There is an inverse relationship between data rate and bus length, that is, the higher the data rate, the shorter the cable length. Conversely, the lower the data rate, the longer the cable may 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 4000 feet and longer. Longer distances are possible by allowing for small signal jitter of up to 5 or 10%.

Figure 8-4 Cable Length vs Data Rate Characteristic