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4. Pulse encoding
Pulse code modulation (PCM) is one of the most commonly used methods of encoding analog data for transmission in instrumentation systems. In PCM the analog signal is sampled and converted into binary form by means of an AdC, and these data are then transmitted in serial form. This is shown in FIG. 20. The bandwidth required for the transmission of a signal using PCM is considerably in excess of the bandwidth of the original signal. if a signal having a band width fd is encoded into an N-bit binary code the minimum bandwidth required to transmit the PCM encoded signal is fd $ ?? N Hz, i.e., N times the original signal bandwidth.
Several forms of PCM are shown in FIG. 21. The non-return to zero (NRZ-L) is a common code that is easily interfaced to a computer. In the non-return to zero mark (NRZ-M) and the non-return to zero space (NRZ-S) codes level transitions represent bit changes. In bi-phase level (BiF-L) a bit transition occurs at the center of every period.
One is represented by a "1" level changing to a "0" level at the center transition point, and zero is represented by a "0" level changing to a "1" level. In bi-phase mark and space code (BiF-M) and (BiF-S) a level change occurs at the beginning of each bit period. In BiF-M one is represented by a mid-bit transition; a zero has no transition. BiF-S is the converse of BiF-M. Delay modulation code dM-M and dM-S have transitions at mid-bit and at the end of the bit time. In dM-M a one is represented by a level change at mid-bit; a zero followed by a zero is represented by a level change after the first zero. No level change occurs if a zero precedes a one. dM-S is the converse of dM-M. Bi-phase codes have a transition at least every bit time which can be used for synchronization, but they require twice the bandwidth of the NRZ-L code. The delay modulation codes offer the greatest bandwidth saving but are more susceptible to error, and are used if bandwidth compression is needed or high signal-to-noise ratio is expected.
FIG. 19 Time-division multiplexing.
Alternative forms of encoding are shown in FIG. 22.
In pulse amplitude modulation (PAM) the amplitude of the signal transmitted is proportional to the magnitude of the signal being transmitted, and it can be used for the transmission of both analog and digital signals. The channel band width required for the transmission of PAM is less than that required for PCM, although the effects of iSi are more marked. PAM as a means of transmitting digital data requires more complex decoding schemes, in that it is necessary to discriminate between an increased number of levels.
other forms of encoding which can be used include pulse-width modulation (PWM), otherwise referred to as pulse-duration modulation (PDM), which employs a constant height variable width pulse with the information being contained in the width of the pulse. In pulse-position modulation (PPM) the position of the pulses corresponds to the width of the pulse in PWM. Delta modulation and sigma-delta modulation use pulse trains, the frequencies of which are proportional to either the rate of change of the signal or the amplitude of the signal itself. For analyses of the above systems in terms of the bandwidth required for transmission, signal-to-noise ratios, and error rate, together with practical details, the reader is directed to Hartley et al. (1967), Cattermole (1969), Steele (1975), and Shanmugan (1979).
5. Carrier wave modulation
Modulation is used to match the frequency characteristics of the data to be transmitted to those of the Transmission channel, to reduce the effects of unwanted noise and interference, and to facilitate the efficient radiation of the signal. These are all effected by shifting the frequency of the data into some frequency band centered around a carrier frequency.
Modulation also allows the allocation of specific frequency bands for specific purposes, such as in a FDM system or in rf transmission systems, where certain frequency bands are assigned for broadcasting, telemetry, etc. Modulation can also be used to overcome the limitations of signal-processing equipment in that the frequency of the signal can be shifted into frequency bands where the design of filters or amplifiers is somewhat easier, or into a frequency band that the processing equipment will accept. Modulation can be used to provide the bandwidth against signal-to-noise trade-offs which are indicated by the Hartley-Shannon theorem.
FIG. 23 Amplitude, frequency, and phase modulation of a carrier wave.
Carrier-wave modulation uses the modulation of one of its three parameters, namely amplitude, frequency, or phase, and these are all shown in FIG. 23. The techniques can be used for the transmission of both analog and digital signals.
