Chuẩn truyền tin Hart trong đo lường và điều khiển tự động mạng công nghiệp

er Pin Connect to 3.3 volt or 5.0 volt.
16 IRXA In Input to Receive Gain Stage See Circuits 
17 ORXAF Out Output from Rcv Gain Stage Low Impedance Point 
18 IRXAC In Receive Filter Continued 
19 OXTL Out Crystal Oscillator Output 460 kHz 
20 IXTL In Crystal Oscillator Input Or External Clock Drive 
21 Ground 
23 INRTS In Request-to-Send Selects between XMIT and RCV 
24 ITXD In Transmit Data 
26 ORXD Out Receive Data 
27 OCD Out Carrier Detect 
Operation 
 Modulator (Transmit): 
 The modulator is operating (and the demodulator is shut down) whenever /RTS 
(INRTS) is low. When TXD (ITXD) is high the modulator output at TXA (OTXA) will 
be a trapezoidally shaped wave at nominally 1200 Hz. When TXD is low the modulator 
output is nominally 2200 Hz. TXA (OTXA) is usually AC-coupled to an amplifier or 
buffer stage. 
 The output voltage levels at TXA depend on the reference voltage applied at IAREF. 
Let VQ, VMIN, and VMAX be as defined in figure 1. 
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Figure 1 
That is, VQ is the quiescent (no modulation) voltage at TXA, and VMAX - VMIN is the 
pp signal voltage swing. VQ, VMIN, and VMAX are given by 
 where VREF is the voltage at pin IAREF. For VREF = 1.235 volt, VQ is 0.5 volt and 
the signal swings from 0.25 volt to 0.75 volt. In a HART Master this is the correct pp 
voltage to be applied to the network. But the OTXA pin does not have enough drive 
capability to drive a HART network directly. A buffer amplifier is usually needed. In a 
field instrument, the 0.5 volt pp OTXA output is usually converted to 1 mA pp. 
 Demodulator (Receiver): 
 The demodulator is operating (and the modulator is shut down) whenever /RTS 
(INRTS) is high. The received signal is first applied to a bandpass filter. Part of this 
filter is off-chip. This is necessary to reduce interference to a level that is within the 
20C15 supply rails. The bandpass filter consists of a 4-pole highpass filter and a single-
pole lowpass. When used with the suggested passive components the overall filter has a 
gain of 1.6 in the passband. Other pins that are part of the receive filter are IRXA, 
ORXAF, and IRXAC. 
 The filter output is applied to two comparators -- one to slice the signal and produce a 
logic-level version of the received FSK and a second to act as a carrier detect. The 
reference for the first comparator is the reference voltage applied at IAREF. The 
reference for the second comparator is applied at ICDREF and is normally set to be 80 
mV below the level at IAREF. The carrier detect comparator is therefore tripped if the 
filter output swings low by 80 mV peak or more. Or, since the filter has a passband gain 
of 1.6, the carrier detect comparator will trip if the input swings by 50 mV peak (100 mV 
pp) or more. These values are chosen to satisfy the HART requirement that the carrier 
detect be OFF for input signals 120 mV pp. 
 Logic circuits following the carrier detect comparator are used to decide whether 
carrier is present. If the input has sufficient amplitude to trip the carrier detect comparator 
on each of 3 or 4 consecutive cycles, then carrier is present and OCD goes high. Once 
carrier is present, if the input signal fails to trip the comparator for a time of about 3 or 4 
cycles, then carrier is no longer present and OCD goes low. 
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 The logic-level FSK out of the first comparator is applied to a logic block that 
generates a high level if a frequency below 1700 Hz is present and a low level if a 
frequency above 1700 Hz is present. The output of this logic block is gated with OCD to 
form the output RXD (ORXD). If carrier is not present the RXD output is always low. 
If carrier is present, then RXD equals the output of the logic block. 
 RXD (ORXD) is normally connected to a UART. The UART sees a low level as an 
assertion of RXD. Therefore, when no carrier is present (no reception occuring) RXD is 
asserted. In some cases this will appear to be an extra character at the end of a message. 
The software that services the UART must handle this condition appropriately. 
 Clock: 
 A 460.8 kHz clock is required. This can be generated by connecting a crystal or 
ceramic resonator across pins OXTL and IXTL (see Circuits). An external clock source 
can also be used. Connect the external source to OXTL and connect IXTL to ground. 
