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42-433: HVD may refer to: High-voltage differential signaling , an electrical signalling method Hosted Virtual Desktop, a type of computer desktop virtualization High-Definition Versatile Disc , a DVD format Holographic Versatile Disc , an optical disc technology High-value detention site , a type of United States military prison Humanistischer Verband Deutschlands ,

84-441: A rule of thumb , the speed in bit/s multiplied by the length in meters should not exceed 10 . Thus a 50-meter cable should not signal faster than 2 Mbit/s . In contrast to RS-422, which has a driver circuit which cannot be switched off, RS-485 drivers use three-state logic allowing individual transmitters to be deactivated. This allows RS-485 to implement linear bus topologies using only two wires. The equipment located along

126-430: A twisted-pair or ribbon cable or traces on a printed circuit board . Electrically, the two conductors carry voltage signals which are equal in magnitude , but of opposite polarity . The receiving circuit responds to the difference between the two signals, which results in a signal with a magnitude twice as large. The symmetrical signals of differential signalling may be referred to as balanced , but this term

168-555: A German humanist organization Khovd Airport , in Mongolia Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with the title HVD . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=HVD&oldid=1141436793 " Category : Disambiguation pages Hidden categories: Short description

210-407: A balanced interface. An interface is a subsystem containing three parts: a driver, a line, and a receiver. These three components complete a full circuit for a signal to travel through and the impedances of this circuit is what determines whether the interface as a whole is balanced or not: "A balanced circuit is a two-conductor circuit in which both conductors and all circuits connected to them have

252-480: A balanced pair of conductors. For short cables and low frequencies, the two methods are equivalent, so cheap single-ended circuits with a common ground can be used with cheap cables. As signalling speeds become faster, wires begin to behave as transmission lines. Differential signalling is often used in computers to reduce electromagnetic interference , because complete screening is not possible with microstrips and chips in computers, due to geometric constraints and

294-1098: A circuit does not determine the signals it can carry and vice versa. The technique minimizes electronic crosstalk and electromagnetic interference , both noise emission and noise acceptance, and can achieve a constant or known characteristic impedance , allowing impedance matching techniques important in a high-speed signal transmission line or high-quality balanced line and balanced circuit audio signal path. Differential pairs include: Differential pairs generally carry differential or semi-differential signals, such as high-speed digital serial interfaces including LVDS differential ECL , PECL , LVPECL , Hypertransport , Ethernet over twisted pair , serial digital interface , RS-422 , RS-485 , USB , Serial ATA , TMDS , FireWire , and HDMI , etc., or else high quality and/or high frequency analog signals (e.g. video signals , balanced audio signals, etc.). Differential signalling often uses length-matched wires or conductors which are used in high speed serial links . Data rates of some interfaces implemented with differential pairs include

336-407: A differential system with the same supply voltage. The voltage difference in the high state, where one wire is at V S {\displaystyle V_{S}\,} and the other at 0 V, is V S − 0 V = V S {\displaystyle V_{S}-0\,\mathrm {V} =V_{S}} . The voltage difference in the low state, where the voltages on

378-408: A fixed reference voltage, both relative to a common ground connection shared by both ends. In many instances, single-ended designs are not feasible. Another difficulty is the electromagnetic interference that can be generated by a single-ended signalling system that attempts to operate at high speed. When transmitting signals differentially between two pieces of equipment it is common to do so through

420-548: A given supply voltage, it provides twice the noise immunity of a single-ended system. To see why, consider a single-ended digital system with supply voltage V S {\displaystyle V_{S}} . The high logic level is V S {\displaystyle V_{S}\,} and the low logic level is 0 V. The difference between the two levels is therefore V S − 0 V = V S {\displaystyle V_{S}-0\,\mathrm {V} =V_{S}} . Now consider

462-468: A high voltage differential (HVD) implementation whose maximum cable length was many times that of the single-ended version. SCSI equipment, for example, allows a maximum total cable length of 25 meters using HVD, while single-ended SCSI allows a maximum cable length of 1.5 to 6 meters, depending on bus speed. LVD versions of SCSI allow less than 25 m cable length not because of the lower voltage, but because these SCSI standards allow much higher speeds than

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504-522: A matching image current in the ground plane, which is required anyway for supplying power, the pair looks like four lines and therefore has a shorter crosstalk distance than a simple isolated pair. In fact, it behaves as well as a twisted pair. Low crosstalk is important when many lines are packed into a small space, as on a typical PCB. High-voltage differential (HVD) signalling uses high- voltage signals. In computer electronics, high voltage normally means 5 volts or more. SCSI -1 variations included

546-523: A positive one, A may be considered the non-inverting signal and B as inverting. The RS-485 standard states (paraphrased): The truth tables of most popular devices, starting with the SN75176, show the output signals inverted. This is in accordance with the A/B naming used by most differential transceiver manufacturers, including: These manufacturers all agree on the meaning of the standard, and their practice

588-407: A set of RS-485 wires are interchangeably called nodes, stations or devices. The recommended arrangement of the wires is as a connected series of point-to-point (multidropped) nodes, i.e. a line or bus , not a star , ring , or multiply connected network. Star and ring topologies are not recommended because of signal reflections or excessively low or high termination impedance. If a star configuration

