A modular connector is an electrical connector that was originally designed for use in telephone wiring, but has since been used for many other purposes. Many applications that originally used a bulkier, more expensive connector have converted to modular connectors. Probably the most well known applications of modular connectors are for telephone jacks and for Ethernet jacks, both of which are nearly always modular connectors.
Modular connectors were originally used in the Registration Interface system, mandated by the Federal Communications Commission (FCC) in 1976 in which they became known as registered jacks. The registered jack specifications define the wiring patterns of the jacks, not the physical dimensions or geometry of the connectors of either gender. Instead, these latter aspects are covered by ISO standard 8877, first used in ISDN systems. TIA/EIA-568 is a standard for data circuits wired on modular connectors.
Other systems exist for assigning signals to modular connectors; physical interchangeability of plugs and jacks does not ensure interoperation, nor protection from electrical damage to circuits. For example, modular cables and connectors have been used to supply low-voltage AC or DC power and no clear standard exists for this application.
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Nomenclature
Modular connectors also go by the names "modular phone jack/plug", "RJ connector" and "Western jack/plug". The term "modular connector" arose from its original use in a novel system of cabling designed to make telephone equipment more modular. This includes the 4P4C handset connector.
It is very common to use a registered jack number to refer to the physical connector itself; for instance, the 8P8C modular connector type is often called RJ45 because the Registered Jack standard of that name was an early user of 8P8C modular connectors. A very popular use of 8P8C today is Ethernet over twisted pair, and that may be the most well known context in which the name RJ45 is known, even though it has nothing to do with the RJ45 standard. Likewise, the 4P4C connector is sometimes called RJ9 or RJ22 and various 6P connectors are called RJ11.
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History
Modular connectors were originally developed and patented by General Cable Corp in 1974. They replaced the hard-wired connections on most Western Electric telephones around 1976. At the same time, they began to replace screw terminals and larger 3 and 4 pin telephone jacks in buildings.
Gender
Modular connectors have gender: plugs are considered to be "male," while jacks or sockets are considered to be "female," a reference to reproductive organs.
Plugs are used to terminate loose cables and cords, while jacks are used for fixed locations on surfaces such as walls and panels, and on equipment. Other than telephone extension cables, cables with a modular plug on one end and a jack on the other are rare. Instead, cables are connected using a male-to-male adapter, which consists of two female jacks wired back-to-back.
Latching tab and orientation
Modular connectors are designed to latch together. A spring-loaded tab on the plug snaps into a jack so that the plug cannot be pulled out. To remove the plug, the latching tab must be depressed. The standard and most common way to install a jack in a wall or panel is with the tab side down. This usually makes it easier to operate the tab when removing the plug, because the person grabs the plug with thumb on top and presses the tab up with the index finger. This orientation is recommended by manufacturers because any dust or fine abrasive or conductive particles that may enter an unused jack will tend to fall away from the electrical contacts, rather than settling onto the contact surfaces.
The modular connector suffers from a design flaw or weakness however, as the fragile latching tab easily snags on other cables and breaks off. When this happens, the connector is still functional, but the crucial latching feature is lost. Some higher quality cables have a flexible sleeve called a boot over the plug, or a special tab design, to prevent this. These cables are often marketed as snagless. Boots are seen mainly on 8P8C data cables, but are also used on other sizes of connectors.
Most protective boots must be installed onto a cable before the modular plug is crimped on. This means that field retrofits of these types of boots are not possible, except by cutting off the existing plug, and then replacing it with a new one. However, protective boots or rigid protective "ramp" adapters are available which can be snapped over an installed unprotected modular plug, to add a measure of protection to the latch tab.
Sizes and contacts
Modular connectors come in four sizes: 4-, 6-, 8-, and 10-position, where a position is a location for a contact. Not all of the positions may have contacts installed. When contacts are omitted, they are typically done so from the outermost pair of contacts inward, such that the number of contacts is almost always an even number. The insulating plastic bodies of 4P and 6P connectors have different widths, whereas 8P or 10P connectors share an even larger body width. The connector body positions with omitted contacts or contacts unattached to wires are unused for the electrical connection, but ensure that the plug fits correctly. For instance, RJ11 cables often have connectors with six positions and four contacts, to which are attached just two wires.
The connectors are designated with two numbers that represent the quantity of positions and contacts, with each number followed by a "P" and "C", respectively: for example, "6P2C" for a connector having six positions and two contacts. Alternate designations omit the "P" and "C" while separating the position and contact quantities with either an "x" ("6x2") or a slash ("6/2").
