The inverse relationship between the data rate and bus length, means the higher the data rate, the shorter the cable length; and conversely, the lower the data rate, the longer the cable can be without introducing data errors. While most RS-485 systems use data rates between 10 kbps and 100 kbps, some applications require data rates up to 250 kbps at distances of 4,000 feet and longer. The longer distances can be achieved by allowing small signal jitter of up to 5 or 10%.

Figure 9-3 Cable Length vs Data Rate Characteristic