In amplitude modulation the amplitude of the carrier varies linearly with the amplitude of the signal to be transmitted. If the data signal d(t) is represented by a sinusoid d(t) = cos 2πfdt then in amplitude modulation the carrier wave c(t) is given by:
c(t) = C(1+m x cos 2πfdt) cos 2πfct
...where C is the amplitude of the unmodulated wave, fc its frequency, and m is the depth of modulation which has a value lying between 0 and 1. if m = 1 then the carrier is said to have 100 percent modulation. The above expression for c(t) can be rearranged as:
...showing that the spectrum of the transmitted signal has three frequency components at the carrier frequency fc and at the sum and difference frequencies ( fc + fd) and ( fc - fd). if the signal is represented by a spectrum having frequencies up to fd then the transmitted spectrum has a bandwidth of 2fd centered around fc. Thus in order to transmit data using AM a bandwidth equal to twice that of the data is required.
As can be seen, the envelope of the AM signal contains the information, and thus demodulation can be effected simply by rectifying and smoothing the signal.
Both the upper and lower sidebands of AM contain sufficient amplitude and phase information to reconstruct the data, and thus it is possible to reduce the bandwidth requirements of the system. Single side-band modulation (SSB) and vestigial side-band modulation (VSM) both transmit the data using amplitude modulation with smaller band widths than straight AM. SSB has half the bandwidth of a simple AM system; the low-frequency response is generally poor. VSM transmits one side band almost completely and only a trace of the other. it is very often used in high speed data transmission, since it offers the best compromise between bandwidth requirements, low-frequency response, and improved power efficiency.
In frequency modulation consider the carrier signal c(t) given by:
[equation not shown]
Then the instantaneous frequency of this signal is given by:
[equation not shown]
The frequency deviation [equation not shown]
$ r z of the signal from the carrier frequency is made to be proportional to the data signal. If the data signal is represented by a single sinusoid of the form:
[equation not shown]
where kf is the frequency deviation constant which has units of Hz/V. Thus:
[equation not shown]
fp # and assuming zero initial phase deviation, then the carrier wave can be represented by:
[equation not shown]
where b is the modulation index and represents the maxi mum phase deviation produced by the data. it is possible to show that c(t) can be represented by an infinite series of frequency components f ! nfd, n = 1, 2, 3,..., given by:
[equation not shown]
where Jn(b) is a Bessel function of the first kind of order n and argument b. Since the signal consists of an infinite number of frequency components, limiting the transmission bandwidth distorts the signal, and the question arises as to what is a reasonable bandwidth for the system to have in order to transmit the data with an acceptable degree of distortion. For 1 % b only J0 and J1 are important, and large b implies large bandwidth. It has been found in practice that if 98 percent or more of the FM signal power is transmit ted, then the signal distortion is negligible. Carson's rule indicates that the bandwidth required for FM transmission of a signal having a spectrum with components up to a frequency of fd is given by 2( fD + fd), where fD is the maxi mum frequency deviation. For narrow-band FM systems having small frequency deviations the bandwidth required is the same as that for AM. Wide-band FM systems require a bandwidth of 2fD. Frequency modulation is used extensively in rf telemetry and in FDM.
In phase modulation the instantaneous phase deviation z is made proportional to the data signal. Thus:
[equation not shown]
and it can be shown that the carrier wave c(t) can be represented by:
[equation not shown]
where b is now given by kp. For further details of the various modulation schemes and their realizations the reader should consult Shanmugan (1979) and Coates (1982).
6. Error detection and correction codes
Errors occur in digital data communications systems as a con sequence of the corruption of the data by noise. FIG. 24 shows the bit error probability as a function of signal-to-noise ratio for a PCM transmission system using NRZ-L coding.
In order to reduce the probability of an error occurring in the transmission of the data, bits are added to the transmitted message. These bits add redundancy to the transmitted data, and since only part of the transmitted message is now the actual data, the efficiency of the transmission is reduced.
FIG. 24 Bit-error probability for PCM transmission using a NRZ-L code.
