5 May
2008
5 May
'08
11:39 p.m.
On Mon, 5 May 2008 20:55:51 +0100 (BST), Tony Duell wrote:
[...]
Quite often I need to transfer data between 2 machines. Maybe to
download a file from this PC, which I've in turn downloaded from a web
site, to run on one of the classics. Maybe to print out some listing
from
a classic. Whatever.
[...]
So, I think the problem reduces to 'how to interconnect RS232
ports'. let
me add some constraints :
Requirements as I understand them:
1
Must work over a distance longer than the RS232 spec
allows (i.e. the
answer is probably not 'A long RS232 cable' :-)).
(see <http://www.lammertbies.nl/comm/info/RS-232_specs.html> for an
interesting discussion on cable lengths v.s. data rate - your data
rates spec'ed below indicate that this is not a problem...
2.
Prefereably no cables at all. One solution I've
come up with is to
use a
couple of line drivers and a long cable between them. A long cable
that
my parents, or the cat, will get tangled up in :-(
i.e. no cables hence 1 does not apply
3.
No line-of-sight between the machines
This excludes optical means.
4.
Must work at 300 and 1200 baud. 110 and 9600 baud
would be a bonus
5.
I only need one pair of machines linked at a time. I
don't need a
network. [...]
Point-to-point - collision avoidance not required.
6.
Must not make use of any flow control lines on the
RS232 port, since
some
of my machines don't support them.
7.
Using classic, or at least repairable, hardwre is a
bonus :-)
8.
I said 'RS232'. I mean asynchronous serial, of
course :-). [...]
9.
I've been looking at some of the license-exempt
radio modules, but
they
either are half-duplex or amke use of the flow control lines
(typically
they buffer <n> bytes internally, then de-assert a flow control line
while they pack up that data and send it to the other end).
Does this mean that you require full-dupex?
-------
If 8 is required, you probably have to go with a dual channel, radio
link of some sort. Zigbee is definitely overkill based on 5. A simple
set of transceivers should do the trick. TI and others have parts and
reference designs. However, my experience has been disappointing in
enclosed and partitioned areas with these devices - at least at legal
power levels...
Item 7 makes me think X10. This is a classic data-over-power-line
method that has been around for good number of years. It is generally
used to control lights, sprinklers, intrusion alarms, etc. and uses a
simple protocol. However, it appears to be half-duplex. The info is
out there on the web.
You might want to look at ST's ST7540 and ST7538 which are current
power-line modems and should do what you want. A small micro to buffer
things will probably take care of 6.
Jules Richardson noted:
Hmm, I've got a deep mistrust of any 'data
over the mains'
technology, but
might that be an option here? I assume *most* of your systems are
physically
plugged into the mains anyway, so it'd meet the ideal requirement
for no extra
cabling. Data rates presumably not lightning fast, of course...
I don't think security is of concern here unless you are extremely
paranoid or are transferring prohibited material (e.g. perhaps with
your collection, ASCII kiddy porn to the impact printer :=P).
CRC