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'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
terminated, since the signal is differential. What I remember as odd was that
back in the late '70's when I first encountered these, specifically in the
context of data line drivers on SMD's and hard disks in general, the AMD pairs
(26LS31/32) were terminated with a 100-ohm resistor between the pair, while the
MOT parts were terminated with a pair of 51-ohm resistors to ground. Both of
them have essentially the same common mode tolerance and they both work
essentially the same.
ISTR 75176 was designed to RS-485 spec's, which allows for multi-drop use. I
don't know the low-level details, but there's a whole range of RS-485 devices
which will work in RS422/432, but the RS-422 devices won't work in RS-485
environments.
Dick
----- Original Message -----
From: "Tony Duell" <ard_at_p850ug1.demon.co.uk>
To: <classiccmp_at_classiccmp.org>
Sent: Tuesday, July 24, 2001 12:26 PM
Subject: Re: Apple ][ boards -- what have I found?
> > > On Sun, 22 Jul 2001, Tony Duell wrote:
> > >
> > > > Memsoft TC2E0260. This board looks hand-soldered, and some components
are
> > > > missing. It's got a 6850 (ACIA), MC3486, MC3487 (Buffers), and TTL glue
> > > > on it. And space for a 2716 EPROM.
> > >
> > My guess is that this is a serial interface intended for RS423 or RS422 use,
> > i.e. longer distances. It's possible the 2716 was intended to support some
> > customed protocol, but the 6850 is essentially a UART, (no synchronous
> > capability) and somewhat rate limited. The 3486/87 are differential drivers
for
> > RS422/423. One wouldn't have used the diff drivers unless one needed to
have
> > the signal travel some considerable distance.
>
> Do these buffers support any kind of multi-drop capability? Could this be
> a low-end custom network interface.
>
> I know that the Gemini (UK Z80 machine, related to the Nascom) had a
> network interface that was little more than an AY-5-1013 UART and a 75176
> transceiver.
> The number on the 40 pin UART chip had been scraped off, but
> it took about 10 seconds to work out what it really was. Perhaps
> something similar existed for the Apple.
>
> -tony
>
>
Received on Tue Jul 24 2001 - 16:52:16 BST