inline comments below.
Dick
----- Original Message -----
From: "Tony Duell" <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Tuesday, July 24, 2001 4:00 PM
Subject: Re: Apple ][ boards -- what have I found?
> >
> > Having looked a MOT databook for communication devices, (1993) but I do see
they
> > have an output enable. The 3487's have got enables on the transmitters, but
in
> > pairs, i.e. you enable two outputs at a time. I'm not sure that these are
> > suitable for multidrop applications, though there are similar devices that
are.
>
> I've found the TI datasheet for these chips. It claims they're direct
> replacements for the Motorola parts, so I assume it's close enough
>
>
> > I'm not convinced that there's a "disconnected" state that would work to
allow
> > more than one transmitter on a pair. The line is intended to be passively
>
> When the OE line is low, then both outputs of each transmitter in the
> pair are high impedance (disconnected, etc). So you can have multiple
> transmitters driving the same pair of lines, if you ensure only one is on
> at a time.
>
I made that assumption once, and was immediately informed that one should use
the RS485-compatible drivers, back in the "old" days. The guy who jumped me was
the app's engineer from MOT, and he was pretty young, but he was only spouting
the party line. I'm not sure that it makes any difference, but one does wonder
why the Fairchild/T.I. folks built all those RS-485-targeted devices if these
would work as well. It was never an issue for me, since I used them internally
between subsystems, but I always liked the 2-resistors on either side of ground
better than the single 100-ohm resistor as shown with the AMD driver/receiver
pair. The hi-z state certainly looks like it should work for multidrop use.
>
> My guess is that on this card, not all the sections of the 3487 are used,
> so the fact that one enable line controls 2 transmitter sections is not
> going to matter too much.
>
> The data sheet gives a value for the short-circuit current. It also says
> that only 1 transmitter should be shorted at a time, and not for more
> than 1s. In a mulitdrop network type application, a collision would be
> detected in a lot less than 1s (we hope!), so my guess is that it would
> be OK to use the 3487 in this way. Not a good design, but a design that
> would work (typical of a lot of Apple and IBM PC plug-in cards, actually...)
>
> When I've got some time (too many other projects...) I'll see how the
> 3486/7 are used on this card.
>
Until you learn what the target application was, you'll like be left in the
dark. While one can figure out what a board might do, until one gets a picture
of what the mfg intended, it remains a mystery why things were done as they
were.
>
> -tony
>
>
Received on Tue Jul 24 2001 - 18:14:36 BST