Date   : Sun, 20 Jun 2004 13:18:48 +0100
From   : Philip Blundell <pb@...>
Subject: Re: PC/Econet

On Sun, 2004-06-20 at 12:57, Jules Richardson wrote:
> On Sun, 2004-06-20 at 10:45, Philip Blundell wrote:
> > On Sun, 2004-06-20 at 10:40, Jules Richardson wrote:
> > > I was actually wondering if a PC parallel port had enough speed to just
> > > drive one of the Master-type Econet modules. Raw-speed-wise I think it
> > > should have, but timing could be a problem due to the nature of the
> > > polled-IO approach to driving a parallel port. Another back-burner
> > > project I'd like to fiddle with sometime though...
> > 
> > Yeah, it'd be interesting to try.  Data throughput probably wouldn't be
> > much of a problem, but again I suspect latency might be an issue.
> 
> Yep. If it really were that easy, why would Acorn invest time and money
> in developing an entire ISA card? 

Well, remember that things have moved on a lot in PC hardware terms in
the intervening two decades.  The fundamental thing that determines
latency is the maximum time that the CPU can spend with IRQs locked out;
on a 286 running DOS, a few milliseconds would probably not be much of a
surprise.  But CPUs are a lot faster nowadays, and operating systems
tend to be much less cavalier about disabling interrupts whenever they
feel like it.

> Of course maybe a 'modern' ISA design with just a 68B54 and a few buffer
> / latch ICs thrown onto a prototyping board could do the job too. 

My suspicion is that you'd end up needing a lot of glue to interface the
68B54 directly to ISA.  Something like a Z80 SIO might be an easier
option.  But, of course, modern-day PCs don't have any ISA slots anyway.

If you wanted to do it properly, the thing to do would probably be to
make a PCI card and do the serial handling in an FPGA.  That's perhaps a
bit too much like hard work, though.

p.