Download miễn phí Bài giảng Chuẩn truyền tin HART trong đo lường và điều khiển tự động mạng công nghiệp



Các liên kết PPI cho phép kéo dài đường truyền đến 3000m và MPI là 1500m, tối đa của
MPI lến đến 15 thiết bị. Tuy nhiên HART có nhược điểm là tốc độtruyền thấp, hiện nay
đến 4800 baud. Ngược lại, HART lại cho phép cảthiết bịtương tựvà sốcó thểlàm việc
trên cùng một mạng. Sau đây sẽtrình bày cụthểhơn những đặc điểm cơbản vềHART.
Tài liệu sau đây vừa trình bày những kiến thức vềHART, đồng thời cũng đưa ra những
mạch điện cụthểsửdụng cho các chuẩn đo lượng hiện đại hiện nay. Sinh viên có thểsử
dụng các phần kiến thức đó đểphục vụcho quá trình làm bài tập, đồán môn học, tốt
nghiệp và các công tác khác sau này


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ater may be necessary when the desired cable length exceeds limits set by the HART
Standards or when there are more than 15 devices. It is a two-port device placed between two
network segments. From a Protocol viewpoint it makes the two segments look like one network.
Although the repeater might also be equipped to repeat the analog 4-20 mA signal, our
discussion here is limited to a device that repeats only HART signals.
To preserve the HART timing, a repeater must repeat in real time. That is, it cannot store
messages for later forwarding. Delays must be limited to a few bit times if various timers are to
work reliably. Another limitation is noise. A repeater cannot simply amplify and re-transmit the
FSK signal, since this would also amplify and re-transmit noise on the network segment. This
narrows the choice of repeater architecture to one in which the incoming signal is demodulated
and then re-modulated. In addition, we must re-modulate with a "clean" signal. The output of a
demodulator will contain jitter due to noise and to the demodulation process. This jittery signal
should not be applied directly to the re-modulator, since this would result in a degraded signal to
one or more receiving devices. Instead, the demodulator output should be detected in UART
fashion (i.e., sample at mid-bit). Some logic is also needed to determine at start-up which bit is a
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start bit and to count out each 11 bits that have passed and identify the next start bit. A block
diagram of the repeater is shown in figure 2.14.
Figure 2.14 -- Repeater Block Diagram
Notice that, except for the line interface circuits and carrier detects, there is just a single signal
path that is turned around as needed. The direction can be determined by a relatively simple
state machine as illustrated in figure 2.15.
Chuẩn truyền tin HART- Highway Addressable Remote Tranducer
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CA = Carrier Detect on Network A
CB = Carrier Detect on Network B
Figure 2.15 -- Direction State Machine
There are 3 states: idle, B>A (Network B to Network A), and A>B (Network A to Network
B). Idle means that both ends are listening and the driver switch is not connected to either
network. CA, CB = 1,0 means that there is carrier at network A and not at network B. And so
on. If both carriers are present, the last state is retained.
A problem with all interface or bridge devices is the time it takes to turn the line around (or to
turn on a signal path). This is usually related to carrier detect and can often be done in less than
one character time. However, the loss of a character increases the number of preamble
characters that may be lost from 2 to 3 (see also Part 2: Startup Synchronization in HART). If
only 5 preamble characters were sent, as is often the case, this leaves only 2 as valid preamble.
Thus, the margin against missing the preamble is reduced. If another device, such as a 2nd
repeater were to be included somewhere in the network, there would likely be frequent failures to
recognize the start of a message. The change to a HART Slave to force it to require more than 5
preamble characters is usually minor. Therefore, the vendor of the Slave device may be willing
to increase the preamble size for the device in the interest of satisfying a customer. At the Master
end the software can be changed so that it always uses a preamble of greater than five characters,
ignoring whatever number the HART Slave says it should use.
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HART Gateways and Alternative Networks
Conventional HART, operating at 1200 bits/second and using a process loop as a network, has
been the focus throughout most of this book. However, the desires of HART process equipment
customers are seldom so limited. The need arises to connect
HART devices in unconventional ways, which is the subject of this section. These
unconventional methods can be divided roughly into two categories: those that still use HART
protocol or some of it, and those that connect HART with networks using entirely different
protocols. An interface between networks having different protocols is called a Gateway [2.8].
Examples would be HART to Devicenet [2.9], HART to Ethernet, HART to Modbus, etc. Some
of these unconventional methods are presented here, in no particular order.
PC as Gateway
About the easiest way to form a Gateway is with a personal computer. This is sometimes done
by systems integrators who need something up and running in the shortest possible time. As
personal computers become less expensive and more reliable, this
becomes more of an option. Small, inexpensive, single-board computers that implement DOS or
Windows CE can also make this a reasonable approach.
As an example, suppose you need an Ethernet-to-HART gateway. This is done as in figure
2.16.
Figure 2.16 -- Using PC As Gateway
You buy the 3 pieces of hardware and write the software. For applications that are more cost-
sensitive or that require greater reliability, a dedicated piece of hardware may be needed. Some
of these are examined as follows.
DeviceNet to HART
DeviceNet is becoming the de facto standard for high speed on-off sensing and control. HART
and DeviceNet have little in common, as indicated in the following table. We wouldn't expect
many similarities, since the two protocols are intended for different purposes.
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Network Type HART DeviceNet
Modulation Method FSK Baseband with Bit Stuffing
Data Rate 1200 BPS 125 kBPS and up
Application Transmit text and floating pt. numbers
Transmit discrete I/O (on-
off, open-closed)
Power and Signal on
Same Pair? Yes (2-wire is possible) No (uses 4 wires)
Number Addressees per
network 15 or 275,000,000,000(*) 63
Equality of Devices Master and Slave Devices equal but prioritized
Message Frame HART (UART-based) CAN
Possible Message Length Long Short but can be continued
Device Power Available Milliwatts to Watts Watts
(*) HART allows only 15 devices on a conventional current loop-type of network.
But there are 275e9 possible addresses.
Table 2.3 -- HART and DeviceNet Comparison
Suppose, however, that someone implementing DeviceNet needed to read the process variable of
a HART flow meter? A dedicated gateway between the two networks might be a possible way.
It might work like this: At the DeviceNet side, the gateway looks like a DeviceNet Server
(produces response) with Cyclic I/O Messaging at perhaps about once per second. At the HART
side it appears to the flowmeter as a HART Master. Once each second it queries the flowmeter to
get the process variable at the HART side and then transmits this variable to all consumers at the
DeviceNet side. At power up, the gateway device would go through the DeviceNet
configuration, receive its assigned DeviceNet address, and become a publisher of information.
Then it would determine the address of the HART flowmeter in preparation to read the process
variable.
A dedicated gateway would probably be designed to work with more than one HART Field
Instrument and would publish the process variable corresponding to each Field Instrument.
HART Over RS485/RS232
Conventional HART uses FSK modulation to translate the frequency spectrum to a region that
is compatible with 4-20 mA. In some applications where there is no current loop, the modulator
and demodulator are simply removed and HART is transmitted as a baseband signal. This is
illustrated in figure 2.17 for RS232 and RS485 ...

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