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To allow the construction of hybrid systems, where the 4–20 mA is generated by the controller, but allows the use of pneumatic valves, a range of current to pressure (I to P) converters are available from manufacturers. These are usually local to the control valve and convert 4–20 mA to 3–15 psi (or 0.2–1.0 bar).
Both the 4–20 mA current and the digital signal are valid signalling protocols between the controller and measuring instrument or final control element. The polling address of the instrument is set to "0". Only one instrument can be put on each instrument cable signal pair. One signal, generally specified by the user, is specified to be the 4 ...
Older teleprinters used a 60 mA current loop. Later machines, such as the Teletype Model 33, operated on a lower 20 mA current level and most early minicomputers featured a 20 mA current loop interface, with an RS-232 port generally available as a more expensive option. The original IBM PC serial port card had provisions for a 20 mA current ...
The sensor circuit is supplied with constant current. A distinguishing feature of the IEPE principle is that the power supply and the sensor signal are transmitted via one shielded wire. Most IEPE sensors work at a constant current between 2 and 20 mA. A common value is 4 mA. The higher the constant current the longer the possible cable length.
For example, an analog 0 to 10 V or 4-20 mA current loop input would be converted into an integer value of 0 to 32,767. The PLC will take this value and translate it into the desired units of the process so the operator or program can read it.
The bipolar LMV431 by Texas Instruments has a V REF of 1.24 V and is capable of regulating voltages up to 30 V at currents from 80 μA to 30 mA; [55] [56] The low-voltage CMOS NCP100 by ON Semiconductor has a V REF of 0.7 V and is capable of regulating voltages up to 6 V at currents from 100 μA to 20 mA. [57] [58]
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This page was last edited on 13 March 2007, at 22:35 (UTC).; Text is available under the Creative Commons Attribution-ShareAlike 4.0 License; additional terms may ...
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