One item you mention certainly supports the notion that this board was used in a
multi-drop scheme of some sort, and that's that the termination resistors were
not populated. Only one station at each end of a transmission line should carry
the termination resistors, hence, it's likely they'd be external to the board.
I'm puzzled at the use of the 6850, however, since it's only async-capable. So
far, however, and maybe until you find out quite a bit more about this setup,
it's anybody's guess.
Since this setup is async, why would you believe that it transmits clock? I
guess it's conceivable that a master might provide a global clock, but, since
the data format is async, I don't see any benefit.
Dick
----- Original Message -----
From: "Tony Duell" <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Wednesday, July 25, 2001 3:12 PM
Subject: Re: Apple ][ boards -- what have I found?
> [MC3487 driver]
>
> > > > 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
>
> My guess (without trying the chip) is that the 'disconnected'
> (high-impedance) mode does work as described. But that you should only
> use multiple drivers if you are sure there is no chance of contention (2
> drivers enabled at once). The RS485 devices should allow for contention
> without damage.
>
> I would believe as well that you'd probably get away with short periods
> of contention using most RS422 drivers. It's not a good design to do
> that, but then a lot of things aren't well designed.
>
> > > 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.
>
> I had a quick look last night, and things are a little clearer. It
> appears that 2 drivers and 2 receivers are used. The outputs of one
> driver are connected to the inputs of a receiver (and ditto for the other
> pair). There is space on the PCB for termination resistors (which are not
> fitted), including a multi-turn preset between the lines of each
> differential pair. The termination components are missing (never
> soldered) on my board, so I have no idea what the values are.
>
> There are 6 numbered solder pads at the front edge of the card. 2 are
> obviously grounded, the other 4 are the 2 differential pairs. I would
> assume these carried a cable to some external connector.
>
> My guess is that one differentinal pair is the TxD and RxD lines of the
> 6850. The other might be clock. I didn't have any data sheets to hand, so
> I can't be sure about this yet. And I've not looked at the enable inputs
> to the 3487 driver (the 3486 receiver seems to be enabled all the time,
> which makes sense). I can't even remember if the 2 drivers used would be
> enabled together or not (i.e. do they share a common enable pin).
>
> I need to do some more checking, obviously, but it's looking increasingly
> likely that this card does interface to some form of multi-drop serial bus.
>
> -tony
>
>
Received on Wed Jul 25 2001 - 18:37:08 BST