Date   : Mon, 06 Aug 2007 22:21:29 +0200 (BST)
From   : johan@... (Johan Heuseveldt)
Subject: Emulating Econet hardware?

Hi,

On Mon 06 Aug, Chris Johns wrote:
> On Mon, 6 Aug 2007, Alex Taylor wrote:

> > Would it be possible to emulate an Econet server and clock using a
> > PC's parallel port, and suitable software and cabling? A little bit
> > like my Commodore 64 setup, where I have a laptop and cable emulating
> > a 1541 floppy drive. (Before anyone pounces on this, please don't
> > start a flamewar about how parallel ports are obsolete, and this
> > should be done with FPGAs/USB/FireWire/ExpressCard).
> 
> IIRC, the problem with making anything to connect PC to Econet is interupt 
> latency. The ADLC only has a few bytes (i think 3 bytes?) of buffer so you 
> need to be quick to read and process the data before it's gone for good.

And sometimes (=many times) the interrupt is only there when the
FIFO buffer is filled with /two/ bytes. That's why the TUBE software
also has two-bytes protocols.
Meaning, the interrupt has to be served within 8bits time of network
speed, which can be as difficult as for an FDC.

> On the BBC this is done with NMIs (and FIQs on the arc), but I don't think 
> the PC has anything that can service an interupt in that sort of speed.
> 
> The Econlink ISA cards had their own processors to handle the econet side 
> of things, but that makes your design a lot more complicated.

Correct. It was an ISA card. I think nowadays the problem remains
with interrupt latency, although modern speeds could make it a bit
better. I think the problem remains though, as certain processor
instructions can take a long time.

The EcoLink card has its own processor system on board. There were
two versions. One with a Z80 from SJ Research, and one with a 6502
for which I don't know the manufaturer. SJ Research too?
A year ago, there was one on Ebay. Anyone seen it too?



> If you have a spare beeb i guess you could connect it via the user port to 
> the PC's parallel port to act as a big "econet card".

Probably the best, and (still) quite a task as a software project! :-)

Greetings,
Johan

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

  The best place is a Riscy place
 
A mouse is an elephant built by the Japanese.