Interconnecting classic computers
ard at p850ug1.demon.co.uk
Fri May 9 17:42:53 CDT 2008
> Tony Duell wrote:
> > Well, I am not going to make the radio modules myself (no way can I
> > meet
> > type approval, even if I meet the technical specs), but I would
> > certainly consider making something _round_ said modules.
> I assumed that you'd start with a module for the RF stuff: but
Indeed I would. There seem to be fairly dumb modules, and those Xbee
things, and the latter are not significantly more expensive or harder to use.
> that's essentially no different to plonking a 7474 on a board
> (you could build it from discretes, but there's a nice
> convenient package ...)
there is one difference. The 74x74 schematics -- to component level --
are in the data sheet. The internals of these radio modules are not
disclosed anywhere from what I can see. Which might matter if either the
system doesn't behave in the way I expect, or if I am trying to do
soemthign unconventional with it.
And of course testing a 74x74 is a lot easier than testing a radio module
with an embedded microprocessor.
> > Now that's something that had not really struck me (which is why this
> > sort
> > of discussion on the list is so useful) -- that the file sizes I am
> > going
> > to be transfering _are_ limited. 128Mbytes is larger than the
> > available
> > mass storage (let alone RAM memory) on most of my classics, so except
> > in
> > very unusual ciercumstances, the data I wish to transfer is going to
> > be a
> > lot smaller than that. Which means a buffer is certainly an option.
> I mentioned 128MB simply because it is the *smallest* SDRAM I have
> to hand in any reasonable quantity (I have a handful of 32MB and
> 64MB). 512MB is cheap too. I don't know at what point you go beyond
> the likely capacity of any classic. There's not a lot of point in
There would seem to be 3 size limits to consider :
The size of the RAM memory of the classic
Size of removeable storage (e.g. floppy disks)
Size of fixed storage (winchesters, etc)
In many cases, individual files are smaller than the RAM memory (since
they're entirely loarded into RAM when in use). Not always of course. And
generally files are smaller than the stnadard dloppy disk capacity of
the machine in question.
Which means that for an awful lot of smaller classics, a 1Myte buffer
would be overkill. 128M would exceed the total hard disk space on just
about any classic that I own.
> going this route with anything that has an ethernet connection. In
Well, assuming the machine you want to connect it to also has an ethernet
interface, and that you have suitable software on both machines already.
> my case that would leave machines like a PET, C=64, RML 380Z
> and so on. 128MB would be overkill for any of those.
> Of course, the other issue is speed. Assuming 19200 baud that's
> ~2KiB/s on a good day. It would take a chunk of time to fill
I get it as about 19 hours (assuming 10 bits/character, so 1920
characters/second). For that sort of thing I'd want to split the trasnfer
up into smaller sections , so that if anything went wrong I wouldn't have
to do the whole lot again.
> > Does it need to be idriectional?
> I wasn't sure how you envisaged using this mechanism. If it's
> always the case that machine A "prints" to machine B (i.e. A
> sends, B collects) then the modules do not strictly need to be
> bidirectional. But I suspect that it would be much more convenient
I was refernting to the buffer having to be bidirectoaal.
I think you only ened this exernal buffer between the output of the
classic and the input of the radio modem. From the radio modem to the
classic is less of a problem -- if the classic supports hardware flow
control, then it can tell the radio modem to stop sending, if there is no
hardware flow control, then it's assumed the classic is going to keep up
with the incoming data stream (no matter what it comes from). . A buffer
woun't be a lot of use there. The complete system is bidriectional, but
there's only a unbidirectional uffer at esch node
On the other hand, adding a buffer would allow the radio modem to run at a
standard speed, and the interface to the classic machine to be run at any
speed desired. And having designed the buffer once, it's not a lot more
work to put 2 of them at each node/
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