 Parameters 
 Clock: 460.8 kHz, Tolerance 1%. 
 Power Supply Current: 400 uA max. (3.3 volt supply), 600 uA max. (5.0 volt 
supply). 
 (NOTE: This is the current during receive. It is about 100 uA to 200 uA less during 
transmit. 
 This can sometimes be used to advantage.) 
 Operating Temperature Range: 0 C to 70 C. (NOTE: This would seem to 
preclude using 
 this part in industrial control applications, which usually require -40 C to 85 C. 
However, this 
 specification is apparently the result of problems in testing the part outside of 0 C to 
70 C; and 
 doesn't reflect the part's true capability. It is designed for and is being widely used 
in equipment 
 specified for -40 C to 85 C.) 
 Reset Minimum Pulse Time: 2 microsecond. 
 Transmit Output Drive Capability: Needs minimum of 30 kohm. 
Circuits 
 Field Instrument, 3.3 Volt Supply: 
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Figure 2 
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 Field Instrument, 5.0 Volt Supply: 
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Figure 3 
Package 
Figure 4 
Package -- 28 Pin PLCC 
 References 
1.1. Control Magazine, "You Gotta Have HART ...", June, 1998, Putman Publishing Company 
555 Pierce Road, Suite 301, Itasca, Illinois 60143 
1.2. Control Engineering Magazine, "4-20 mA Transmitters Alive and Kicking," October 1998. 
1.3 ARC Study as reported in I&CS Magazine, "Pressure Transmitter: A Unit For Every 
Application," November, 1999. 
1.4. Rosemount Inc., 12001 Technology Drive, Eden Prairie, MN 55344. 
1.5. HART Communication Foundation, 9390 Research Blvd., Suite II-250, Austin, TX, 78759 
1.6. HART Field Communications Protocol: A Technical Overview, HCF LIT 20, rev. 2, 1994, 
HART Communication Foundation. 
1.7. Fieldbus Standard for Use in Industrial Control Systems, ISA SP-50, Instrument Society of 
America. 
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 2.1 Questions on real-time programming in Programming Questions, 
2.2 Ramamritham, K., et. al., "Using Windows NT for Real-Time Applications: Experimental 
Observations and Recommendations," 1998 IEEE Real-Time Technology and Applications 
Symposium. 
2.3 HART Physical Layer Test Procedure, HCF_TEST-2, Revision 1.0, 1995, HART 
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2.4 BS 6305:1992, "General Requirements for Apparatus for Connection to Public Switched 
Telephone Networks Run By Certain Public Telecommunications Operators," British Standards 
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2.5 Ott, H.W., Noise Reduction Techniques in Electronic Systems, Wiley, 1976. 
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Environment". 
2.7 ENV50141:1993, "Electromagnetic Compatibility -- Basic Immunity Standard -- 
Conducted Disturbances Induced by Radio-Frequency Fields -- Immunity Test." 
2.8 Stahlings, W., ed., Tutorial: Local Network Technology, IEEE Computer Society Press, 
1983. 
2.9 DeviceNet Standard. Originally developed by Allen-Bradley, now Open DeviceNet 
Vendor's Association (ODVA) ODVA, PMB 499 * 20423 State Road 7 #F6 * Boca Raton, FL 
33498-6797 USA. 
2.10 NT International, Minneapolis, MN, 612-502-0200. 
3.1 MATLAB, Mathworks, Inc., Natick, MA. 
3.2 ISO/IEC 7498-1:1994(E), "Open Systems Interconnection -- Basic Reference Model: The 
Basic Model," ISO/IEC, Case Postale 56, CH-1211 Geneva 20, Switzerland. 
3.3. Bennet, W.R., and Rice, S.O., "Spectral Density and Autocorrelation Functions Associated 
with Binary Frequency-Shift Keying", Bell System Technical Journal, Sept., 1963. 
3.4 Proakis, J.G., Digital Communications, McGraw-Hill, 1983. 
3.5 Shanmugam, K.S., "Digital and Analog Communication Systems," 1979, John Wiley & 
Sons. 
3.6 Private Communication with Doug Arntson of Rosemount Inc. 
3.7 Private Communication with Jay Warrior of Rosemount Inc.