Topology
Uses serial communication lines such as RS-485, RS-422 and RS-232, as well as TCP/IP networks, for data transmission.
When serial communication lines are used, a network can contain only one master device, which polls the other slave devices. No slave device can request or send data to another device on its own. The master can query each slave in turn or initiate a simultaneous broadcast to all slaves. A single network can include up to 247 slave devices.
A TCP/IP network can contain a virtually unlimited number of masters and slaves, and any device can act as both a master and a slave at the same time. The network may also include dedicated gateways that give devices on the TCP/IP network access to devices connected via a serial line, or give a master with a serial interface access to the TCP/IP network.
Modbus protocol modes
For data transmission over serial lines, the Modbus protocol provides two transmission modes: RTU and ASCII.
ASCII mode is intended for slow communication lines, where each byte of a packet is transmitted as two ASCII characters. A new packet begins with a special control character, and pauses of several seconds (depending on the settings) may occur between the characters of a single packet without causing transmission errors.
RTU mode roughly doubles the amount of data that can be transmitted over a serial line. Packet data is sent over the network in binary form, without modification. In RTU mode, a short interval of silence is maintained on the line before a packet is transmitted, and the packet is then sent as a continuous data stream. At 19200 bit/s this makes it possible to transmit up to 1400 bytes of data per second (with a repeated request for 126 variables) or to poll one or several devices up to 40 times per second (when requesting a single variable).
RS-485 network infrastructure for the Modbus protocol
In the typical case, when Modbus devices are connected via an RS-485 line, each device is daisy-chained to the others with a single cable. The cable must contain at least one twisted pair carrying the differential signals D0 and D1, plus a COM conductor (short for Common — the RS-485 receiver ground of all devices). The RS-485 receiver ground must be connected to protective earth only at the master controller. LT terminators — resistors rated at 120 ohms — are installed at each end of the communication line.
Cables
Two cable types can be used: specialized shielded industrial cables for RS-485 networks, or multi-pair shielded copper twisted-pair cable of category 5 or higher.
A wide range of specialized RS-485 cables is available today, typically with a conductor cross-section of 0.7-0.8 mm. To interconnect the devices, the chosen cable only needs one twisted pair plus one additional conductor; multi-pair cable is, however, perfectly acceptable.
Ordinary category 5 shielded twisted pair is entirely sufficient for RS-485 communication. Category 6 and 7 twisted-pair cables have a larger cross-section, and their use is justified only in networks with a large number of devices and/or lines longer than 300 m.