Firstly, the interface presupposes a common ground between the DTE and DCE. This is a reasonable assumption where a short cable connects a DTE and DCE in the same room, but with longer lines and connections between devices that may be on different electrical busses, this may not be true. We have seen some spectacular electrical events causes by "uncommon grounds".

Secondly, a signal on a single line is impossible to screen effectively for noise. By screening the entire cable one can reduce the influence of outside noise, but internally generated noise remains a problem. As the baud rate and line length increase, the effect of capacitance between the cables introduces serious crosstalk until a point is reached where the data itself is unreadable.

Crosstalk can be reduced by using low capacitance cable. Also, as it is the higher frequencies that are the problem, control of slew rate in the signal (i.e., making the signal more rounded, rather than square) also decreases the crosstalk. The original specifications for RS-232 had no specification for maximum slew rate.

The standards for RS-232 and similar interfaces usually restrict RS-232 to 20kbps or less and line lengths of 15m (50 ft) or less. These restrictions are mostly throwbacks to the days when 20kbps was considered a very high line speed, and cables were thick, with high capacitance.

However, in practice, RS-232 is far more robust than the traditional specified limits of 20kbps over a 15m line would imply. Most 56kbps DSUs are supplied with both V.35 and RS-232 ports because RS-232 is perfectly adequate at speeds up to 200kbps.

The 15m limitation for cable length can be stretched to about 30m for ordinary cable, if well screened and grounded, and about 100m if the cable is low capacitance as well.

Data Rate (bps)     Maximum Cable Length
57,600 or less           30m/100 feet
115,200                    25m/80 feet
230,400                    13m/40 feet
460,800                    6m/20 feet
921,600                    3m/10 feet

RS-422, RS-485, V.11 and other balanced interfaces.

The limitations of RS-232 are largely eliminated by the balanced line interface.

A pair of wires A and B is used to carry each signal. The data is encoded and decoded as a differential voltage between the two lines.

As a differential voltage, in principle the interface is unaffected by differences in ground voltage between sender and receiver.

Furthermore, if lines A and B are close together, they will be affected almost identically by external electromagnetic noise. If the lines are also twisted together, then neither line is permanently closer to a noise source than the other. Hence the well known "twisted pair" is extremely effective in eliminating noise from the signal.

Balanced systems are used by LAN topologies like Ethernet and Token Ring. They can support line speeds over 100Mbps and work reliably at distances of several kilometers.

As line speeds and lengths go up, the problem of signal reflections becomes important. Lines must be properly terminated by a resistor that makes the cable look electrically like it is infinitely long (an infinitely long cable, of course, can have no reflected signals because the far end is infinitely far away). These terminating resistor values depend on the geometry of the cable itself. So you will see cable designated as 75 Ohm cable or 50 ohm cable, etc. What this means is that by installing a 50 ohm resistor, say, between the signal pair, this particular type of cable will have the electrical characteristics of an infinitely long cable. Note that the designation "50 ohm cable" has nothing to do with the electrical impedance of the physical cable itself.

V.35 Interface.

The V.35 interface was originally specified by CCITT as an interface for 48kbps line transmissions. It has been adopted for all line speeds above 20kbps, and seems to have acquired a life of its own. It was discontinued by CCITT in 1988, and replaced by recommendations V.10 and V.11.

V.35 is a mixture of balanced (like RS422) and common earth (like RS232) signal interfaces. The control lines including DTR, DSR. DCD, RTS and CTS are single wire common earth interfaces, functionally compatible with RS-232 level signals. The data and clock signals are balanced, RS-422-like signals.

The control signals in V.35 are common earth single wire interfaces because these signal levels are mostly constant or vary at low frequencies. The high frequency data and clock signals are carried by balanced lines. Thus single wires are used for the low frequencies for which they are adequate, while balanced pairs are used for the high frequency data and clock signals.

If your DSU supports RS-232 as well as V.35 you are always better off financially by using the RS-232 option. An additional complication with V.35 is that the V.35 plug is too large to fit on many add-in cards, such as those used by PCs. Thus there is very often a non standard cable used to connect a V.35 system, terminating in a DB25 at one end and a V.35 plug at the other. It is very easy to use the wrong cable, and quite difficult to debug if you do.