Modbus RTU Network Characteristics

Overview

This section defines the characteristics of the Modbus RTU network over serial communication line. The Modbus RTU unit complies with the serial line specification. For information on serial line specification and implementation, see Serial Line Specification and Implementation Guide published on www.Modbus.org.

Standard Diagram

The standard diagram corresponds to the Modbus RTU specification on the www.Modbus.org site and in particular to the two-wire multidrop serial bus diagram.

The simplified diagram is as follows:

Characteristics for Connection to Modbus RTU RS 485 Bus

The RS 485 standard allows variants with characteristics like:

Line terminator
Number of servers
Bus length

Characteristics	Value
Maximum number of stations (without repeater)	32 stations, which is 31 servers
Type of trunk cable	Single, shielded, twisted pair cable, with 120 Ω characteristic impedance, and at least a third conductor
Maximum bus length	1000 m (3300 ft) at 19200 bps
Maximum length of tap-offs	20 m (66 ft) for one tap-off 40 m (131 ft) divided by the number of tap-offs on the multiple junction box
Line terminator	120 Ω resistor +/- 5% at both ends of the bus
Common polarity	Common polarity is connected to the protective ground in at least one point on the bus.