HART & MODBUS PROTCOL

1. Write the salient feature of HART protocol which is generally not found in other
protocol
•The HART Communication Protocol (Highway Addressable Remote Transducer) is a hybrid
analog digital industrial automation open protocol.
• legacy 4–20 mA analog instrumentation current loops, sharing the pair of wires used by the
analog-only host systems.
• HART is widely used in process and instrumentation systems ranging from small automation
applications up to highly sophisticated industrial applications.
Unit-5 MODBUS & HART Module
applications up to highly sophisticated industrial applications.
• HART protocol has made a good transition protocol.
•The protocol was developed by built off the Bell 202 early communications standard in the mid-
1980s.
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2. Discuss the features of HART for smart instrumentation
Smart (or intelligent) instrumentation protocols are designed for applications where
actual data is collected from instruments, sensors, and actuators by digital communication
techniques. These components are linked directly to programmable logic controllers
(PLCs) and computers.
The HART (highway addressable remote transducer) protocol is a typical smart
instrumentation fieldbus that can operate in a hybrid 4–20 mA digital fashion.instrumentation fieldbus that can operate in a hybrid 4–20 mA digital fashion.
HART is, by no means, the only protocol in this sphere. There are hundreds of smart
implementations produced by various manufacturers – for example, Honeywell – that
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3. compete with HART. This chapter deals specifically with HART.
At a basic level, most smart instruments provide core functions such as:
• Control of range/zero/span adjustments
• Diagnostics to verify functionality
• Memory to store configuration and status information (such as tag numbers
etc)
Accessing these functions allows major gains in the speed and efficiency of the
installation and maintenance process. For example, the time consuming 4–20 mA loop
check phase can be achieved in minutes, and the device can be readied for use in the
process by zeroing and adjustment for any other controllable aspects such as the dampingprocess by zeroing and adjustment for any other controllable aspects such as the damping
value.
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4. HART protocol
This protocol was originally developed by Rosemount and is regarded as an open
standard, available to all manufacturers.
Its main advantage is that it enables an instrumentation engineer to keep the existing
4–20 mA instrumentation cabling and to use, simultaneously, the same wires to carry
digital information superimposed on the analog signal.
This enables most companies to capitalize on their existing investment inThis enables most companies to capitalize on their existing investment in
4–20 mA instrumentation cabling and associated systems and to add further capability
of
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5. HART without incurring major costs.
HART is a hybrid analog and digital protocol, as opposed to most fieldbus systems,
which are purely digital.
The HART protocol uses the frequency shift keying (FSK) technique based on the Bell
202 communications standard. Two individual frequencies of 1200 and 2200 Hz,
representing digits 1 and 0 respectively, are used.
The average value of the sine wave (at the 1200 and 2200 Hz frequencies), which is
superimposed on the 4–20 mA signal, is zero. Hence, the 4–20 mA analog information is
not affected.
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6. The HART protocol can be used in three ways:
• In conjunction with the 4–20 mA current signal in point-to-point mode
• In conjunction with other field devices in multidrop mode
• In point-to-point mode with only one field device broadcasting in burst
Mode
Traditional point-to-point loops use zero for the smart device polling address.
Setting the smart device polling address to a number greater than zero creates a multidrop
loop.loop.
The smart device then sets its analog output to a constant 4 mA and communicates only
digitally.
The HART protocol has two formats for digital transmission of data:
• Poll/response mode
• Burst (or broadcast) mode
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7. In the poll/response mode the master polls each of the smart devices on the highway
and requests the relevant information.
In burst mode the field device continuously transmits process data without the need for the
host to send request messages. Although this mode is fairly fast (up to 3.7 times/second), it
cannot be used in multidrop networks.
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8. Physical layer
The physical layer of the HART protocol is
based on two methods of communication.
• Analog 4–20 mA
• Digital frequency shift keying (FSK
•The basic communication of the HART protocol is the 4–20 mA current system.
This analog system is used by the sensor to transmit an analog value to the HART PLC or
HART card in a PC.
• In a 4–20 mA system, the sensor outputs a current value somewhere between 4 and 20 mA• In a 4–20 mA system, the sensor outputs a current value somewhere between 4 and 20 mA
that represents the analog value of the sensor.
• For example, a water tank that is half full – say 3400 kiloliters – would put out 12 mA. The
receiver would interpret this 12 mA as 3400 kiloliters. This communication is always point-to-
point, i.e. from one device to another. It is not possible to do multidrop communication using
this method alone.
