| Modbus Overview |
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 The Modbus protocol was originally developed by Modicon in 1978 to exchange information between products on the factory floor. This protocol became a de facto standard for exchanging data and communication information between PLC systems.
Modbus devices communicate over a serial network in a master/slave (request/response) type relationship using one of two transmission modes: ASCII (American Standard Code for Information Interchange) mode or RTU (Remote Terminal Unit) mode.
In ASCII mode, two eight-bit bytes of information are sent as two ASCII characters. The primary advantage of ASCII mode is the flexibility of the timing sequence. Up to a one second interval can occur between character transmissions without causing communication errors.
In RTU mode, data is sent as two four-bit, hexadecimal characters, providing for higher throughput than in ASCII mode for the same baud rate.
Enhancements to Modbus include Modbus Plus and Modbus/TCP protocols, both of which allow Modbus information to be encapsulated in a network structure to support peer-to-peer communications. Modbus Plus communicates via a single twisted pair of wires and uses a token passing sequence for peer communication sequences. Modbus/TCP is an open standard designed to facilitate Modbus message transfer using TCP/IP protocol and standard Ethernet networks.
TopWorx's Modbus devices are designed to operate as slave devices on a Modbus network for discrete valve control.
| Modbus Network Highlights |
| Type of Network |
|
| ASCII/RTU |
Device Bus |
| ModbusPlus |
Control Bus |
| Physical Media |
Shielded twisted pairs in one shielded cable |
| Network Topology |
Bus, tree, star with drops |
| Maximum Devices |
|
| ASCII/RTU |
One to one communications |
| ModbusPlus |
32 (up to 64 with repeater) |
| Maximum Distance |
|
| ASCII/RTU |
350m |
ModbusPlus
(up to 3 repeaters may be used) |
1500m (6000m with repeaters)
(min. 1m between devices) |
| Communication Methods |
|
| ASCII/RTU |
Master-Slave Query-Response Cycle
(LRC error checking for ASCII)
(CRC error checking for RTU) |
| ModbusPlus |
Peer to Peer (Token passing logical ring) |
| Primary usage |
|
| ASCII/RTU |
Serial Communications for PLC,
Variable Speed Drives, Control Systems, etc. |
| ModbusPlus |
Linking MODBUS and/or RS232/RS485 devices in a peer-to-peer network |
| Power Supply |
12VDC, not used for devices |
| Power and Communications |
Communications only on bus
12VDC, max. 300mA (100mA typical) |
| Device Power Supply |
Devices must be powered separately from communications bus |
Wiring Types
(types used varies on application) |
Shielded Twisted Pair
#18AWG (0.8mm) |
| Grounding aspects |
Floating communications bus |
| Shielding |
Grounded at one end |
| Area Classification |
General Purpose |
| Device Addressing |
Switch or software selectable |
| Governing Body |
MODBUS.ORG |
| Website |
www.modbus.org |
| Conventional I/O System |
Modbus Network |
Advantages
|
Advantages
|
| Technology is already understood |
Well understood and documented protocol |
| Slightly lower device cost |
Widely supported protocol by many host PLC, DCS and process systems |
| Independent wiring from devices to the control system means wiring problems with one device don’t affect other field devices |
Protocol is already used in many facilities |
Drawbacks
|
Drawbacks
|
| Higher installed cost |
Limited use as a device bus |
| Point-to-point wiring is expensive |
Limited diagnostic capabilities for device applications |
Many wiring connections:
| - |
are labor intensive to install |
| - |
create many points of failure |
| - |
increase complexity when troubleshooting |
| - |
require large amounts of cabinet or rack space for installation of terminal blocks |
| - |
create time-consuming initial checkout and startup |
|
Separate power required for device operations |
Expansion requires duplicating the entire wiring scheme for each additional point
|
Recommended
|
|
When similar Modbus devices are being used |
|
When Modbus network is pre-existing |
|
When Modbus protocol is well understood and is being used extensively as a facility standard |
|
| TopWorx Comments |
| Strengths |
Modbus is well accepted and well understood by many in the world of industrial communications.
Modbus delivers cost-effective simplicity with a bit of added functionality supporting limited diagnostic information. |
| Limitations |
The selection and availability of field devices that support the Modbus protocol is limited, especially in the process industries.
From the field device perspective, Modbus is a bit cumbersome to configure with today’s advanced process control systems.
Modbus does not support field devices effectively in intrinsically safe applications. |
| When to Use Modbus |
Generally speaking, TopWorx recommends Modbus when:
| - |
device populations are primarily discrete |
| - |
end users already have an existing control system that supports Modbus |
| - |
end users have a legacy control system that does not support other common protocols |
| - |
plants are not intrinsically safe |
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