Modbus is a serial communications protocol published by
Modicon in 1979 for use with its programmable logic controllers (PLCs). It has become a de
facto standard communications protocol in industry, and is now the most commonly available means of connecting industrial electronic devices. The main reasons for the extensive use of Modbus over other communications protocols
are:
- it is openly published and royalty-free
- it can be implemented in days, not months
- it moves raw bits or words without placing many restrictions on vendors
Modbus allows for communication between many devices connected to the same network, for example a system that measures
temperature and humidity and communicates the results to a computer. Modbus is often used to
connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data
acquisition (SCADA) systems. Versions of the Modbus protocol exist for serial port and Ethernet.
For serial connections, two variants exist, with different representations of numerical data and slightly different protocol
details. Modbus RTU is a compact, binary representation of the data. Modbus ASCII is human readable, and more verbose. Both of
these variants use serial communication. The RTU format follows the commands/data
with a cyclic redundancy check checksum, while the ASCII format uses a
longitudinal redundancy check checksum. Nodes configured for the RTU
variant will not communicate with nodes set for ASCII, and the reverse.
For connections over TCP/IP (e.g. ethernet), the more recent variant Modbus/TCP exists. It is easier to implement than Modbus/ASCII or Modbus/RTU
because it does not require a checksum calculation.
Data model and function calls are identical for all three communication protocols; only the encapsulation is different.
An extended version, Modbus Plus (Modbus+ or MB+), also exists, but remains proprietary to Modicon. It requires a dedicated
co-processor to handle fast HDLC-like token rotation. It uses twisted pair at 1 Mbit/s and includes transformer isolation at each
node, which makes it transition/edge triggered instead of voltage/level triggered. Special interfaces are required to connect
Modbus Plus to a computer, typically a card made for the ISA, PCI or PCMCIA bus.
Each device intended to communicate using Modbus is given a unique address. Any device can send out a Modbus command, although
usually only one master device does so. A Modbus command contains the Modbus address of the device it is intended for. Only the
intended device will act on the command, even though other devices might receive it. All Modbus commands contain checking
information, ensuring that a command arrives undamaged. The basic Modbus commands can instruct an RTU to change a value in one of
its registers, as well as commanding the device to send back one or more values contained in its registers.
There are many modems that support Modbus. Some of them were specifically designed for this protocol. Different
implementations use wires, wireless communication and even SMS or GPRS. Typical problems the designers have to overcome include high latency and timing
problems.
Variations:
Almost all implementations have variations from the official standard. Different varieties may not communicate correctly
between different suppliers equipment. Some of the most common variations are:
- Data Types
- Floating Point IEEE
- 32 bit integer
- 8 bit data
- mixed data types
- bit fields in integers
- multipliers to change data to/from integer. 10, 100, 1000, 256 ...
- Protocol extensions
- 16 bit slave addresses
- 32 bit data size (1 address = 32 bits of data returned.)
- word swapped data
Limitations
Modbus was designed in the late 1970's to communicate to programmable logic
controllers. The number of data types are limited to those understood by PLCs at the time. Large binary objects are not
supported. No standard way exists for a node to find the description of a data object, for example, to determine if a register
value represents a temperature between 30 and 175 degrees. Since Modbus is a master/slave protocol, there is no way for a field
device to "report by exception" - the master node must routinely poll each field device, and look for changes in the data. This
consumes bandwidth and network time in applications where bandwidth may be expensive, such as over a low-bit-rate radio link.
Modbus is restricted to addressing 254 devices on one data link, which limits the number of field devices that may be connected
to a master station. Modbus transmissions must be contiguous which limits the types of remote communications devices to those
that can buffer data to avoid gaps in the transmission.
External links
Free software exists for Modbus:
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