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Since you can send more data in small­er pack­ets and unlike Mod­bus, it is an event-dri­ven pro­to­col, mean­ing that net­work devices are able to trans­mit unso­licit­ed respons­es and con­ti­nu­ity is not required, using DNP3 can save lots of band­width. While Mod­bus has a spe­cif­ic vari­ant for TCP/IP com­munca­tion, DNP3 needs to be wrapped with­in TCP/IP. This means that RTUs can send updates when a change of sta­tus hap­pens, with­out wait­ing to be polled by the master.īoth pro­to­cols can be used over var­i­ous media, such as RS-232, RS-485, and TCP/IP. Anoth­er dif­fer­ence is that DNP3 also sup­ports unso­licit­ed mes­sag­ing. Mod­bus is an appli­ca­tion lay­er pro­to­col, where­as DNP3 con­sists of both an appli­ca­tion and data link lay­er. Depend­ing on the avail­able ports, slaves/servers usu­al­ly oper­ate with one or the oth­er protocol. Nowa­days, the ver­sa­til­i­ty, speed and scal­a­bil­i­ty of Eth­er­net net­works are usu­al­ly pre­ferred over ser­i­al, but some still pre­fer the sim­plic­i­ty, wiring and cost effi­cien­cy of Mod­bus RTU. Any addi­tion­al mas­ters would destroy the net­work com­mu­ni­ca­tions. With Eth­er­net, you also to deal with less ter­mi­na­tion and con­fig­u­ra­tion issues. The biggest advan­tage of Mod­bus TCP/IP over Mod­bus RTU is that you can use more than one polling device, unlike Mod­bus RTU which only allows for a sin­gle mas­ter device. Alter­na­tive­ly, there are also gate­ways that con­vert Mod­bus RTU into a Mod­bus TCP mes­sages by chang­ing th e mes­sage bytes. In the Mod­bus TCP stan­dard a 6 byte MBAP head­er is added to the mes­sage and the two byte CRC is removed. The Mod­bus RTU mes­sage is usu­al­ly sent through a ser­i­al port con­nec­tor and then con­vert­ed by an Ethernet/ mod­bus gateway. Mod­bus RTU over TCP refers to tak­ing the RTU mes­sage is encap­su­lat­ed for E ther­net com­mu­ni­ca­tion with­out actu­al­ly chang­ing the mes­sage itself. Mes­sages begin with a Slave ID (one byte) and end with a CRC (two bytes). Mod­bus RTU is meant for ser­i­al com­mu­ni­ca­tions (RS232 or RS485 are the most com­mon). Modbus TCP/IP vs Modbus RTU over TCP vs Modbus RTU Mod­bus over UDP - In rar­er cas­es Mod­bus RTU is used over UDP on IP net­works to save the over­head required for TCP. Mod­bus over TCP/IP or Mod­bus over TCP or Mod­bus RTU/IP - This vari­ant encap­su­lates Mod­bus RTU for its use in TCP/IP/Ethernet networks. This Mod­bus vari­ant is used for com­mu­ni­ca­tions over TCP/IP net­works and does not require a check­sum cal­cu­la­tion, as low­er lay­ers already pro­vide it. The ASCII for­mat relies on lon­gi­tu­di­nal redun­dan­cy check (LRC) check­sum for error checking. The Mod­bus ASCII for­mat is uti­lized for ser­i­al com­mu­ni­ca­tion and makes use of ASCII char­ac­ters for the com­mu­ni­ca­tion pro­to­col. A Mod­bus RTU mes­sage must be trans­mit­ted con­tin­u­ous­ly and are seper­at­ed by inac­tive (silent) periods. The pro­to­col uses a com­pact bina­ry rep­re­sen­ta­tion of the data and tails the mes­sages with a cyclic redun­dan­cy check­sum (CRC) to check for errors and ensure data reli­a­bil­i­ty. over RS-232, RS-422 or RS-485 ser­i­al ports) and thus the most com­mon one. This ver­sion is used for ser­i­al com­mu­ni­ca­tion (e.g.

Mod­bus does not pro­vide pro­tec­tion against unau­tho­rized com­mands or data interception.Mod­bus trans­mis­sions must be con­tigu­ous, which requires remote com­mu­ni­ca­tion devices to be able to store data to avoid gaps in trans­mis­sion.There is no stan­dard way for a node to find the descrip­tion of a data object, for exam­ple, to deter­mine if a reg­is­ter val­ue rep­re­sents a volt­age in a cer­tain range.The Mod­bus pro­to­col can only address a max­i­mum of 254 devices on a data link, lim­it­ing the num­ber of devices that can con­nect to a mas­ter sta­tion (again with the excep­tion of Eth­er­net TCP/IP).Since Mod­bus was designed for the com­mu­ni­ca­tion with PLCs in the late 1970s, the num­ber of data types is lim­it­ed to those under­stood by PLCs at that time.This delays the net­work and con­sumes band­width, which can be a prob­lem in appli­ca­tions where band­width is expen­sive, such as a low bit rate radio link. Thus the mas­ter node must rou­tine­ly poll each field device and look for the data changes itself.

Since Mod­bus is a master/slave based pro­to­col, it is not pos­si­ble for a field device to report changes by itself (with the excep­tion of Mod­bus over Eth­er­net net­works).What are common limitations with Modbus ?