USB or RS232 scope controll

On 14 Oct 2006 22:08:36 -0700, in uk.sci.astronomy , "w_tom"
wrote:

Mark nitpicks at irrelevant points. Yes, RS-232 existed before CRTs.
So what? RS-232 was also intended for CRTs (or Teletypes) to modems -

This is true.

the fact: what RS-232 was intended. RS-232 is common mode signaling -
1962 technology - that is inferior for

Inferior to what? Later technologies? Well golly.

and was never intended for
instrumentation.

It may have never been intended for it, but its perfectly capable of
doing it.

Yes, USB has distance limitations. Stated was that RS-232 or USB
were both acceptable options - meaning distance is not relevant.

Except that RS232 by design (cf RS-232C/CCITT v.24) can do 50ft
without assistance (in practice much more), whereas USB can't do more
htan about 15. If you want to do long lengths, RS232 will probably
work where USB won't.

Is RS-232 still found in computers? Yes - and so is COBOL and Time
Sharing. So what? All three were obsoleted long ago.

Does one have to always use the latest shiny technology? Are we using
laser knives to cut our steak? Have hardware stores stopped selling
screwdrivers? Have you heard of Occams Razor?


--
Mark McIntyre

"Roger Hamlett" wrote in
:


"Llanzlan Klazmon the 15th" wrote in message
7.6...
"Roger Hamlett" wrote in
:


"Simon" wrote in message
...
is there any advantage to having the extra baud rate
of USB over RS232 for controlling Meade Scopes?.

I am only guessing now but I imagine there isnt
huge amounts of data needed to controll the scope,
and that RS232 would cope fine, but I dont know
for sure and your experience/advicewould be appreciated.

simon
None whatsoever.
There is also a massive 'downside', in that RS232, used at reasonable
data rates, can go 100', while USB, requires a repeater module to go
more than 10'...
The 'better' connection, would be something like Ethernet, which
handles
the distance of RS232, with the advantage of better isolation, and a
higher data rate if required. USB, was designed as a 'local' PC
connection, to talk to devices like printers close to the PC. It has
very poor error checking, and recovery, and (unfortunately), has been
'pushed'

Actually running RS232 over 100' is also pushing it. RS232 also has no
inherent error detection or correction. It relies on some sort of error
checking at the data level such as parity or CRC check which are not
defined in the RS232 standard. The standard also specifies cable length
of
less than 45' but of course you can get away with a fair bit more than
that using low capacitance cables. Even way back when, V.35 or X.21
which
used balanced transmission would have been a far superior technical
choice. To get those on a PC you had to buy an add in card whereas most
PC's came with two RS232 ports as standard. I don't know why the scope
drive manufacturers don't use 10 or 100BaseT and TCP/IP. As you say,
that
would be a much more sensible choice and not particularly expensive.

Klazmon.
The 'spec', is 50 feet max, using the standard 'test' cable, or length to
give 2500pF max. The second part is commonly forgotten, but implies that
using low capacitance cable, you can go further and stay in spec. Using
standard Cat-5 cable, at 17pF/ft, allows just under 150 feet to be
'legal'. The 50' limit, is also for 19200bps. The allowed length rises
massively as you drop the data rate. At 9600bps (commonly used for
scopes), the allowed length rises more than linearly (look at Texas
SLLA067A, for a graph of the relationship - it actually gives the same
signalling behaviour at 9600bps, with 500' of cable, as for the 50' at
19200bps...).
The thing about RS232, is that being async, the interface will only lose
data, from interference, not hang.
The problem with USB, is that the way
the interface is designed (mainly in the software driver), there is no
'timeout' in the USB stack.

Well that is pretty silly software design.

Both suffer similar data loss from noise, but
the RS232 receiver will simply recover and start looking for the next
byte. USB for certain control transfers, can get completely hung. :-(
Personally, I hate USB. It is fine, used for what it is designed for, but
the way it has been 'pushed', leads to more problems than almost anything
else on the PC today...

Interesting. I have also seen people complaining bitterly about the USB
connectors too. Mainly due to fragilty and I don't mean ill treatment just
repeatedly plugging and uplugging is likely to end in grief. I'm not
certain this is true but since I heard that, I have been extra careful with
camera connectors etc.

Klazmon.



to do all sorts of things it was not designed to do, because it is seen
as the 'in thing', and the chipsets are cheap. the only advantages to
using a USB connection, are first that if you are using a USB camera,
with a simple hub, you only then require one connection, and secondly,
that a lot of new laptops, do not have a serial connection.
Unfortunately, it is only when you then discover signalling artefacts
on
the images, or need multiple repeaters, that you start to realise the
problems of USB...

Best Wishes