630-528: A similar auto crossover feature). PCIe and USB SuperSpeed also support lane polarity inversion. Another way to deal with polarity errors is to use polarity-insensitive line codes . RS-485 RS-485 supports inexpensive local networks and multidrop communications links, using the same differential signaling over twisted pair as RS-422 . It is generally accepted that RS-485 can be used with data rates up to 10  Mbit/s or, at lower speeds, distances up to 1,200 m (4,000 ft). As

672-473: A sufficient margin for a reliable data transmission even under severe signal degradation across the cable and connectors. This robustness is the main reason why RS-485 is well suited for long-distance networking in noisy environment. In addition to the A and B connections, an optional, third connection may be present (the TIA standard requires the presence of a common return path between all circuit grounds along

714-480: A wide range of computer and automation systems. In a computer system, SCSI -2 and SCSI-3 may use this specification to implement the physical layer for data transmission between a controller and a disk drive. RS-485 is used for low-speed data communications in commercial aircraft cabins' vehicle bus . It requires minimal wiring and can share the wiring among several seats, reducing weight. These are used in programmable logic controllers and on factory floors. RS-485

756-459: Is a multi-point specification, however, this is not necessary or desirable in many cases. RS-485 and RS-422 can interoperate with certain restrictions. Converters between RS-485 and RS-232 are available to allow a personal computer to communicate with remote devices. By using repeaters very large RS-485 networks can be formed. TSB-89A, Application Guidelines for TIA/EIA-485-A does not recommend using star topology. RS-485 signals are used in

798-539: Is also used in Digital Command Control (DCC) for model railways . The external interface to the DCC command station is often RS-485 used by hand-held controllers or for controlling the layout in a networked PC environment. 8P8C modular connectors are used in this case. RS-485 does not define a communication protocol ; merely an electrical interface. Although many applications use RS-485 signal levels,

840-417: Is different from Wikidata All article disambiguation pages All disambiguation pages High-voltage differential signaling Differential signalling is a method for electrically transmitting information using two complementary signals . The technique sends the same electrical signal as a differential pair of signals, each in its own conductor . The pair of conductors can be wires in

882-680: Is differential, it resists electromagnetic interference from motors and welding equipment. In theatre and performance venues, RS-485 networks are used to control lighting and other systems using the DMX512 protocol. RS-485 serves as a physical layer for the AES3 digital audio interconnect. RS-485 is also used in building automation as the simple bus wiring and long cable length is ideal for joining remote devices. It may be used to control video surveillance systems or to interconnect security control panels and devices such as access control card readers. It

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924-428: Is in widespread use. The issue also exists in programmable logic controller applications. Care must be taken when using A/B naming. Alternate nomenclature is often used to avoid confusion surrounding the A/B naming: RS-485 standard conformant drivers provide a differential output of a minimum 1.5 V across a 54-Ω load, whereas standard conformant receivers detect a differential input down to 200 mV. The two values provide

966-415: Is more appropriately applied to balanced circuits and balanced lines which reject common-mode interference when fed into a differential receiver. Differential signalling does not make a line balanced, nor does noise rejection in balanced circuits require differential signalling. Differential signalling is to be contrasted to single-ended signalling which drives only one conductor with signal, while

1008-433: Is transmitted on A and B. C is a ground reference. This section also defines the logic states 1 (off) and 0 (on), by the polarity between A and B terminals. If A is negative with respect to B, the state is binary 1. The reversed polarity (A positive with respect to B) is binary 0. The standard does not assign any logic function to the two states. RS-485, like RS-422, can be made full-duplex by using four wires. Since RS-485

1050-414: Is twice the difference of the single-ended system. If the voltage noise on one wire is uncorrelated to the noise on the other one, it takes twice as much noise to cause an error with the differential system as with the single-ended system. In other words, differential signalling doubles the noise immunity. In single-ended signalling, the transmitter generates a single voltage that the receiver compares with

1092-447: Is unavoidable, special RS-485 repeaters are available which bidirectionally listen for data on each span and then retransmit the data onto all other spans. Ideally, the two ends of the cable will have a termination resistor connected across the two wires. Without termination resistors, signal reflections off the unterminated end of the cable can cause data corruption. Termination resistors also reduce electrical noise sensitivity due to

1134-443: Is used as the physical layer underlying many standard and proprietary automation protocols used to implement industrial control systems , including the most common versions of Modbus and Profibus . DH 485 is a proprietary communications protocol used by Allen-Bradley in their line of industrial control units. Utilizing a series of dedicated interface devices, it allows PCs and industrial controllers to communicate. Since it

1176-589: Is wise to add some current limiting to the SC connection. Grounds between buildings may vary by a small voltage, but with very low impedance and hence the possibility of catastrophic currents – enough to melt signal cables, PCB traces, and transceiver devices. RS-485 does not specify any connector or pinout. Circuits may be terminated on screw terminals , D-subminiature connectors, or other types of connectors. The standard does not discuss cable shielding but makes some recommendations on preferred methods of interconnecting