There are two forms of error coding, known as forward error detection and correction coding (FEC), in which the transmitted message is coded in such a way that errors can be both detected and corrected continuously, and automatic repeat request coding (ARQ), in which if an error is detected then a request is sent to repeat the transmission. In terms of data-throughput rates FEC codes are more efficient than AQR codes because of the need to retransmit the data in the case of error in an ARQ code, although the equipment required to detect errors is somewhat simpler than that required to correct the errors from the corrupted message. ARQ codes are commonly used in instrumentation systems.
Parity-checking coding is a form of coding used in ARQ coding in which (n - k) bits are added to the k bits of the data to make an n-bit data system. The simplest form of coding is parity-bit coding in which the number of added bits is one, and this additional bit is added to the data stream in order to make the total number of ones in the data stream either odd or even. The received data are checked for parity.
This form of coding will detect only an odd number of bit errors. More complex forms of coding include linear block codes such as Hamming codes, cyclic codes such as Bose Chanhuri-Hocquenghen codes, and geometric codes. Such codes can be designed to detect multiple-burst errors, in which two or more successive bits are in error. In general the larger the number of parity bits, the less efficient the coding is, but the larger are both the maximum number of random errors and the maximum burst length that can be detected.
FIG. 25 shows examples of some of these coding techniques. Further details of coding techniques can be found in Shanmugan (1979), Bowdell (1981), and Coates (1982).
7. Direct Analog Signal Transmission
Analog signals are rarely transmitted over transmission lines as a voltage since the method suffers from errors due to series and common mode inductively and capacitively coupled interference signals and those due to line resistance. The most common form of analog signal transmission is as current.
Current transmission as shown in FIG. 26 typically uses 0-20 or 4-20 mA. The analog signal is converted to a current at the transmitter and is detected at the receiver either by measuring the potential difference developed across a fixed resistor or using the current to drive an indicating instrument or chart recorder. The length of line over which signals can be transmitted at low frequencies is primarily limited by the voltage available at the transmitter to overcome voltage drop along the line and across the receiver. With a typical voltage of 24 V the system is capable of transmitting the current over several kilometers. The percentage error in a current transmission system can be calculated as 50 # the ratio of the loop resistance in ohms to the total line insulation resistance, also expressed in ohms. The accuracy of current transmission system systems is typically 0.5 percent.
The advantage of using 4-20 mA instead of 0-20 mA is that the use of a live zero enables instrument or line faults to be detected. In the 4-20 mA system zero value is represented by 4 mA and failure is indicated by 0 mA. it is possible to use a 4-20 mA system as a two-wire transmission system in which both the power and the signal are transmitted along the same wire, as shown in FIG. 27. The 4 mA standing current is used to power the remote instrumentation and the transmitter. With 24 V drive the maximum power available to the remote station is 96 mW. Integrated-circuit devices such as the Burr-Brown XTR 100 are available for providing two-wire transmission. This is capable of providing a 4- 20 mA output span for an input voltage as small as 10 mV, and is capable of transmitting at frequencies up to 2 kHz over a distance of 600 m. Current transmission cannot be used over the public telephone system because it requires a dc transmission path, and telephone systems use ac amplifiers in the repeater stations.
FIG. 25 Error-detection coding
FIG. 26 4-20 mA current transmission system.
FIG. 27 Two-wire trans mission system.
FIG. 28 Position telemetry using an inductive "synchro."
Position telemetry transmits an analog variable by reproducing at the receiver the positional information available at the transmitter. Such devices employ null techniques with either resistive or inductive elements to achieve the position telemetry. FIG. 28 shows an inductive "synchro." The ac power applied to the transmitter induces EMF in the three stator windings, the magnitude of which are dependent upon the position of the transmitter rotor. If the receiver rotor is aligned in the same direction as the transmitter rotor then the EMF induced in the stator windings of the receiver will be identical to those on the stator windings of the transmitter. There will therefore be no resultant circulating cur rents. If the receiver rotor is not aligned to the direction of the transmitter rotor then the circulating currents in the stator windings will be such as to generate a torque which will move the receiver rotor in such a direction as to align itself with the transmitter rotor.
