Limitations of Modbus RTU
1.Maximum number of of devices - 32
Specification supports up to 247 slaves
Limited to 32 devices,per RS-485 standard
2.Limited distances
RS 232 - 25 ft (7.5 m) RS 485- 4000 ft(1.2 km)
3.Data access constraints
Single modbus master support only No provision for simultaneous SCADA /diagnostic access Difficult to implement “bumpless” SCADA redundancy