Re: Mystery hardware query

[dempson@actrix.gen.nz (David Empson)]

Steven Hirsch <snhirsch@gmail.com> wrote:

> David Empson wrote:
> 
> >>> At its peak, our network included about 17 Apple IIe workstations on the
> >>> Nestar network.
> 
> >> Just out of curiosity, what data bandwidth did the network support?
> 
> > It was an 8-bit parallel bus, with handshaking. Somewhat like a parallel
> > printer interface, with a data strobe and an acknowledge. I don't recall
> > details about the protocol and I'm not sure if I ever analysed its
> > performance in close detail.
> > 
> > Each sector or block was transferred over the network as a single burst,
> > and the average performance of the network was pretty bad with a lot of
> > computers tring to use it at once, probably due to the contention phase
> > and polling time.
> > 
> > Some specialised applications were able to hog the network for short
> > periods in order to transfer a larger amount of data. One example was
> > the network login screen, which loaded a fairly large amount of data
> > quickly.
> 
> What you describe sounds very much like the original Corvus "flat cable"
> network.  It was wired in a star configuration with each computer having a
> 34-pin ribbon cable back to a device called a multiplexer.  This connected to
> a hard disk and acted as an arbitrator.  I have a bunch of Corvus hardware and
> software (probably just about everything they ever made), but haven't had it
> setup for a while.
> 
> The flat-cable networks were replaced in 1982 or thereabouts with the
> twisted-pair OmniNet, which was a daisy-chain topology using a form of
> collision-detect / backoff protocol.

Nestar used a 16-pin ribbon cable in a multidrop bus configuration,
physically wired as a daisy chain between adjacent computers. I'm not
sure about the electrical interface - it was bidirectional, which would
complicate the interface if each card had to buffer and retransmit.
Individual computers could be switched off, which suggests passive
signal forwarding rather than active retransmission.

There were probably some multidrop pins dedicated to the arbitration
process, and active transmission by the computer which is sending data,
by enabling its line drivers.

Don't know how it dealt with fan-out problems or transmission line
effects like signal reflections. I don't recall anything like a
terminator as a separate component, or as a switch setting on the "end"
cards.

The configuration lent itself well to classroom layout, with all the
computers around the outside wall (or in rows with a single "return"
cable to get back to the wall) and not requiring a massive lump of
cables arriving at a common point.

My school got our original Nestar network (20 MB hard drive and
workstation cards) in 1983, but it was second hand and I don't know when
it was originally purchased. We later acquired a second 20 MB hard drive
and a dual 8 inch floppy drive from another school getting rid of their
network.

-- 
David Empson
dempson@actrix.gen.nz