8. Frequency Transmission
By transmitting signals as frequency the characteristics of the transmission line in terms of amplitude and phase characteristics are less important. On reception the signal can be counted over a fixed period of time to provide a digital measurement. The resolution of such systems will be one count in the total number received. Thus for high resolution it is necessary to count the signal over a long time period, and this method of transmission is therefore unsuitable for rapidly changing or multiplexed signals but is useful for such applications as batch control, where, for example, a totalized value of a variable over a given period is required.
FIG. 29(a) Shows a frequency-transmission system.
Frequency-transmission systems can also be used in two-wire transmission systems, as shown in FIG. 29(b), where the twisted pair carries both the power to the remote device and the frequency signal in the form of current modulation. The frequency range of such systems is governed by the bandwidth of the channel over which the signal is to be transmitted, but commercially available integrated circuit V-to-f converters, such as the Analog devices Ad458, convert a 0-10 V dc signal to a frequency in the range 0-10 kHz or 0-100 kHz with a maximum non-linearity of 0.01 percent of FS output, a maximum temperature coefficient of +/-5 ppm/K, a maximum input offset voltage of +/-10 mV, and a maximum input offset voltage temperature coefficient of 30 nV/K. The response time is two output pulses plus 2 n. A low-cost f to V converter, such as the Analog devices Ad453, has an input frequency range of 0-100 kHz with a variable threshold voltage of between 0 and 12 V, and can be used with low-level signals as well as high-level inputs from TTL and CMoS. The converter has a full-scale output of 10 V and a non-linearity of less than +/- 0.008 percent of FS with a maximum tempera ture coefficient of !50 ppm/K. The maximum response time is 4 ms.
9. Digital signal transmission
Digital signals are transmitted over transmission lines using either serial or parallel communication.
For long-distance communication serial communication is the preferred method. The serial communication may be either synchronous or asynchronous. In synchronous communication the data are sent in a continuous stream with out stop or start information. Asynchronous communication refers to a mode of communication in which data are transmitted as individual blocks framed by start and stop bits.
Bits are also added to the data stream for error detection.
Integrated circuit devices known as universal asynchronous receiver transmitters (UARTS) are available for converting parallel data into a serial format suitable for transmission over a twisted pair or coaxial line and for reception of the data in serial format and reconversion to parallel format with parity-bit checking. The schematic diagram for such a device is shown in FIG. 30.
FIG. 29 (a) Frequency transmission system; (b) two-wire frequency-transmission
system.
FIG. 30 (a) Universal asynchronous receiver transmitter (UART). (b) Serial
data format. (c) Transmitter timing (not to scale). (d) Receiver timing (not
to scale). (e) Start bit timing.
Because of the high capacitance of twisted-pair and coaxial cables the length of line over which standard 74 series TTL can transmit digital signals is limited typically to a length of can be used for data transmission rates up to 1200 bits/s. The receiver uses a frequency discriminator whose threshold is set midway between the two frequencies. The recommended frequency shift is not less than 0.66 of the modulating frequency. Thus a modem operating at 1200 bits/s has a recommended central frequency of 1700 Hz and a frequency deviation of 800 Hz, with a 0 represented by a frequency of 1300 Hz and a 1 by a frequency of 2100 Hz. At a transmission rate of 200 bits/s it is possible to operate a full-duplex system. At 600 and 1200 bits/s half-duplex operation is used incorporating a slow-speed backward channel for supervisory control or low-speed return data.
At bit rates above 2400 bits/s the bandwidth and group delay characteristics of telephone lines make it impossible to transmit the data using FSK. it is necessary for each signal to contain more than one bit of information.
This is achieved by a process known as phase shift keying (PSK), in which the phase of a constant-amplitude carrier is changed. FIG. 32(a) shows the principle of PSK and 3 m at 2 Mbit/s. This can be increased to 15 m by the use of open-collector TTL driving a low-impedance terminated line.