•If two or more devices put some current on the line at the same time, the
resulting current value would not be valid for either device
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9. For multidrop communications, the HART protocol uses a digital/analog modulation
technique known as frequency shift keying (FSK).
This technique is based on the Bell 202 communication standard. Data transfer rate is 1200 baud
with a digital ‘0’ frequency (2200 Hz) and a digital ‘1’ frequency (1200 Hz).
Category 5 shielded, twisted pair wire is recommended by most manufacturers. Devices can be
powered by the bus or individually.
If the bus powers the devices, only 15 devices can be connected. As the average DC current ofIf the bus powers the devices, only 15 devices can be connected. As the average DC current of
an AC frequency is zero, it is possible to place a 1200 Hz or 2200 Hz tone on top of a 4–20 mA
signal.
The HART protocol does this to allow simultaneous communications on a multidrop system
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13. Data link layer
Two-dimensional error checking, including vertical and longitudinal parity checks, is
implemented in each frame. Each character or frame of information has the following
parameters:
• 1 start bit
• 8 data bits
• 1 odd parity bit
• 1 stop bit
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15. Application layer
The application layer allows the host device to obtain and interpret
field device data.
There are three classes of commands:
• Universal commands
• Common practice commands
• Device specific commands
Examples of these commands are listed below.
Universal commands
• Read manufacturer and device type• Read manufacturer and device type
• Read primary variable (PV) and units
• Read current output and per cent of range
• Read up to 4 predefined dynamic variables
• Read or write 8-character tag, 16-character descriptor, date
• Read or write 32 character message
• Read device range, units and damping time constant
• Read or write final assembly number
• Write polling address
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17. Common practice commands
• Read selection of up to 4 dynamic variables
• Write damping time constant
• Write device range
• Calibrate (set zero, set span)
• Set fixed output current
• Perform self-test
• Perform master reset
Trim pv zero
• Write PV units
• Trim DAC zero and gain
• Write transfer function (square root/linear)• Write transfer function (square root/linear)
• Write sensor serial number
• Read or write dynamic variable assignments
Instrument specific commands
• Read or write low flow cut-off value
• Start, stop or clear totalizer
• Read or write density calibration factor
• Choose PV (mass flow or density)
• Read or write materials or construction
information
• Trim sensor calibration
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18. Troubleshooting
Beside the actual instruments that require calibration, the only major problem that can
occur with HART is the cable length calculation.
The HART protocol is designed to work over existing analog signal cables but the
reliable length of cable depends on:
• Loop load resistance
• Cable resistance
• Cable capacitance
• Number and capacitance of field devices
• Resistance and position of other devices in the loop
The main reason for this is that network must pass the HART signal frequencies
(1200 Hz and 2200 Hz) without excessive loss or distortion. A software package such as
H-Sim can be used to calculate whether you are operating with the correct signal level. In
addition, you should ensure that you have the correct bandwidth of at least 2500 Hz. You
can do this by ensuring that the product of the cable resistance and capacitance is less
than 65 microseconds.
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19. Modbus® is a transmission protocol (note – a protocol only), developed by Gould
Modicon (now Schneider Electric) for process control systems. It is, however, regarded
as a ‘public’ protocol and has become the de facto standard in multi-vendor integration.
In contrast to other buses and protocols, no physical (OSI layer 1) interface has been
defined.
Modbus is a simple, flexible, publicly published protocol, which allows devices to
exchange discrete and analog data. End users are aware that specifying MODBUS as the
required interface between subsystems is a way to achieve multi-vendor integration
with the most purchasing options and at the lowest cost.
Small equipment makers are also aware that they must offer MODBUS with EIA-232
and/or EIA-485 to sell their equipment to system integrators for use in larger projects.
System integrators know that MODBUS is a safe interface to commit to, as they can be
sure of finding enough equipment on the market to both realize the required designs
and handle the inevitable ‘change orders,’ which come along. However, Modbus suffers
from the limitations imposed by EIA-232/485 serial links, including the following:
• Serial lines are relatively slow – 9600 to 115 000 baud means only
0.010 Mbps to 0.115 Mbps. Compare that to today’s common ‘control
network’ speeds of 5 to 16 Mbps – or even the new Ethernet speeds of 100
Mbps, and 1 Gbps and 10 Gbps
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20. Modbus overview 97
• While it is easy to link 2 devices by EIA-232 and 20–30 devices by EIA-485, the only
solution to link 500 devices with EIA-485 is a complex hierarchy of masters and slaves in a
deeply nested tree structure. Such solutions are never simple or easy to maintain.
