It is impor­tant not to con­fuse com­mu­ni­ca­tion pro­to­cols with stan­dards for elec­tri­cal char­ac­ter­is­tics of phys­i­cal com­mu­ni­ca­tion media. Dif­fer­ent com­mu­ni­ca­tion media are used depend­ing on project needs. The stan­dards’ main dif­fer­ences are the com­mu­ni­ca­tion speeds, the max­i­mum num­ber of con­nect­ed devices and the phys­i­cal dis­tance between con­nect­ed nodes.

RS-232The RS-232 (Rec­om­mend­ed Stan­dard 232) or also known EIA-232 (Elec­tron­ic Indus­tries Alliance-232) stan­dard is used only in point-to-point com­mu­ni­ca­tions, i.e. it only sup­ports com­mu­ni­ca­tions between two devices, which in the case of the Mod­bus pro­to­col would be a mas­ter and a slave device. The max­i­mum speed of RS-232 is around 115Kbp/s with a max­i­mum dis­tance between net­work devices of about 30m.

RS-422, or also TIA/EIA-422, was intend­ed to replace the old­er RS-232C stan­dard with a stan­dard that used dif­fer­en­tial sig­nalling to pro­vide high­er speeds, longer cable lengths and less noise. At short dis­tances, data trans­mis­sion rates can reach up to 10 Mbit/s. At low­er rates, data can be send through cables with a length of up to 1,500 meters.

RS-485The RS-485 (Rec­om­men­dad Stan­dard-485) or EIA-485 (Elec­tron­ic Indus­tries Alliance-485) stan­dard is one of the most wide­ly used stan­dards for ser­i­al com­mu­ni­ca­tion in indus­tri­al appli­ca­tions. The main dif­fer­ence with RS-232 is that it allows more than two devices on the net­work, enabling to have sev­er­al slaves. It achieves rates of up to 12Mbps and in rar­er cas­es up to 50Mbps, while the max­i­mum dis­tance with­in the net­work is 1200m, and the max­i­mum num­ber of devices on the net­work is 32.

Eth­er­netDepend­ing on the vari­a­tion used, tran­simis­sion speeds with Eth­er­net range from 100Mbps and up to 10Gbps, while max­i­mum dis­tance can vary from 100m to 200m depend­ing on project con­di­tions and the type of cable used. In some cas­es, it is pos­si­ble to use fiber optic net­works, which enable longer dis­tances and high­er com­mu­ni­ca­tion rates, as well as wire­less communication.