In order to drive digital signals over long lines special purpose line driver and receiver circuits are available. Integrated circuit driver/Receiver combinations such as the Texas instruments SN75150/SN75152, SN75158/SN75157, and SN75156/SN75157 devices meet the internationally agreed EIA Standards RS-232C, RS-422A, and RS-423A, respectively (see Section 9.2).
FIG. 31 Frequency-shift keying.
FIG. 32 (a) Principle of phase-shift keying; (b) two-, four-, and eight-state
shift keying.
9.1 Modems
In order to overcome the limitations of the public telephone lines digital data are transmitted down these lines by means of a modem. The two methods of modulation used by modems are frequency-shift keying (FSK) and phase shift keying (PSK). Amplitude-modulation techniques are not used because of the unsuitable response of the line to step changes in amplitude. Modems can be used to transmit information in two directions along a telephone line.
Full-duplex operation is transmission of information in both directions simultaneously; half-duplex is the transmission of information in both directions but only in one direction at any one time; and simplex is the transmission of data in one direction only.
The principle of FSK is shown in FIG. 31. FSK uses two different frequencies to represent a 1 and a 0, and this:
FIG. 32(b) shows how the information content of PSK can be increased by employing two-, four-, and eight-state systems. It should now be seen that the number of signal elements/s (which is referred to as the Baud rate) has to be multiplied by the number of states to obtain the data transmission rate in bits/s. Thus an eight-state PSK operating at a rate of 1200 baud can transmit 9600 bits/s. For years, 9600 bps was the fastest transmission over telephone cables using leased lines, i.e., lines which are permanently allocated to the user as opposed to switched lines. At the higher data transmission rates it is necessary to apply adaptive equalization of the line to ensure correct operation and also to have in-built error-correcting coding. Details of various schemes for modem operation are to be found in Coates (1982) and Blackwell (1981). The international Telephone and Telegraph Consultative Committee (CCiTT) has made recommendations for the mode of operation of modems operating at different rates over telephone lines. These are set out in recommendations V21, V23, V26, V27, and V29.
These are listed in the References. Since the publication of the second edition of this guide, much improvement in telephone modems has been made. Typically, today's modems utilize the V90 protocol and operate at maximum speeds of up to 56 Kbps.
FIG. 33 Modem operation.
FIG. 33 shows the elements and operation of a typical modem. The data set ready (dSR) signal indicates to the equipment attached to the modem that it is ready to transmit data. When the equipment is ready to send the data it sends a request to send (RTS) signal. The modem then starts transmitting down the line. The first part of the transmission is to synchronize the receiving modem. Having given sufficient time for the receiver to synchronize, the transmitting modem sends a clear to send (CTS) signal to the equipment, and the data are then sent. At the receiver the detection of the transmitted signal sends the data carrier detected (dCd) line high, and the signal transmitted is demodulated.
9.2 Data transmission and Interfacing standards
To ease the problem of equipment interconnection various standards have been introduced for serial and parallel data transmission. For serial data transmission between data terminal equipment (dTE), such as a computer or a piece of peripheral equipment, and data communication equipment (dCE), such as modem, the standards which are currently being used are the RS-232C standard produced in the USA by the Electronic industries Association (EIA) in 1969 and their more recent RS-449 standard with its associated RS-422 and RS-423 standards.
Table 1 Pin assignments for RS-232 [not shown]
The RS-232C standard defines an electro-mechanical interface by the designation of the pins of a 25-pin plug and socket which are used for providing electrical ground, data interchange, control, and clock or timing signals between the two pieces of equipment. The standard also defines the signal levels, conditions, and polarity at each interface connection.
Table 1 gives the pin assignments for the interface, and it can be seen that only pins 2 and 3 are used for data transmission. Logical 1 for the driver is an output voltage between -5 and -15 V, with logical zero being between +5 and +15 V. The receiver detects logical 1 for input voltages < -3 V and logical 0 for input voltages >3 V, thus giving the system a minimum 2 V noise margin. The maximum transmission rate of data is 20,000 bits/s, and the maximum length of the interconnecting cable is limited by the requirement that the receiver should not have more than 2500 pF across it. The length of cable permitted thus depends on its capacitance/unit length.
