Practical Radio Engineering and Telemetry for Industry
Follow Us. App Download. US UK. Thank you for subscribing! Please check your email to confirm your subscription. Our Stores. Results for - idc. All the latest offers delivered right to your inbox! We Accept. Shipping Methods business days Minimum 10 business days. The advantage of this code over binary is that only one bit changes every time the value is incremented. This reduces the ambiguity in measuring consecutive angular positions. The Gray code is shown in Table 2. Binary coded decimal BCD is an extension of the 4-bit binary code. BCD encoding converts each separate digit of a decimal number into a 4-bit binary code.
Consequently, the BCD uses 4 bits to represent one decimal digit. Although 4 bits in the binary code can represent 16 numbers from 0 to 15 only the first 10 of these, from 0 to 9, are valid for BCD. BCD is commonly used on relatively simple systems such as small instruments, thumbwheels, and digital panel meters. Special interface cards and integrated circuits ICs are available for connecting BCD components to other intelligent devices. They can be connected directly to the inputs and outputs of PLCs. A typical application for BCD is the setting of a parameter on a control panel from a group of thumbwheels.
Each thumbwheel represents a decimal digit from left to right; thousands, hundreds, tens and units digits. The interface connection of each digit to a PLC requires 4 wires plus a common, which would mean a total of 20 wires for a 4-digit set of thumbwheels. The number of wires, and their connections to a PLC, can be reduced to 8 by using a time division multiplexing system as shown in Figure 2. A similar arrangement is used in reverse for the digital display on a panel meter, using a group of four 7-segment LCD or LED displays. The start, stop and parity bits used in asynchronous transmission systems are usually physically generated by a standard integrated circuit IC chip that is part of the interface circuitry between the microprocessor bus and the line driver or receiver of the communications link.
The outputs of a UART are not designed to interface directly with the communications link. Additional devices, called line drivers and line receivers, are necessary to give out and receive the voltages appropriate to the communications link. The timing pulses are derived from the microprocessor master clock through external connections. Handshaking is discussed in more detail in Chapter 3. This removes the burden of programming the above routines in the microprocessor and, instead, they are handled transparently by the UART.
The bits to be transmitted are loaded into a shift register, then shifted out on the negative transition of the transmit data clock. This pulse rate sets the baud rate. Some authors refer to the packet as a serial data unit SDU. Between the transmitter holding register and the shift register is a section called the SDU serial data unit formation.
This section constructs the actual packet to be loaded into the shift register. In full duplex communications, the software needs to only test the value of the transmitter buffer empty TBE flag to decide whether to write a byte to the UART. In half-duplex communications, the modem must swap between transmitter and receiver states.
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The UART receiver continuously monitors the incoming serial line waiting for a start bit. This takes place according to the format described in the user programmable data format register.
After assembly of the byte, it is moved into a FIFO first in first out buffer. Another major function of the UART is to detect errors in the data received. Most errors are receiver errors. Typical errors are:. This is called a break, and receivers can be equipped with a break detect to detect this condition. It is useful for interrupting the receiver, even in the middle of a stream of characters being sent. The break detect time is a function of the baud rate. Serialization errors are reported in the serialization status register as shown in Figure 2.
The frequency of the master signal is designed to be many times higher than that of the baud rate. This ratio of master serial clock to baud rate is called the clocking factor typically Instead of sampling the input line at the baud rate frequency, the improved start bit detector samples the incoming line at the rate of the master clock. This minimizes the possibility of an error due to slippage of sampling a stream of serial bits and sampling the wrong bit.
The earliest serial ports used or chips, which interrupted the main processor for every character to be transmitted or received. This worked well for the speeds of that time.
This has since been replaced by the chip which works in a similar fashion but supported faster PC bus speeds, and later by which has a byte buffer thereby reducing the number of CPU interruptions by a factor of A more recent development is to use an enhanced serial port, which provides a buffer of about bytes and has its own processor to reduce the interruptions to the main CPU by a factor of The is a high speed serial universal asynchronous receiver transmitter UART.
The advantage of the over the older and UARTs is that it has a byte buffer. The can operate at speeds from 1 to k baud. The is commonly used on RS even though the RS standard only allows communication at speeds up to Due to the availability and low cost of chips, the manufacturers of computers and add-on COM ports have included the as standard equipment. The old UART The advantages of the byte buffer on the are twofold:. There are seven major organizations worldwide involved in drawing up standards or recommendations, which affect data communications.
ANSI is a non-profit, non-governmental body supported by over trade organizations, professional societies, and companies.
It consists of representatives from the Postal, Telephony, and Telegraphy organizations PTTs , common carriers and manufacturers of telecommunications equipment. In Europe, administrations tend to follow the ITU defined recommendations closely. Although the US manufacturers did not recognize them in the past, they are increasingly conforming to ITU recommendations. The ITU defines a complete range of standards for interconnecting telecommunications equipment.
The RS is a voluntary standards organization in the USA, specializing in the electrical and functional characteristics of interface equipment. It mainly represents the manufacturers of electronic equipment.
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It concentrates on electrical standards. The ISO draws members from all countries of the world and concentrates on coordination of standards internationally. An interface standard defines the electrical and mechanical details that allow equipment from different manufacturers to be connected and able to communicate.
The RS have produced several well known data interface standards, which will be discussed in this chapter.
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