Serial digital signals

Parallel digital input representing a voltage of 1.85V (Table 6.18) 

High transmission rates can be achieved, if necessary, over the relatively short distances required in a process plant. The PCM equivalent of the 4-20 mA analog transmission system shown in Fig. 6.1 can operate at up to 9,600 baud for distances up to 3000 m. The standard RS-232C transmission link is limited to about 15 m at rates up to 20,000 baud. Higher speed interfaces (such as versions of the IEEE-488 connection) used for computer control systems can handle up to 20,000 bytes/s (which for an 8-bit system is about 1.6 x 105 baud). However, in this case, the distance between devices is limited to about 2 m(4). The more recent RS-422A standard allows the transmission of data rates of 107 baud over distances not exceeding 16.4 m and 105 baud over distances not exceeding 1220 m(9S). 

In many telemetry systems the PCM signal is frequency modulated on to a carrier wave. This is termed frequency shift keying (FSK). In other arrangements, the output of the transducer is converted into fixed step changes of the phase of the modulating signal. A device for this purpose is termed a modulator/demodulator or modem. 

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Many functions of an automobile are controlled electronically or with electromechanical-electrohydraulic controls. This change in vehicle design has made a wide variety of options available for people who require advanced vehicle controls. Many automobiles use electronic fuel injection. Electronic fuel injection systems convert the position of the accelerator pedal to a serial digital signal which is used by a microcontroller to inject the optimal fuel-air mixture into the automobile at the proper time during the piston stroke. The electronic signal for the accelerator position can be provided by another control device (e.g., joystick, slide-bar). 

Following the source encoder, the resulting serial digital signal is applied to the input of the channel encoder, a device that modulates the transmitted RF wave with the reduced-rate audio information. Auxiliary serial data, such as program information and receiver control functions, also can be input to the channel encoder (or combined within the source encoder) for simultaneous transmission. 

Fig. 11. Particle size analyzer based on ultrasonic spectroscopy where DSP = digital signal processor, and RS 232 is a standard serial connection that allows...

Fig. 6.75. Parallel-to-serial conversion of digital signals (t, - time elapsed after initial clock pulse)...

Self-adaptive control, 436-38, 662-67 Self-regulating systems, 7, 34, 180 Self-tuning regulator, 437-38, 662-67 references, 447, 673 Serial transmission, digital signals, 561 Servo problem (see Feedback control)... 

A modern commercial mass spectrometer will incorporate a computer or will offer a facility to communicate with a computer via a data link, for example, to a plant control system using serial, ethernet or analog, and/or digital signals. This allows data to be stored and manipulated, often in real-time , so that results can be displayed to the user in a convenient form or to be utilized for process control either automatically via a closed loop or for operator fine-tuning . A mass spectrum may be displayed as a continuous scan over a specified mass range similar to a strip-chart, but this is difficult to interpret, and is not directly quantitative. 

The ICs discussed in this chapter will be strictly the D/A and A/D conversion circuits. The ICs usually perform either D/A or A/D conversion. There are serial interface ICs, however, typically for digital signal processing applications, that perform both A/D and D/A processes. 

Communicates with data processor over a serial link to transmit analytical data and calculations and receives new or modilied applications programs. A digitized form of the detector signal may also be transmitted lor remote reconstruction of a real-time chromatogram. 

The A/D board can reside either in the detector, an interface box, or in the computer. If the board is in the computer, it will have analog input terminals similar to a strip chart recorder or it will be connected to an interface box that will have this type of connector. If the A/D card is in the detector, the detector will have some type of digital interface port and a cable to connect it to the computer. In the personal computer world, once the signal is digitized outside the computer, it will be sent to the computer over one of three types of communication cables parallel, serial, or GPIB. 

After the analog signal has been converted to digital form the next step is to make the digital data available to the computer. There are two major modes of signal transmission serial transmission and parallel transmission. 

In the serial mode, the digital word (number) is sent to the computer one bit at a time. Now a binary counter provides a parallel output since each of the output bits has its own data output channel and the value of each output bit is simultaneously available. To use a serial transmission scheme, this parallel output must be put into serial form. One way to accomplish this is to use a Universal Asynchronous Receiver Transmitter (UART). The detailed operation of the UART will not be given here as it is not germane to the subject of this book. It is sufficient to say that the heart of the UART is a shift register and the shift register is strobed by a signal from the computer that displaces the binary number, bit by bit, sequentially from the register to the computer. 

RS-232-C In asynchronous transmissions, a recommended standard interface established by the Electrical Industries Association. The standard defines the specific lines, timing, and signal characteristics used between the computer and the peripheral device and uses a 25-pin or 9-pin DB connector. RS-232-C is used for serial communications between a computer and a peripheral such as a printer, modem, digitizing tablet, or mouse. 

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