• Serial links with Modbus are inherently single-master designs. That means,
only one device can talk to a group of slave devices – so only that one
device (the master) is aware of all the current real-time data.
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21. Modbus has the advantage of wide acceptance among
instrument manufacturers and users with many systems in operation. It can therefore be
regarded as a de facto industrial standard with proven capabilities.
Certain characteristics of the Modbus protocol are fixed, such as frame format, frame
sequences, handling of communications errors and exception conditions and the functions
performed. Other characteristics are selectable.
These are transmission medium, transmission characteristics and transmission mode, viz. RTU or
ASCII. The user characteristics are set at each device and cannot be changed when the system isASCII. The user characteristics are set at each device and cannot be changed when the system is
running.
The two transmission modes in which data is exchanged are:
• ASCII – readable; used, for example, for testing. (ASCII format)
• RTU – compact and faster; used for normal operation. (Hexadecimal
format)
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22. The RTU mode (sometimes also referred to as Modbus-B for Modbus Binary) is the
preferred Modbus mode. The ASCII transmission mode (sometimes referred to as
Modbus-A) has a typical message that is about twice the length of the equivalent RTU
message.
Modbus also provides an error check for transmission and communication errors.
Communication errors are detected by character framing, a parity check, a redundancy
check or a sixteen bit cyclic redundancy check (CRC-16). The latter varies depending on
whether the RTU or ASCII transmission mode is being used.whether the RTU or ASCII transmission mode is being used.
Modbus packets can also be sent over local area and wide area networks by
encapsulating the Modbus data in a TCP/IP packet
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23. address field between 1 and 247; although practical limitations will limit the maximum
number of slaves.
.
The second field in each message is the function field, which also consists of a single
byte of information. In a host request, this byte identifies the function that the target PLC
is to perform.
If the target PLC is able to perform the requested function, the function field of its
response will echo that of the original request. Otherwise, the function field of the request
will be echoed with its most-significant bit set to one, thus signaling an exception
response
The third field in a message frame is the data field, which varies in length according toThe third field in a message frame is the data field, which varies in length according to
which function is specified in the function field. In a host request, this field contains
information the PLC may need to complete the requested function. In a PLC response,
this field contains any data requested by that host.
The last two bytes in a message frame comprise the error-check field. The numeric
value of this field is calculated by performing a cyclic redundancy check (CRC-16) on the
message frame. This error checking assures that devices do not react to messages that
may have been damaged during transmission.
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25. Read coil or digital output status (function
code 01)
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26. This function allows the host to obtain the ON/OFF status of one or more logic coils in
the target device
The data field of the request consists of the relative address of the first coil followed
by the number of coils to be read
The coil data bytes are packed with one bit for
the status of each consecutive coil
(1=ON, 0=OFF).
In the following example, the host requests the
status of coils 000A (decimal 00011)
and 000B (decimal 00012). The target device’s
response indicates both coils are ON.
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27. Read input status
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28. In the following example, the host requests the status of discrete inputs with
the relative address of the first discrete
input followed by the number of discrete inputs to be read. The data field of the
response
frame consists of a count of the discrete input data bytes followed by that many
bytes of
discrete input data.
The discrete-input data bytes are packed with one bit for the status of each
consecutive
discrete input (1=ON, 0=OFF).
In the following example, the host requests the status of discrete inputs with
offsets
0000 and 0001 hex i.e. decimal 10001 and 10002. The target device’s response
indicates
that discrete input 10001 is OFF and 10002 is ON.
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29. Read Holding Register
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30. The data field of the request frame consists of the relative address of the first holding
register followed by the number of registers to be read. The data field of the response
time consists of a count of the register data bytes followed by that many bytes of holding
register data.
The contents of each requested register (16 bits) are returned in two consecutive data
bytes (most significant byte first).
In the following example, the host requests the contents of holding register hexadecimalIn the following example, the host requests the contents of holding register hexadecimal
offset 0002 or decimal 40003. The controller’s response indicates that the numerical
value of the register’s contents is hexadecimal 07FF or decimal 2047.
The first byte of theresponse register data is the high order byte of the first addressed
register
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31. Reference:
Book:Practical industrial data Networks(Designing ,Installation,Trouble shooting)
Authors:Steve Mackay CPEng, BSc(ElecEng), BSc(Hons), MBA
Edwin Wright MIPENZ, BSc(Hons), BSc(Elec Eng)
DeonReynders Pr.Eng, BSc(ElecEng)(Hons), MBA
John Park ASD
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