From: rick@ee.uwm.edu Subject: Standard Speed/Length RS-232 Cable Limits Date: 14 Oct 1992 18:12:51 GMT
michaelw@mcshh.Hanse.DE (Michael Will) writes:
>I want to place my 386-machine running linux 0.98 in the cellar, since
>it is rather loud (harddrives, fan) and I do not need graphics at the
>moment.
>
>The cable for rs232 should not be longer than 12meters, right?
>
>Alas, I need it longer. I'd say it is about 20meters. [...]
Here's the 'official' numbers (according to Cisco's router inst. guide):
--- begin --- Quote of Cisco's "IGS Hardware and Installation Reference"
Console Cable Considerations
When setting up your system, consider electromagnetic interference
and, when using RS-232 connections, distance limitations
Distance Limitations
As with all signaling systems, RS-232 can travel a limited
distance at any given bit rate; generally, the slowere the
data rate, the greater the distance. Table 2-1 shows the
standard relationship between bit rate and distance.
Table 2-1 RS-232 Speed Versus Distance
Data Rate Distance (Feet) Distance (Meters)
---------------------------------------------------------
2400 200 60
4800 100 30
9600 50 15
19200 25 7.6
38400 12 3.7
=======================================================================
NOTE: RS-232 is often used in violation of these specifications.
If you understand the electrical problems that can arise
and can compensate for them, you might be able to get good
results at distances far greater than those shown in Table 2-1;
however, do so at your own risk.
--- end --- Quote of Cisco's "IGS Hardware and Installation Reference"
Remember, these distances are the RS-232 specification *requirements*.
Your devices' serial line drivers may *already* exceed them. (Also
remember that *both* serial line drivers must in some way exceed this
requirement for communication in both directions.)
Since Michael wanted 9600 at 20 Meters, he just may be able to get it
with no additional hardware, since it's practically guaranteed out to
15 Meters. Just remember to use well-shielded cable, running in fairly
straight lines, and being as short as possible without physical strain.
As an alternative, I'm sure there are commercially available line-drivers
for this purpose... though it would be a fairly simple do-it-yourself job.