1218-409: The lower impedance . The value of each termination resistor should be equal to the cable characteristic impedance (typically, 120 ohms for twisted pairs). The termination also includes pull up and pull down resistors to establish bias for each data wire for the case when the lines are not being driven by any device. This way, the lines will be biased to known voltages and nodes will not interpret

1260-672: The balanced line for proper operation) called SC , G or reference , the common signal reference ground used by the receiver to measure the A and B voltages. This connection may be used to limit the common-mode signal that can be impressed on the receiver inputs. The allowable common-mode voltage is in the range −7 V to +12 V, i.e. ±7 V on top of the 0–5 V signal range. Failure to stay within this range will result in, at best, signal corruption, and, at worst, damage to connected devices. Care must be taken that an SC connection, especially over long cable runs, does not result in an attempt to connect disparate grounds together – it

1302-573: The benefits of differential signalling include: Differential signalling works for both analog signalling, as in balanced audio , and in digital signalling, as in RS-422 , RS-485 , Ethernet over twisted pair , PCI Express , DisplayPort , HDMI and USB . The electronics industry , particularly in portable and mobile devices, continually strives to lower supply voltage to save power. A low supply voltage, however, reduces noise immunity. Differential signalling helps to reduce these problems because, for

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1344-430: The fact that screening does not work at DC. If a DC power supply line and a low-voltage signal line share the same ground, the power current returning through the ground can induce a significant voltage in it. A low-resistance ground reduces this problem to some extent. A balanced pair of microstrip lines is a convenient solution because it does not need an additional PCB layer, as a stripline does. Because each line causes

1386-418: The following: The type of transmission line that connects two devices (chips, modules) often dictates the type of signalling. Single-ended signalling is typically used with coaxial cables , in which one conductor totally screens the other from the environment. All screens (or shields) are combined into a single piece of material to form a common ground. Differential signalling, however, is typically used with

1428-480: The noise from undriven lines as actual data; without biasing resistors, the data lines float in such a way that electrical noise sensitivity is greatest when all device stations are silent or unpowered. The EIA once labeled all its standards with the prefix RS ( Recommended Standard ), but the EIA/TIA officially replaced RS with EIA/TIA to help identify the origin of its standards. The EIA has officially disbanded and

1470-407: The older HVD SCSI. The generic term high-voltage differential signalling describes a variety of systems. Low-voltage differential signalling (LVDS), on the other hand, is a specific system defined by a TIA/EIA standard. Some integrated circuits dealing with differential signals provide a hardware option (via strapping options , under firmware control, or even automatic) to swap the polarity of

1512-419: The other is connected to a fixed reference voltage. Contrary to popular belief, differential signalling does not affect noise cancellation. Balanced lines with differential receivers will reject noise regardless of whether the signal is differential or single-ended, but since balanced line noise rejection requires a differential receiver anyway, differential signalling is often used on balanced lines. Some of

1554-613: The same impedance to ground and to all other conductors." Balanced interfaces were developed as a protection scheme against noise. In theory, it can reject any interference so long as it is common-mode (voltages that appear with equal magnitude and the same polarity in both conductors). There exists great confusion as to what constitutes a balanced interface and how it relates to differential signalling. In reality, they are two completely independent concepts: balanced interfacing concerns noise and interference rejection, while differential signalling only concerns headroom. The impedance balance of

1596-472: The speed, format, and protocol of the data transmission are not specified by RS-485. Interoperability of even similar devices from different manufacturers is not assured by compliance with the signal levels alone. The RS-485 differential line consists of two signals: Because a mark (logic 1) condition is traditionally represented (e.g. in RS-232) with a negative voltage; and space (logic 0) represented with

1638-510: The standard is now maintained by the TIA as TIA-485, but engineers and applications guides continue to use the RS-485 designation. The initial edition of EIA RS-485 was dated April 1983. RS-485 only specifies the electrical characteristics of the generator and the receiver: the physical layer . It does not specify or recommend any communications protocol ; Other standards define the protocols for communication over an RS-485 link. The foreword to

1680-593: The standard references The Telecommunications Systems Bulletin TSB-89 which contains application guidelines, including data signaling rate vs. cable length, stub length, and configurations. Section 4 defines the electrical characteristics of the generator (transmitter or driver), receiver, transceiver, and system. These characteristics include: definition of a unit load, voltage ranges, open-circuit voltages, thresholds, and transient tolerance. It also defines three generator interface points (signal lines); A, B and C. The data

1722-566: The two differential signals, called differential pair swapping , polarity reversion , differential pair inversion , polarity inversion , or lane inversion . This can be utilized to simplify or improve the routing of high-speed differential pairs of traces on printed circuit boards in hardware development, to help to cope with common cabling errors through swapped wires, or easily fix common design errors under firmware control. Many Ethernet PHY transceivers support this as auto polarity detection and correction (not to be confused with

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1764-413: The wires are exchanged, is 0 V − V S = − V S {\displaystyle 0\,\mathrm {V} -V_{S}=-V_{S}} . The difference between high and low logic levels is therefore V S − ( − V S ) = 2 V S {\displaystyle V_{S}-(-V_{S})=2V_{S}\,} . This

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