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What is the maximum cable length for real-time RS-232 data?

Cable length is one of the most misunderstood items in the RS-232 world. The RS-232 standard was originally developed decades ago for a 19200 baud rate, and defines the maximum cable length as 50 feet, or the cable length equal to a capacitance of 2500 pF. The capacitance rule is often forgotten; using a cable with low capacitance allows you to span longer distances without going beyond the limitations of the standard. Also, the maximum cable length mentioned in the standard is based on 19200 baud rate; if baud is reduced by a factor of 2 or 4, the maximum length increases dramatically. Using typical underwater cables, allowable combinations of cable length and baud rate for Sea-Bird instruments communicating with RS-232 are shown below:

Maximum Cable Length (meters) Maximum Baud Rate*
1600 600
800 1200
400 2400
200 4800
100 9600
50 19,200
25 38,400
16 57,600
8 115,200

*Note: Consult instrument manual for baud rates supported for your instrument.