Re: I Need help copying a 15MB disk image across floppy disks?

Tim Auton <tim.auton@xxxxxxxxxxxxxxxxx> wrote:

Rowland McDonnell <real-address-in-sig@xxxxxxxxxxxxxxx> wrote:
Stephen <srmoll@xxxxxxxxx> wrote:
On 8 Mar, 06:23, Chris Ridd <chrisr...@xxxxxxx> wrote:
(Rowland McDonnell) said:
[snip]
The serial ports of the Mac and PC when wired together weren't a great
match. The voltages were different,

Well, yes, but the ranges overlap. I can't find a proper reference -
the pukka standards have to be purchased, and the file I have here to
tell me about the standards claims that RS232 input sensitivity is
+/-3V, when I happen to know it's +/-2.5V.

I "know" it's +/- 3V and my copy of The Art of Electronics

My copy of Horowitz and Hill's `The Art of Electronics' (1st ed., 1980)
says the input voltage threshold is +/- 2V; figure 9.29, page 406. It's
obviously wrong. I can't see any other statement of the input voltage
spec in that book - are you looking at the 2nd edition?

I've read (back in about 1989-1991, probably) the actual pukka RS232
spec, and that's what I recall: +/- 2.5V as the input threshold, to
complement the +/- 3V minimum driver output voltage.

Noise exists, and this is a communications protocol. The input
thresholds *MUST* be lower than the minimum output spec, or it won't
work reliably. Given that the minimum output voltage is +/- 3V (from
what I recall, anyway), you must have a lower input threshold.

But note the qualifiers I put in, and read on...

and every
website I've ever seen and just found with a quick web search agrees.

Hmm... Well, I've looked some more - and it seems my memory has screwed
up again. The minimum output voltage spec seems to be +/- 5V, with the
minimum input threshold being +/- 3V. I wonder where I got my wires
crossed? But what I've read really does not ring any bells - my memory
of the standard I read is still pretty definite about what it says, even
though I'd've thought I'd've had one of those `Ping!' moments on
`finding out the truth'. I shall have to visit a library and get the
real standard to find out. RS232 is a very old standard. It was meant
to connect teletypes to old-tyme modems, not to work with any form of
microelectronics. RS232 didn't make sense in the 1980s; it was already
obsolete when IBM came up with its first PC.

Rowland.

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