11.4. Lengths of serial cables
The RS-232 standard 9600bps port will
drive 15 metres of shielded cable. More precisely, an
RS-232 line driver will operate against a
capacitance of up to 2500 picoFarad with low enough skew to allow a
9600bps signal to be recovered.
If you select a cable with lower capacitance you can drive
further distances. For example,
ANSI/TIA/EIA-568-A
unshielded twisted pair category 5 cable has a maximum capacitiance
of 55pF per metre, so this popular
"UTP cat 5" cable can be safely
driven up to 45m. Beyond that you should check the cable
manufacturers specifications for the actual "shunt
capacitance" (a common figure is 47.5
pF/m, giving a maximum cable length of about
50m). However long runs of unshielded cable
will pick up noise easily, as the RS-232 signals
are not balanced. Some cable manufacturers offer shielded low
capacitance cables which can be driven up to
100m.
Similarly, if you select a lower data rate you can drive
further distances. Table 11-1
shows the maximum distances over standard shielded cable at
differing data rates.
Table 11-1. Data rates and the maximum distances recommended in
RS-232
Data rate (bps) | Distance (m) |
---|
2400 | 60 |
4800 | 30 |
9600 | 15 |
19200 | 7.6 |
38400 | 3.7 |
56000 | 2.6 |
If you are comfortable in working beyond specifications then
you might note that the experience of enterprise network operators
has been that structured cabling layout in buildings is limited by
the 100m distance limitation of fast ethernet over category 5
cable, not by the practical distances achieved by RS-232
asynchronous signals at 9600bps over category 5 cable.
For longer distances use an RS-232 line
driver; these will typically drive up to 2000 meters over category
3 UTP cable. For greater distances consider
using fiber optical modems, the global telephony system, the mobile
telephony system, satellite or radio.