中文版
Description
This publication update contains the following new information.
• New table for the HART Analog Input Module.
• New table for the HART Analog Output Module.
• New section that describes the HART Analog Input Module(16 bits).
• New section that describes the HART Analog Output Module (14 bits).
• Replace Module Group Numbers in Table 24-1. MAU Subsystem.
• Add Module Group Numbers for the Remote Node Controller Module.
• New Appendix C.
HART (Highway Addressable Remote Transducer) is a digital communication
protocol designed for industrial process measurement applications. Field
measurement devices (transmitters) interface a process control system via an analog
4-to-20 mA current loop. HART uses a low-level frequency-shift-keyed sine wave
signal that is superimposed on the standard 4-to-20 mA process measurement
current loop. Since the HART sine wave signal is small and its average value is zero,
the current loop analog 4-to-20 mA signal is not significantly affected by the
presence of the HART signal. Using HART allows a field device to provide more
than one measurement, which is a feature not available when using only the 4-to-20
mA analog current signal.
“Smart” field devices may be described as field devices in which the analog 4-to-20
mA signal, digital communication, and sometimes power, co-exist on the same pair
of wires. The Ovation HART Analog Input (HAI) module is a standard form factor
Ovation I/O module, which will permit Ovation to communicate with HART
devices.
The HART Analog Input Module utilizes the standard +24V Ovation main power
supply to provide the power required for the logic circuitry.
The HART Analog Input Module utilizes a +24 auxiliary power supply to provide
the power required for the field circuitry. This includes all 4-20 mA loop power,
A/D conversion, and the remaining output channel components.
Due to the low signal levels of the HART communications, it is recommended that
a low noise (</=1.2 mV rms) power supply, such as an external linear power supply,
be used for the auxiliary power.
If an external supply is utilized to provide auxiliary power, see Appendix D for the
steps to be taken before connecting the external power supply to the Ovation I/O
base unit terminal block.
In addition, all modules utilizing the auxiliary power supplies, including the HART
modules, MUST utilize shielded I/O cables in order to suppress coupled noise and
transients into the HART modules. This includes modules on the same branch,
utilizing the auxiliary power, or modules on other branches utilizing the same
auxiliary power. This recommendation applies regardless of the type of power
supply chosen by the user.
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