The newer RS-449 interface standard which is used for higher data-transmission rates defines the mechanical characteristics in terms of the pin designations of a 37-pin interface. These are listed in Table 40.2. The electrical characteristics of the interface are specified by the two other associated standards, RS-422, which refers to communication by means of a balanced differential driver along a balanced interconnecting cable with detection being by means of a differential receiver, and RS-423, which refers to communication by means of a single-ended driver on an unbalanced cable with detection by means of a differential receiver. These two systems are shown in FIG. 34.
The maximum recommended cable lengths for the balanced RS-422 standard are 4000 ft at 90 kbits/s, 380 ft at 1 Mbits/s and 40 ft at 10 Mbits/s. For the unbalanced RS-423 standard the limits are 4000 ft at 900 bits/s, 380 ft at 10 kbits/s and 40 ft at 100 kbits/s. In addition, the RS-485 standard has been developed to permit communication between multiple addressable devices in a ring format, with up to 33 addresses per loop.
For further details of these interface standards the reader is directed to the standards produced by the EiA and IEEE. These are listed in the References. Interfaces are also discussed in Part 5.
[ not shown] Table 1 Pin assignments for RS-232
[ not shown] Table 2 Pin assignments for RS-449 FIG. 34 RS-422 and RS-423 driver/receiver systems.
[ not shown] Table 3 Pin assignment for IEEE-488 interface FIG. 35 IEEE-488 bus system.
The IEEE-488 Bus (IEEE 1978), often referred to as the HPiB Bus (the Hewlett-Packard interface Bus or the GPiP, General Purpose interface Bus), is a standard which specifies a communications protocol between a controller and instruments connected onto the bus. The instruments typically connected on to the bus include digital voltmeters, signal generators, frequency meters, and spectrum and impedance analyzers. The bus allows up to 15 such instruments to be connected onto the bus. Devices talk, listen, or do both, and at least one device on the bus must provide control, this usually being a computer. The bus uses 15 lines, the pin connections for which are shown in Table 40.3. The signal levels are TTL and the cable length between the controller and the device is limited to 2 m. The bus can be operated at a frequency of up to 1 MHz. The connection diagram for a typical system is shown in FIG. 35. Eight lines are used for addresses, program data, and measurement data transfers, three lines are used for the control of data transfers by means of a handshake technique, and five lines are used for general interface management.
CAMAC (which is an acronym for Computer Automated Measurement and Control) is a multiplexed interface system which not only specifies the connections and the communications protocol between the modules of the system which act as interfaces between the computer system and peripheral devices but also stipulates the physical dimensions of the plug-in modules. These modules are typically AdCs, dACs, digital buffers, serial to parallel converters, parallel to serial converters, and level changers. The CAMAC system offers a 24-bit parallel data highway via an 86-way socket at the rear of each module. Twenty-three of the modules are housed in a single unit known as a "crate," which additionally houses a controller. The CAMAC system was originally specified for the nuclear industry, and is particularly suited for systems where a large multiplexing ratio is required. Since each module addressed can have up to 16 sub-addresses a crate can have up to 368 multiplexed inputs/outputs. For further details of the CAMAC system the reader is directed to Barnes (1981) and to the CAMAC standards issued by the Commission of the European Communities, given in the References.
The S100 Bus (also referred to as the IEEE-696 inter face (IEEE 1981)) is an interface standard devised for bus oriented systems and was originally designed for interfacing microcomputer systems. Details of this bus can be found in the References.
Two more bus systems are in common usage: USB or Universal Serial Bus, and IEEE 1394 "FireWire." These are high-speed serial buses with addressable nodes and the ability to pass high bandwidth data. USB is becoming the preferred bus for microcomputer peripherals, while Fire-Wire is becoming preferred for high bandwidth data communications over short distances, such as testbed monitoring.
In addition to telephone modems, new methods of information transfer that have reached widespread use since 1990 include iSdN, the digital subscriber line ADSL (asynchronous) and SDSL (synchronous), and high-bandwidth cable modems.
