PROFIBUS Manual

RS 485 according to ANSI TIA/EIA 485-A is a proven and much used transmission technology that can be used to connect up to 32 stations across distances of up to 1.2 km with one segment (= one piece of cable). RS 485 transmission technology uses two-core cable. Installation is easy and allows for fast data communications up to a bit rate of 12 Mbit/s (the maximum distance is reduced as the bit rate increases).

Does this mean that only 32 stations can be connected to a PROFIBUS network?. No, the limit of 32 stations via RS 485 is overcome with repeaters and FOC connections. The individual segments then remain electrically isolated. The whole network can therefore comprise significantly more stations and bridge greater distances than specified by RS 485.  

Within any one electrical segment, all devices and repeaters count toward the 32-station limit. This means that, if two repeaters are coupled to one segment, only 30 devices can be connected to that segment. In practice it is advisable always to hold 10% of each segment's capacity in reserve for future expansions and/or the connection of diagnostic tools.

The PROFIBUS Organization provides detailed assembly guidelines for electrical transmission technology according to RS 485.