On Thu, 26 Dec 2002, Sellam Ismail wrote:
> On Thu, 26 Dec 2002, Tony Duell wrote:
>
> > The first confusion is to whether it's a 7404 output (as specified here)
> > or a 7416 output (as shown in the diagram). The main difference to you is
> > that the '16 is an open-collector device. If it's that, you need to wire
> > a pull-up resistor (1k or so) between the output signal line and a +5V
> > supply (take that from the Apple). If it's a '04, then you don't need
> > these resistors.
>
> That is indeed one main element of my confusion. Why do simple errors
> like this make it past the technical writers?
>
> How do I know for sure which to use? Is there some way I can check with a
> simple volt meter or something?
You should be able to tell... check if it gives a close to +5V output for
a high signal (relative to the 'return line') with, say, a 470ohm
resistor to ground. you may also want to check that it gives a close to
0V output for a low signal If so, you don't need the pullup resistors.
> > The M200 has a fair amount of electronics in it, and I can assure you
> > that the outputs come from logic ICs, not from mechanical contacts. You
> > don't need optoisolators.
>
> This is the opposite of what John tells me. You're both highly competent.
> To whom shall I defer?
You can always trace back the data lines inside the reader, to see if they
run to mechanical contacts or some sort of semiconductors. Just be
careful that you don't generate a ground loop if the ground on the reader
and the Apple are at different voltage levels and somehow associated -
like something on the lo-v side of the power supply connected to earth
ground.
Pat
--
Purdue Universtiy ITAP/RCS
Information Technology at Purdue
Research Computing and Storage
http://www-rcd.cc.purdue.edu
Received on Thu Dec 26 2002 - 16:16:00 GMT