Profibus-DP

9.4 Profibus-DP timingThe cycle time of a Profibus-DP network running at 1.5 Mbaud containingUD73 and MD24 slaves can be approximately calculated as follows:Cycle time (ms) = 0.63 * Nwhere:N = number of nodes on the network.Consider a network with 50 nodes and 4 words per node. There is a mastercontroller node, which transmits to and receives from each node on everycycle. The network length is 50m. The theoretical minimum networkupdate time is 31.50ms. The minimum update time for an identical networkusing Interbus-S is also given below.Network Min update time (ms) Network data rateProfibus-DP 31.50 1.5MInterbus-S 10.95 500kAs can be seen from the above tables, Profibus-DP does not achieve as highupdate rate as Interbus-S. Profibus-DP uses node addresses, which take upadditional bandwidth on the network. The Profibus-DP network uses only asingle twisted pair, thus requiring all data to travel along the same cable.The advantages of a Profibus-DP network are that the addressing structureallows for easy expansion of existing networks. Nodes can be added, andsimply assigned the next available address. No changes are required toexisting program, nor to the IN and OUT data arrays within the PLC. Anotheradvantage is that multiple master controller configurations are permitted onthe same network.Profibus–DPIssue code: pbnu2 31