24 Jul
2001
24 Jul
'01
9:52 a.m.
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(a)p850ug1.demon.co.uk>
To: <classiccmp(a)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