What is the maximum cable length for an RS-232 communication port?
For the original RS-232 standard, the cable between the driver and the receiver was specified for a maximum of 15 meters in length. This part of the standard was changed in a revision to a maximum capacitive load of 2500 pF. The maximum cable length is determined by the capacitance per unit length of the cable which is provided in the cable specifications. The total capacitance is approximated by the sum of the mutual capacitance between the signal conductors and the conductor to shield capacitance (or stray capacitance in the case of unshielded cable). Cm = Mutual capacitance between conductors. Cs = Conductor to interface cable shield capacitance (if shielded cable is used) – 2*Cm or stray capacitance to earth (if unshielded cable is used) – 0.5*Cm. Cc= Cm+Cs = Total capacitance per unit length Example: Assume the user has decided to use non-shielded cable when interconnecting the equipment. The cable mutual capacitance (Cm) of the cable is found in the cables specifications to be 20
