MAX232A

The old RS-232C interface specification calls for a maximum cable length of 50 feet.The newer EIA-232D specification calls for a length of cable that produces a capacitance of 2500 pF. This change from feet of cable to pF of load capacitance recognizes one of the problems in RS-232C. Not only does RS-232C ignore the effects of cable capacitance, but it contains a 50-ft distance limit that is often ignored by designers.

The EIA-232D specification considers cable length indirectly, through the effect of load capacitance, but does not specify a maximum length. Because the capacitance of inexpensive cable can range from 12 pF/ft for a single twisted pair to 30 pF/ft for low-noise shielded, multiple-twisted-pair cable, there is some confusion.

Cable capacitance is important because EIA-232D transmission are ac signals. Higher capacitance demands higher peak currents from the transmitter, resulting in higher average supply current for a given data rate, as shown in Fig. 3-A. Also, the cable impedance forms an ac divider with the transmitter output impedance. Higher cable capacitance lowers the divider shunt component. In turn, this reduces signal amplitude at the receiver end of the cable, as shown in Fig. 3-B. This signal loss becomes a problem when the receiver-end voltage falls below the specified 5-V minimum input level (required by most digital equipment).

Finally, cable capacitance limits the slew rate available from a given transmitter, as shown in Fig. 3-C. The slew rate determines the transition time between the +3-V and -3-V signal levels. (Slew rate is discussed further in Chapter 6.) Transition time limits the maximum data rate according to the specifications in RS-232C, EIA-232D, and CCITT V28, as shown in Fig. 3-D.

The simplest way to calculate the maximum allowable cable length for an EIA-232D interface IC (transceiver) is to divide the 2500-pF load specification by the capacitance per foot specified for the cable. Then, for cable lengths up to that limit, you can rest assured that the transceiver will operate properly and provide the maximum specified data rate.

Figure 3-E shows how the data rate can be extended past the specification limits. Longer cable adds to the line capacitance, lengthening the signal transition time, thus limiting the data rate. However, you can implement cable lengths and data rates well beyond the limits implied by the EIA-232D specification.