References
Barnes, R. C. M., "A standard interface: CAMAC," in Minicomputers: A Handbook for Engineers, Scientists, and Managers (ed. Y. Parker), Abacus, London (1981), pp. 167-187.
Bentley, J., Principles of Measurement Systems, Longman, London (1983).
Bowdell, K., "interface data transmission," in Microcomputers: A Handbook for Engineers, Scientists, and Managers (ed. Y. Parker), Abacus, London (1981), pp. 148-166.
Blackwell, J., "Long distance communication," in Minicomputers: A Handbook for Engineers, Scientists, and Managers (ed. Y. Parker), Abacus, London (1981), pp. 301-316.
Cattermole, K. W., Principles of Pulse Code Modulation, iliffe, London (1969).
CCITT, Recommendation V 24. List of definitions for interchange circuits between data-terminal equipment and data circuit-terminating equipment, in CCITT, Vol. 8.1, Data Transmission over the Telephone Network, international Telecommunication Union, Geneva (1977).
Coates, R. F. W., Modern Communication Systems, 2d ed., Macmillan, London (1982).
EEC Commission: CAMAC, A Modular System for Data Handling. Revised Description and Specification, EUR 4100e, HMSo, London (1972).
EEC Commission: CAMAC, Organization of Multi-Crate Systems. Specification of the Branch Highway and CAMAC Crate Controller Type A, EUR 4600e, HMSo, London (1972).
EEC Commission: CAMAC, A Modular Instrumentation System for Data Handling. Specification of Amplitude Analog Signals, EUR 5100e, HMSo, London (1972).
EIA, Standard RS-232C Interface between Data Terminal Equipment and Data Communications Equipment Employing Serial Binary Data Interchange, EiA, Washington, d.C. (1969).
EIA, Standard RS-449 General-purpose 37-position and 9-position Interface for Data Terminal Equipment and Data Circuit- terminating Equipment Employing Serial Binary Data Interchange, EIA, Washington, d.C. (1977).
Hartley, G., P. Mornet, F. Ralph, and d. J. Tarron, Techniques of Pulse Code Modulation in Communications Networks, Cambridge University Press, Cambridge (1967).
HMSo, Private Point-to-point Systems Performance Specifications (Nos W. 6457 and W. 6458) for Angle-Modulated UHF Transmitters and Receivers and Systems in the 450-470 Mcls Band, HMSo, London (1963).
HMSo, Performance Specification: Medical and Biological Telemetry Devices, HMSo, London (1978).
HMSo, Performance Specification: Transmitters and Receivers for Use in the Bands Allowed to Low Power Telemetry in the PMR Service, HMSo, London (1979).
HMSo, international Telecommunication Union World Administrative Radio Conference, 1979, Radio Regulations. Revised International Table of Frequency Allocations and Associated Terms and Definitions, HMSo, London (1980).
IEEE, IEEE-488-1978 Standard Interface for Programmable Instruments, IEEE, New York (1978).
IEEE, IEEE-696-1981 Standard Specification for S-100 Bus Interfacing Devices, IEEE, New York (1981).
Johnson, C. S., "Telemetry data systems," Instrument Technology, 39-53, Aug. (1976); 47-53, oct. (1976).
Keiser, G., Optical Fiber Communication, McGraw-Hill international, London (1983).
Senior, J., Optical Fiber Communications, Principles and Practice, Prentice Hall, London (1985).
Shanmugan, S., Digital and Analog Communications Systems, John Wiley, New York (1979).
Steele, R., Delta Modulation Systems, Pentech Press, London (1975).
Warnock, J. d., Section 16.27 in The Process Instruments and Controls Hand book 3rd ed., (ed. by d. M. Considine), McGraw-Hill, London (1985).
Wilson, J., and J. F. B. Hawkes, Optoelectronics: An Introduction, Prentice Hall, London (1983).
Further reading:
Strock, O. J., Telemetry Computer Systems: An Introduction, Prentice-Hall, London (1984).
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