Applies To:
  • CitectSCADA 1.00 1.01 1.10 1.11 1.20 2.00 2.01

Question: How long can I make my RS-232 cable? 

The maximum permissible length of a cable depends on the quality of the cable and environment surrounding it. The cable should be a shielded, low-capacitance cable, ideally designed specifically for serial data transmission. The shield should be grounded at one end of the cable only. While good shielding provides reasonable protection against "noise" (Electro-Magnetic Interference, or EMI), cables should still be routed away from noise sources wherever possible. Avoid laying cables in close proximity to transformers, generators, motors, fluorescent lights, etc.

The total capacitance of a cable affects the integrity of transmitted data. As a rule of thumb, the total capacitance of a cable (including the connectors) should not exceed 2500pF. Serial interface cable is usually rated in pico Farads per foot.

Max Length = (2500 - C1 - C2)/Ccable

Cc1 = Capacitance of connector 1 in pF,
Cc2 = Capacitance of connector 2 in pF,
Ccable = Capacitance of cable in pF/ft, and
Max Length = the maximum length of the cable in ft.


Max Length = (2500 - 100 -100)/50 = 46 ft.

Better quality cables will yield greater spans.

In situations where low-capacitance cables are not available, or very long cable runs are required, an RS-422 interface is recommended. PC/16i Digiboards are available with RS-422 interface. RS-422 to RS-232 converters are also available for use with units that do not have RS-422 capability. The RS-422 interface has a range of up to 4000 ft.