Internally, the contacts have sharp prongs that when crimped, pierce the insulation and connect with the wire conductor, a mechanism known as insulation displacement. Ethernet cables, in particular, may have solid or stranded wire conductors and the sharp prongs are different in the 8P8C connectors made for each type of wire. A modular plug for solid (single strand) wire often has three slightly splayed prongs on each contact to securely surround and grip the conductor. Modular plugs for stranded or tinsel wire have prongs that are designed to connect to multiple wire strands. Connector plugs are designed for either solid or stranded wire; a plug for one wire type might not make reliable contact when crimped to a cable with wires of the other type.
The contact positions are numbered sequentially starting from 1. When viewed head-on with the retention mechanism on the bottom, jacks will have contact position number 1 on the left and plugs will have it on the right. Contacts are numbered by the contact position. For example, on a six-position, two-contact plug, where the outermost four positions do not have contacts, the innermost two contacts are numbered 3 and 4.
Interchangeability
Some modular connectors are indexed: their dimensions are intentionally non-standard, preventing connections with connectors of standard dimensions. The means of indexing may be non-standard cross-section dimensions or shapes, retention mechanism dimensions, or retention mechanism quantity. For example, a Modified Modular Jack (MMJ) using an offset latching tab was developed by Digital Equipment Corporation (DEC) to prevent accidental interchange of data and telephone cables.
The dimensions of modular connectors are such that a narrower plug can be inserted into a wider jack that has more positions than the plug, leaving the jack's outermost contacts unconnected. However, not all plugs from all manufacturers have this capability, and some jack manufacturers warn that their jacks are not designed to accept smaller plugs without damage. If an inserted plug lacks slots to accommodate the jack's contacts at the outermost extremes, it may permanently deform those outermost contacts of an incompatible jack. Excessive resistance may be encountered when inserting an incompatible plug, as the outermost contacts in the jack are forcibly deformed.
Special modular plugs have been manufactured (for example, the Siemon UP-2468) which have extra slots beyond their standard contacts, to accommodate the wider jack's outermost contacts without damage. These special plug connectors can be visually identified by carefully looking for the extra slots molded into the plug. The molded plastic bodies of the special plugs may also be colored with a light blueish tinge, to aid in quick recognition.
The special plugs are preferred for test equipment and adapters, which may be rapidly connected to a large number of corresponding connectors in quick succession for testing purposes. Use of the special plugs avoids inadvertent damage to the equipment under test, even when a narrower plug is inserted into a nominally incompatible wider jack.
The contact spacing is always 1.02 mm (center to center).
Termination
Termination of modular connector cables is very similar, regardless of the number of positions and contacts in the plug. To prevent damage to the plug (and the expensive crimp die-set), the crimp tool must be carefully matched to the plug being attached. For example, termination of a cable with an 8P8C plug involves using a hand crimper or crimp machine containing an 8P8C die-set or an A67T standard die-set. An 8P8C crimp die-set usually looks similar to an 8P8C jack, except for the eight teeth lining the top portion of the die. When the tool is operated, the die compresses around the 8P8C plug. As the die compresses, these teeth force the plug contacts down into the conductors of the cable being terminated, permanently attaching the plug to the cable.
There are two sub-types of plug, that differ only in the type of contacts used. One contact is suitable for solid (single strand) copper conductors and the other is suitable for stranded or tinsel wire copper conductors. The crimper may also permanently deform part of the plastic plug body in such a way that it grips the outer sheath of the cable. This helps to keep the plug securely fastened to the end of the cable, by providing strain relief.
Pinout
The contact assignments (pinout) of modular connectors vary by application. Telephone network connections are standardized by registered jack designations, and Ethernet over twisted pair is specified by the TIA/EIA-568 standard. For other applications, standardization may be lacking; for example, multiple conventions exist for the use of 8P8C connectors in RS-232 applications.
For this reason, D-sub-to-modular adapters are typically shipped with the D-sub contacts (pins or sockets) terminated but not inserted into the connector body, so that the D-sub-to-modular contact pairing can be performed by the equipment maker.
4P4C
The 4P4C connector is the standard modular connector used on both ends of telephone handset cords, and is therefore often called a handset connector.
This handset connector is not a registered jack, because it was not intended to connect directly to the telephone lines. However it is often referred to as RJ9, RJ10, or RJ22.
Handset wiring
Handsets and often headsets for use with telephones commonly use a 4P4C connector. The two center pins are commonly used for the receiver, and the outer pins connect the transmitter, so that a reversal of pin connection is unaffected. Standard handset receivers functions normally when their polarity is reversed, but the electret microphone transmitter used in most modern handsets may not. Many telephones include polarity protection, so that the polarity may be reversed without affecting operation. Handsets manufactured before 1985 using a carbon microphone transmitter are not sensitive to polarity. Some hands-free headsets also may have a 4P4C connector, but the wiring may differ from the above diagram.
