Three different methods are specified for the bit-transmission layer:
With electrical transmission pursuant to EIA-485, twisted pair cables with impedances of 150 ohms are used in a bus topology. Bit rates from 9.6 kbit/s to 12 Mbit/s can be used. The cable length between two repeaters is limited from 100 to 1200 m, depending on the bit rate used. This transmission method is primarily used with PROFIBUS DP.
With optical transmission via fiber optics, star-, bus- and ring-topologies are used. The distance between the repeaters can be up to 15 km. The ring topology can also be executed redundantly.
With MBP (Manchester Bus Powered) transmission technology, data and field bus power are fed through the same cable. The power can be reduced in such a way that use in explosion-hazardous environments is possible. The bus topology can be up to 1900 m long and permits branching to field devices (max. 60 m branches). The bit rate here is a fixed 31.25 kbit/s. This technology was specially established for use in process automation for PROFIBUS PA.
For data transfer via sliding contacts for mobile devices or optical or radio data transmission in open spaces, products from various manufacturers can be obtained, however they do not conform to any standard.
PROFIBUS DP uses two core screened cable with a violet sheath, and runs at speeds between 9.6 kbit/s and 12 Mbit/s. A particular speed can be chosen for a network to give enough time for communication with all the devices present in the network. If systems change slowly then lower communication speed is suitable, and if the systems change quickly then effective communication will happen through faster speed. The RS485 balanced transmission used in PROFIBUS DP only allows 126 devices to be connected at once; however, more devices can be connected or the network expanded with the use of hubs or repeaters.
PROFIBUS PA is slower than PROFIBUS DP and runs at fixed speed of 31.2 kbit/s via blue sheathed two core screened cable. The communication may be initiated to minimise the risk of explosion or for the systems that intrinsically need safe equipment. The message formats in PROFIBUS PA are identical to PROFIBUS DP.
Note: PROFIBUS DP and PROFIBUS PA should not be confused with PROFINET.