXX (don't
> know the number, I'm not near a catalog at the moment) handle the
> 250Kbps stream ok?
>
> For the slightly more advanced version, I want to have isolation between
> the input and each output to avoid ground loops in larger systems. My
> thought is to add an optoisolator to each output of the buffer chip in
> the above circuit. First off, would a garden variety 4N25 (from memory,
> might be a wrong part number) or similar work at those speeds? Second,
> to truly avoid loops, each transmitter chip would have to be on its own
> power supply to avoid connecting the grounds of all the chips together,
> correct? Is there a semi-easy way to do this? At the moment the only
> thing I can think of are independant 120V-5V switching power supplies
> for each chip, or some sort of DC-DC convertor for each chip. This will
> get expensive and large very quickly. Is there any other way?
>
> Thanks,
>
> Josh
> --
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