Data port
The Macintosh 128K, Macintosh 512K and Macintosh Plus from Apple as well as the Amiga 1000 from Commodore used 4P4C connectors to connect the keyboard to the main computer housing. The connector provided power to the keyboard on the outer two contacts and received data signals on the inner pair. The cable between the computer and the keyboard was a coiled cord with an appearance very similar to a telephone handset cable. The connector on the Amiga 1000 used crossover wiring, similar to a telephone handset. The connector wiring on the Apple computers, however, required a polarized straight through pinout. Using a telephone handset cable instead of the supplied cable could short out the +5 volt DC supply and damage the Apple computer or the keyboard.
Some consumer equipment such as DirecTV set top boxes include a 4P4C low-speed data port connector. Such connectors can be adapted for use with a computer's serial port so that control commands can be sent from the computer to the set top box.
6P6C
Modular plugs are described as containing a number of potential contact "positions" and the actual number of contacts installed within these positions. RJ11, RJ14, and RJ25 all use the same six-position modular connector, thus are physically identical except for the different number of contacts (two, four and six respectively).
The 6P2C, 6P4C, and 6P6C modular connectors are probably most well known for their use as RJ11, RJ14, and RJ25 registered jacks respectively.
RJ11 is a physical interface often used for terminating telephone wires. It is probably the most familiar of the registered jacks, being used for single line POTS telephone jacks in most homes across the world.
RJ14 is similar, but for two lines, and RJ25 is for three lines. RJ61 is a similar registered jack for four lines. The telephone line cord and its plug are more often a true RJ11 with only two contacts.
RJ11 wiring
Cables sold as RJ11 often actually use 6P4C RJ14 connectors (six position, four contacts), with four wires running to a central junction box. Two of its six possible contact positions connect tip and ring, and the other two contact positions are then unused. 6P2C and 6P6C can also be found in stores.
The contacts other than the two central tip and ring contacts are in practice used for various things such as a ground for selective ringers, low voltage power for a dial light, or for 'anti-tinkle' circuitry to prevent pulse dialing phones from ringing the bell on other extensions. With tone dialing, anti-tinkle measures are not required.
Pinout
The pins of the 6P6C connector are numbered 1 to 6, counting left to right when holding the connector tab side down with the opening for the cable facing the viewer.
- ^[a] While the old solid color code was well established for pair 1 and usually pair 2, there are several conflicting conventions for pair 3 (and sometimes even pair 2). The colors shown above were taken from a vendor of "silver satin" flat 8-conductor phone cable that claims to be standard. 6-pair solid (old) bellwire cables previously used by the Bell System use white for pair 3 tip but some vendors' cable may substitute orange for white. At least one other vendor of flat 8-conductor cable uses the sequence blue, orange, black, red, green, yellow, brown and white/slate.
- ^[b] This color scheme originates in the (withdrawn) national standard DIN 47100. The scheme shown here is the correct color code for interfacing with the RJ connector standards.
However, with German domestic telephone equipment (and that in some neighbouring countries), 6P4C plugs and sockets are typically only used to connect the telephone cable to the phone base unit, whereas the mechanically different TAE plug is used at the other end of the cable. Older base units may accommodate the additional connectors of TAE (E, W, a2, b2) and may feature non-RJ standard sockets that can be connected "straight" to TAE plugs. Further, flat DIN 47100 cables typically place the wires in ascending order. When used directly with 6P4C plugs, the colors will be scrambled.
Powered version of RJ11
In the powered version, Pins 2 and 5 (black and yellow) may carry low voltage AC or DC power. While the phone line itself (tip and ring) supplies enough power for most telephone terminals, old telephone terminals with incandescent lights in them (such as the classic Western Electric Princess and Trimline telephones) need more power than the phone line can supply. Typically, the power on Pins 2 and 5 comes from a transformer plugged into a wall near one jack, supplying power to all of the jacks in the house. Trimline and Princess phone dial lights are rated at 6.3 volts and the transformer output is typically around 5 volts, providing a long service life for the incandescent lamps.
Compatibility with structured cabling
With the rise of Ethernet local area networks operating over Cat5e and Cat6 unshielded twisted pair cable, structured cabling networks adhering to TIA/EIA-568-B, ISO/IEC 11801 or ISO/IEC 15018 (home networks) are widely used for both computer networking and analog telephony, but these standards specify the T568-A or T568-B pin-outs compatible with Ethernet. The 8P8C ("RJ45") jack used by structured cabling physically accepts the 6-position connector used by RJ11, RJ14 and RJ25, but only RJ11 and RJ14 have full electrical compatibility. Ethernet compatible pin-outs "split" the third pair of RJ25 across two separate cable pairs, rendering that pair unusable by an analog phone. This was necessary to preserve the electrical properties of those pairs for Ethernet, which operates at much higher frequencies than analog telephony.
Both the third and fourth pairs of RJ61 are similarly split. Because of this incompatibility, and because they were never very common to begin with, the TIA/EIA-568-B conventions are displacing RJ25 and RJ61 for telephones with more than two lines.
