Modbus is a communication protocol which is used with programmable logic controllers. This communication protocol was published by a firm named Modicon in 1979. It has now become the most de facto standard protocol in the industry. Modbus is now very commonly used as a means to connect the industrial electronic devices. This protocol is very easily available as it is very extensively used. There are certain important reasons for the use of Modbus rather than use of other communication protocol. These are as follows:
1.This protocol is published openly and is royalty free.
2.It is an easiest way to setup an industrial network.
3.This protocol moves raw bits without any restrictions.
RS-485 is name of standard for data transmission that use only 2 wires.
One wire for sending the data (TxD) and another wire for receiving data (RxD).
RS-485 can be configured to work in Half-duplex or Full-duplex mode.
The example of equipments that use Half-duplex communication are
Fax, Printer, Intercom , walkie-talkie , etc.
Full-duplex equipments such as Telephone or Mobile phone.
RS-485 standard was specified by EIA (Electronics Industries Association)
The advantage of RS-485.
Nowadays, the data transmission used in industrial sector are almost RS-485 bus.
The reasons that people use RS-485 bus in industry instead of RS232 bus are as
1. Price of equipment to create RS-485 bus system is cheap and it require
only +5V (VDD) power supply.
2. The ability of inter-connection, RS-485 can connect with a large number of
internal equipments and it also support the work like networking system.
It can support up to 32 drivers (transmitter) and 32 receivers while RS-232 can
connect to only 1 transmitter and 1 receiver.
3. The distance that RS-485 bus can connect to the equipment is as far as 4,000 feet
or 1.2 kilometre while RS-232 is limit to only 50 to 100 feet.
4. Baud rate of RS-485 is up to 10 Mbps (in theory only) compare to the baud rate
of RS-232 that is only 20 Kbps. At the longest signal-carrier line, the data will be
transmitted at the slowest baud rate.
Characteristic of RS-485 bus.
- The longest range of signal line is 4,000 feet with baud rate of 90 Kbps and
and with the length of 50 feet , the baud rate can be up to10 Mbps.
The signal carrier wire that can be used on RS-485 bus system are
twisted pair and triaxial.Twisted pair line is work fine on RS-485 bus and it also cheap. It comprise
of 2 signal wires that covered by an insulator.Below is picture of twisted pair cable.
Twisted pair can decrease low-frequency noise that arise from magnetic induction
of the carrier wire because twisting two wires cause the confutation of magnatic field
produced from each wire.Otherwise twisted pair also help to decrease the effect that come from external noise too.
Another wire that admired to use on RS-485 bus is Triaxial.
It can protect the noise from the external system but its price is more expensive
than Twisted pair.Triaxial cable is similar to coaxial cable except that it consist of two shield-layers
(inner shield and outer shield) while coaxial is consist of one shield layer.
(shield layer is used for prevent the external noise that may disturb signal transmission)
Both triaxial and coaxial have the “center conductor” that is a wire used for transmit
the signal. Below are the pictures of Triaxial cable and Coaxial cable.
Pictures of this article are come from