Modbus Rtu Cable Length

Go back to modbus rs485 cabling rules 9.

Modbus rtu cable length.

The end resistance must be used only at the ends of the main cable. However the cable recommended for long lengths can be expensive so if there is an existing ethernet infrastructure in place it may be more affordable to convert from bacnet ms tp or modbus rtu over rs 485 to bacnet tcp ip or modbus tcp ip over ethernet. The cable gauge only changes the cables impedance. Modbus is popular in industrial environments because it is openly published and royalty free.

Modbus has become a de facto standard communication protocol and is now a commonly available means of connecting industrial electronic devices. Under ideal conditions cable lengths of 4000 ft. Modbus is a comms protocol how to frame the messages checksums etc or actually two such protocols modbus rtu and modbus ascii. Figure 4 120 ohm resistance connection.

Of slaves also depend on the data logger specification. Modbus has been a standard for interfacing remote i o and plcs into the process dlc system for over 20 years allowing most legacy control systems to interface easily with modbus. Modbus cable interface quickly and easily with most existing plant control architectures by using belden cabling for modbus. Modbus is a data communications protocol originally published by modicon now schneider electric in 1979 for use with its programmable logic controllers plcs.

Common the signal common in eia tia standards. Connection to personal computer. Modbus rtu mode is the most common implementation but modbus tcp ip is gaining ground and ready to surpass it. Modbus is restricted to addressing 247 devices on one data link which limits the number of field devices that may be connected to a master station once again ethernet tcp ip proving the exception.

Modbus transmissions must be contiguous which limits the types of remote communications devices to those that can buffer data to avoid gaps in the. The maximum length of the communication cable should be no longer than 800m recommended the maximum number of devices that can be connected over this network can be 32 for modbus rtu including the master device trackso data logger in our case apart from the network no. The persmissible cable and drop will be determined by the electrical specification of the network on which the modbus is transmitted such as rs232 rs485 etc in conjunction with the baud rate faster rate shorter cable run. To move modbus into the 21st century an open modbus tcp ip specification was.

1200 m or more are possible. In a 2w or 4w rs485 modbus network signal and optional power supply common dce a modbus device for example a programmable controller adapter which implements an rs232 data circuit terminating equipment also named data communication equipment. If the total length of the main cable is less than 50 m termination resistances can be avoided at the ends of the main cable.