PDF AN-214 Data sheet ( Hoja de datos )

Número de pieza	AN-214
Descripción	Transmission Line Drivers and Receivers
Fabricantes	National Semiconductor
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No Preview Available ! Transmission Line Drivers and Receivers for TIA/EIA Standards RS-422 and RS-423 Introduction With the advent of the microprocessor, logic designs have become both sophisticated and modular in concept. Fre- quently the modules making up the system are very closely coupled on a single printed circuit board or cardfile. In a majority of these cases a standard bus transceiver will be adequate. However because of the distributed intelligence ability of the microprocessor, it is becoming common prac- tice for the peripheral circuits to be physically separated from the host processor with data communications being handled over cables (e.g. plant environmental control or security system). And often these cables are measured in hundreds or thousands of feet as opposed to inches on a backplane. At this point the component wavelengths of the digital signals may become shorter than the electrical length of the cable and consequently must be treated as transmission lines. Further, these signals are exposed to electrical noise sources which may require greater noise immunity than the single chassis system. It is the object of this application note to underscore the more important design requirements for balanced and unbalanced transmission lines, and to show that National’s DS1691 driver and DS78LS120 receiver meet or exceed all of those requirements. The Requirements The requirements for transmission lines and noise immunity have been adequately recognized by National Semiconduc- National Semiconductor Application Note 214 John Abbott John Goldie August 1993 tor’s application note AN-108 and TIA/EIA standards TIA/ EIA-422-B (balanced) and TIA/EIA-423-B (unbalanced). In this application note the generic terms of RS-422 and RS-423 will be used to represent the respective TIA/EIA standards. A summary review of these notes will show that the controlling factors in a voltage digital interface are: 1. The cable length 2. The data signaling rate 3. The characteristic of the interconnection cable 4. The rise time of the signal RS-422 and RS-423 contain several useful guidelines rela- tive to the choice of balanced circuits versus unbalanced circuits. Figure 1 and Figure 2 are the digital interface for balanced (1) and unbalanced (2) circuits. Even though the unbalanced interface circuit is intended for use at lower modulation rates than the balanced circuit, its use is not recommended where the following conditions exist: 1. The interconnecting cable is exposed to noise sources which may cause a voltage sufficient to indicate a change of binary state at the load. 2. It is necessary to minimize interference with other sig- nals, such as data versus clock. 3. The interconnecting cable is too long electrically for unbalanced operation (Figure 3). http://www.DataSheet4U.net/ Legend: Rt = Transmission line termination and/or receiver input impedance VGROUND = Ground potential difference A, B = Driver interface A', B' = Load interface C = Driver circuit ground C' = Load circuit ground FIGURE 1. Balanced Digital Interface Circuit 00585401 TRI-STATE® is a registered trademark of National Semiconductor Corp. © 2002 National Semiconductor Corporation AN005854 www.national.com datasheet pdf - http://www.DataSheet4U.net/ 1 page Characteristics (Continued) DRIVER BALANCED (RS-422) The balanced driver characteristics as specified by RS-422 are as follows: 1. A driver circuit should result in a low impedance (100Ω or less) balanced voltage source that will produce a differential voltage applied to the interconnecting cable in the range of 2V to 10V. 2. With a test load of 2 resistors, 50Ω each, connected in series between the driver output terminals, the magni- tude of the differential voltage (VT) measured between the 2 output terminals shall not be less than either 2.0V or 50% of the magnitude of VO, whichever is greater. For the opposite binary state the polarity of VT shall be reversed (VT ). The magnitude of the difference in the magnitude of VT and VT shall be less than 0.4V. The magnitude of the driver offset voltage (VOS) measured between the center point of the test load and driver circuit ground shall not be greater than 3.0V. The mag- nitude of the difference in the magnitude of VOS for one binary state and VOS for the opposing binary state shall be less than 0.4V. 3. During transitions of the driver output between alternat- ing binary states, the differential signal measured across a 100Ω test load connected between the driver output terminals shall be such that the voltage monotonically changes between 0.1 and 0.9 of VSS within 10% of the unit interval or 20 ns, whichever is greater. Thereafter the signal voltage shall not vary more than 10% of VSS from the steady state value, until the next binary transi- tion occurs, and at no time shall the instantaneous mag- nitude of VT or VT exceed 6V, nor less than 2V. INTERCONNECTING CABLE The characteristics of the interconnecting cable should result in a transmission line with a characteristic impedance in the general range of 100Ω to frequencies greater than 100 kHz, and a DC series loop resistance not exceeding 240Ω. The cable may be composed of twisted or untwisted pair (flat cable) and is not further specified within the standards. 1. Conductor size of the 2 wires 24 AWG or larger, and wire resistance not to exceed 30Ω per 1000 feet per conduc- tor. 2. Mutual pair capacitance between 1 wire in the pair to the other should be less than 20 pF/ft. 00585408 00585409 tb = Time duration of the unit interval at the applicable modulation rate. tr ≤ 0.1 tb when tb ≥ 200 ns tr ≤ 20 ns when tb < 200 ns VSS = Difference in steady state voltages VSS = \|Vt − Vt\| FIGURE 7. Balanced Driver Output Signal Waveform RECEIVER The receiver characteristics are identical for both balanced (RS-422) and unbalanced (RS-423) circuits. The electrical characteristics of a single receiver without termination or optional fail-safe provisions are specified as follows: 1. Over an entire common-mode voltage range of −7V to http://www.DataSheet4U.net/ +7V, the receiver shall not require a differential input voltage of more than 200 mV to correctly assume the intended binary state. The common-mode voltage (VCM) is defined as the algebraic mean of the 2 voltages appearing at the receiver input terminals with respect to the receiver circuit ground. Reversing the polarity of VT shall cause the receiver to assume the opposite binary state. This allows for operations where there are ground differences caused by IR drop and noise of up to ±7V. 2. To maintain correct operation for differential input signal voltages ranging between 200 mV and 6V in magnitude. 3. The maximum voltage present between either receiver input terminal and receiver circuit ground shall not ex- ceed 10V (3V signal plus 7V common-mode) in magni- tude nor cause the receiver to operationally fail. Addi- tionally, the receiver shall tolerate a maximum differential signal of 12V applied across its input termi- nals without being damaged. 4. The total load including up to 10 receivers shall not have a resistance less than 90Ω for balanced, and 450Ω for unbalanced at its input points and shall not require a differential input voltage of greater than 200 mV for all receivers to assume the correct binary state. 5 www.national.com datasheet pdf - http://www.DataSheet4U.net/ 5 Page Fail-Safe Operation (Continued) 00585422 00585423 00585424 FIGURE 18. Fail-Safe Using the DS88LS120 Threshold Offset for Unbalanced Lines http://www.DataSheet4U.net/ 00585425 00585426 00585427 00585428 FIGURE 19. Fail-Safe Using the DS88LS120 Threshold Offset for Balanced Lines Conclusion This application note provides a brief overview of TIA/ EIA-422-B and TIA/EIA-423-B. At the time of publication of this application note the Rev. B standards were draft stan- dard proposals only. For complete/current information on the respective standards the reader is referenced to the respec- tive standards, as minor differences may exist between this document and the final versions. 11 www.national.com datasheet pdf - http://www.DataSheet4U.net/ 11 Page

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