Date   : Thu, 28 Jul 2005 11:32:36 +0200 (BST)
From   : Johan Heuseveldt <johan@...>
Subject: Re: Econet-Ethernet bridge

Hi Jules,

On Wed 27 Jul, Jules Richardson wrote:
> On Wed, 2005-07-27 at 15:00 +0000, Jules Richardson wrote:
> > On Wed, 2005-07-27 at 15:50 +0200, Johan Heuseveldt wrote:

> > > So, Econet for Beebs is at max 200 kHz, which is bit speed. Byte speed
> > > will then, lets say, 20 to 25 kHz. (20/10, 20/9, 20/8)
> > > Is that (really) a problem for parallel/Centronics?
> > 
> > I *think* you can get about 100KB/s through a parallel port, so it
> > sounds like that *could* work comfortably with buffering at the byte
> > level.
> > 
> > Just depends whether it's possible to work at the byte level across that
> > interface boundary - which depends on the nature of the Econet protocol
> > itself (if it needs to work at the bit level at any time then control
> > via the parallel port - operating at byte level - might be difficult)
> 
> I just grabbed the 68B54 datasheet, and it looks like the whole thing is
> byte-accessed anyway, and the chip itself handles
> serialisation/deserialisation to/from a bit stream. 

In that respect, the chip is like any other serial device, but already a
bit more than that: TxData, RxData, TxClk, RxClk, RTS, CTS, DCD.
(only DTR isn't used)

I've always had the feeling that people (I talked to in the past) did't
fully appreciate the special features of the ADLC. There were other
designs for a network using a normal serial chip (6850 and 6551 I think,
and even the user port IIRC) and people praise them for being cheap, and
had many bad words addressed to Acorn. But they hadn't the faintest idea
what is involved in network communications, and seemed unwillingly to be
educated.

With a normal serial line (6850) all the data IS by defenition valid on
system level. With a network this isn't. The more machines are connected
to the network, the more likely it is that the data is /not/ for this
machine. And with a normal serial device, the whole data stream /must/ be
dealt with; by the processor.

Any design with normal serial/parallel I/O, puts a heavy load on the
processor time, and thus the user. People were fooled by demonstrations
with just 2 to 4 machines. See the picture?

If NFS has determined from the first byte (or two) that this packet isn't
addressed to this machine, it gives the 68B54 a command to completely
ingore that frame. Any new interrupt is from a new frame coming in,
completely freeing the processor from any overheads of the invalid frame.
But that's just the start. There are many, many pages on the 68B54! :-)

So any new design in new programmable hardware, is /not/ quite the same
as impleting an 6850/6551 in such device.

> Main problem might be the lack of output lines (other than the 8 data
> bits) of a parallel port - seems like the 68B54 needs quite a few
> control lines as well as the data bus: RS0, RS1, R/-W, -RESET, -CS, and
> E.

And additional hardware for clocking the 68B54 from the clock remote
signals, and sensing clock presence (nDCD input), and contention logic
(nCTS input), and the line drivers of course.
In addition to that, it must be possible to 'disconnect' or 'mask' the NMI
from the system NMI line. This is done by not propagating the low NMI signal
from the 68B54 to the system's NMI line with extra hardware again.

Yes, quite a few extra's to deal with!

> (although I think E can just be a free-running local clock, and it
> *might* be possible to hold -CS permanently active)

Hm, you need to check the docs on that, now you have them! :-)

But I think the answer is no: In an 6500/6800 environment the trailing
edge of E (which is the leading edge of the next cycle) is used to clock,
or latch the data in the selected register, which should be addressed (CS)
in relation to that E clock to meet all the timings!
E and CS are closely /related/ to each other!


Hope this helps.


greetings,
Johan

-- 
Johan Heuseveldt <johan@...              >
  aka  waarland

  The best place is a Riscy place
 
"Flight by machines heavier than air is unpractical and
insignificant, if not utterly impossible". - Simon Newcomb.