These are 3 signal-wire cables (RS 232 has 2 signal-wires plus
ground). These standards read differential signals rather than plain
levels and therefore allow to extend by about a factor 10 the maximal
range that RS 232 can reach without a modem.
That's certainly of great value in a computer center or even on a small
size campus - but can in no way replace analog communications over any
kilometers using a modem.
Jacques
On 09/12/2010 04:48 PM, Jonathan E. Brickman wrote:
On 09/12/2010 09:06 AM, Jacques Goldberg wrote:
Sorry to disappoint you Jonathan
The rate reported by wvdialconf is not the transmission rate! It is
(supposedly, never checked, irrelevant) the rate at which the
software communicates with the serial connection to the modem chip
located in the PC.
You will see the true rate when you launch a wvdial command.
wvdial will report the word "CONNECTED" and the rate, and it will
never reach the theoretical 56kbauds limit.
Gotcha.
Just to tease you, though, I am currently working flawless with a
serial connection at 460,800 bauds. But not the way you think of.
I use a Xilinx Spartan 3AN FPGA development board as a data
acquisition system. It has an RS232 port which can go even at least
twice faster than 460,800. I have connected it to a 10-15$
RS232-to-USB adapter with a shielded cable 20cm long only or it won't
work, and the USB side of the adaptor is connected to my laptop with
a 40cm shielded USB cable or it will not work.
Actually while pushing the above to its limits to match the data
acquisition system specifications, I "discovered" that the 115200
bauds limit which everybody knows about is just what was set in the
initial version of National Semiconductors's 16550. There are more
recent versions of this chip which could easily work at 800 kbauds
but they cost 1 or 2 dollars more per piece, a tremendous saving for
PC manufacturers.
This is quite interesting to me. About three years ago I was asked to
try to replace the server of an old multi-user system; all I was told
was that the server was running Windows 95. I found some hardware
with Windows 98 drivers available, ghosted the hard drive to the new
hardware, and completed a Windows 98 in-place upgrade. And then when
I ran the server binary, I was in for a shock.
It was DOS-based, not Windows-based. Last updated in 1984. Clearly
trying to use COM1: in an RS-483 mode. Deep memories in dark storage
opened up, a book I had half-memorized in 1986; RS-483 was a standard
serial-port mode then, the book described RS-232/RS-422 of that time.
I looked at the remaining bits of the old server -- someone had
attempted to work it before me -- and found just a basic ISA 16550A
board, nothing unusual at all.
The server process had been running successfully in a DOS shell under
Windows 95 for at least ten years, so I figured maybe Windows 98 might
be OK too. It had to also cooperate on a 10based2 Windows 2003
network, so I didn't want to try to take it to FreeDOS, which I might
have otherwise. The big catch, though, was the Dell hardware I had
found to rebuild it all on, had no discrete 165**0* chips at all; its
serial ports must have been part of larger multifunction chips. But
to sum it up, it did work, and very well. The most interesting thing
is, when I later looked up RS-483, I learned that it is still in use
to synchronize many large-arena PA systems...and that it can
communicate up to 1 Mbps. I wonder what kind of cable is needed for
the latter.
J.E.B.
