Active Oldest Votes 14 +150 Capacitance in the cable begins to affect serial communications, typically many manufacturers/devices set a limit of approximatly 15m or 50ft on cable lengths for anything up to 9600 baud, and about 4m for 38.4k. 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.
A serial cable is a cable used to transfer information between two devices using a serial communication protocol. The form of connectors depends on the particular serial port used. A cable wired for connecting two DTEs directly is known as a null modem cable.
Maximum cable lengths[edit]
The maximum working length of a cable varies depending on the characteristics of the transmitters and receivers, the baud rate on the cable, and the capacitance and electrical impedance of the cable. The RS-232 standard states that a compliant port must provide defined signal characteristics for a capacitive load of 2500 pF. This does not correspond to a fixed length of cable since varying cables have different characteristics. Empirically tested combinations of bit rate, serial ports, cable type, and lengths may provide reliable communications, but generally RS-232-compatible ports are intended to be connected by, at the most, a few tens of metres of cable. Other serial communications standards are better adapted to drive hundreds or thousands of metres of cable.
Serial Cable Maximum Run Length
See also[edit]
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- LapLink cable (can be seen as a parallel equivalent to a serial null modem cable)
- Rollover cable (also known as a 'Yost' cable.)
References[edit]
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
Serial Interface Cable
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.
