MODBUS

It is design as a means of communicating with many devices over a

single twisted pair wire. Is the most popular industrial protocol being

used today, for good reasons. It is simple, inexpensive, universal and

easy to use. Even though MODBUS has been around since past century-

nearly 30 years almost all major industrial instrumentation and

automation equipment vendors continue to support it in new products.

MODBUS is still the protocol that most vendors choose to implement in

new and old devices.

MODBUS AS A MASTER SLAVE

MODBUS is a master slave system, where the master communicates

with one or multiple slave.

The master typically is a PLC [programmable logic controller], PC, DCS

[distribution control system] or RTU [remote terminal unit].

A MODBUS RTU slaves are often field devices, all of which connect to

the network in a multidrop configuration. When a MODBUS RTU master

wants information from a device, the master sends a message that

contains the devices address, data it wants, and a checksum for error

detection. Every other device on the network sees the message, but

only the device is addressed responds.

MODBUS/TCP

MODBUS/TCP is often referred to as MODBUS over Ethernet.

MODBUS/TCP is simply MODBUS packet encapsulated in standard

TCP/IP packets. This enables MODBUS/TCP device to immediately and

easily communicate over existing Ethernet and fiber network.

MODBUS/TCP also allows many more addresses than RS485, the use of

multiple masters and speeds in the gigabit range. MODBUS/TCP

networks can have as many slaves as the physical layer can handle.

Often this number is somewhere around 1,024.

Ethernets rapid adoption within the process control and automation

industry has allowed MODBUS/TCP to become the most widely used,

fastest growing and supported industrial protocol over Ethernet.

Although PLC vendors of all size have adopted their own proprietary

protocols over Ethernet, almost all of them support MODBUS/TCP.

RS 232

RS232 is a popular communications protocol for connecting modem

and data acquisition devices to computer.RS232 can be plugged straight

into the computers serial port [also known as the COM or COMMS port]

examples of data acquisition device are GPS receivers, data loggers and

other measurement instruments.

RS STANDARD

The standard of RS232 was defined to connect a computer to a

modem. Any other used is outside of the standard.

RS232 serial communications used fixed voltage values and generally

allow you to connect to only one device. The fixed voltage signals are

subject to noise and distance problems.

RS 485

RS 485 specifications include the ability to connect to 32 devices.

Modem RS 485 device driver can often exceed this standard. Not all

converters are equal, however. Extensive use of our converters by our

customer has shown our converters to be reliable even when other

converters are not.

FIELDBUS

One of the most important aspects of foundation field bus is its ability

to collect and deliver vast amounts of information. Not only process

variables and control signals. But other types of instrument and

process data as well.