8P8C (8 position 8 contact)
The 8 position 8 contact (8P8C) connector is a modular connector commonly used to terminate twisted pair and multi-conductor flat cable. These connectors are commonly used for Ethernet over twisted pair, registered jacks and other telephone applications, RS-232 serial using the EIA/TIA-561 and Yost standards, and other applications involving unshielded twisted pair, shielded twisted pair, and multi-conductor flat cable.
An 8P8C modular connection has two paired components: the male plug and the female jack, each with eight equally-spaced conductors. On the plug, these conductors are flat contacts positioned parallel with the connector body. Inside the jack, the contacts are suspended diagonally toward the insertion interface. When an 8P8C plug is mated with an 8P8C jack, the contacts meet and create an electrical connection. Spring tension in the jack's contacts ensures a good interface with the plug and allows for slight travel during insertion and removal.
Although commonly referred to as an RJ45 in the context of Ethernet and category 5 cables, it is incorrect to refer to a generic 8P8C connector as an RJ45. A telephone-system-standard RJ45 plug has a key which excludes insertion in an un-keyed 8P8C socket. The registered jack (RJ) standard specifies a different mechanical interface and wiring scheme for an RJ45S from TIA/EIA-568-B which is often used for modular connectors used in Ethernet and telephone applications. 8P8C modular plugs and jacks look very similar to the plugs and jacks used for FCC's registered jack RJ45 variants, although the RJ45S is not compatible with 8P8C modular connectors.
The original RJ45S jack mates with a keyed 8P2C modular plug, and has pins 4 and 5 (the middle positions) wired for ring and tip of a single telephone line and pins 7 and 8 shorting a programming resistor. It was intended for high speed modems, and is obsolete.
Telephone installers who wired RJ45S modem jacks or RJ61X telephone jacks were familiar with the pin assignments that were part of the standard. However, the standard un-keyed modular connectors became ubiquitous for computer networking, and informally inherited the name RJ45. While RJ45S uses a keyed variety of the 8P body, meaning it may have an extra tab that a common modular connector cannot mate with, the visual difference is subtle and likely caused the confusion.
Because telephone RJ61 and data RJ45/RJ48 connectors were not widely used and 8P8C connectors in computers became ubiquitous, RJ45 is used to refer to 8P8C un-keyed modular connectors. This practice is followed by electronics catalogs and many electronic equipment manuals. In common usage, RJ45 may also refer to the pin assignments for the attached cable, which are actually defined as T568A and T568B in wiring standards such as TIA/EIA-568.
Standardization
The shape and dimensions of an 8P8C modular connector are specified for U.S. telephone applications by the Administrative Council for Terminal Attachment (ACTA) in national standard ANSI/TIA-1096-A. This standard does not use the short term 8P8C and covers more than just 8P8C modular connectors, but the 8P8C modular connector type is the eight position connector type described therein, with eight contacts installed. The international standard is ISO-8877.
For data communication applications (LAN, structured cabling), International Standard IEC 60603 specifies in parts 7-1, 7-2, 7-4, 7-5, and 7-7 not only the same physical dimensions, but also high-frequency performance requirements for shielded and unshielded versions of this connector for frequencies up to 100, 250 and 600 MHz, respectively.
Pinout
Connectors are frequently terminated using the T568A or T568B assignments that are defined in TIA/EIA-568. The drawings to the right show that the copper connections and pairing are the same, the only difference is that the orange and green pairs (colors) are swapped. A cable wired as T568A at one end and T568B at the other (Tx and Rx pairs reversed) is a "crossover" cable. Before the widespread acceptance of auto MDI-X capabilities a crossover cable was needed to interconnect similar network equipment (such as hubs to hubs). A cable wired the same at both ends is called a "patch" or "straight-through" cable, because no pin/pair assignments are swapped. Crossover cables are sometimes still used to connect two computers together without a switch or hub, however most network interface cards (NIC) in use today implement auto MDI-X to automatically configure themselves based on the type of cable plugged into them. If a "patch" or "straight" cable is used to connect two computers with auto MDI-X capable NICs, one NIC will configure itself to swap the functions of its Tx and Rx wire pairs.
Types and incompatibility
Two types of 8P8C plugs and installation tools (used for crimping the plug onto a cable) are commonly available: Western Electric/Stewart Stamping (WE/SS) and Tyco/AMP. While both types look remarkably similar, the tooling used to install the two different plug types is mutually exclusive and cannot be interchanged between the two types. WE/SS compatible plugs are available from a large number of manufacturers, whereas Tyco/AMP plugs are produced exclusively by Tyco Electronics. Both types of modular plugs will plug into the same standard 8P8C modular jack.
WE/SS and Tyco/AMP 8P8C plugs have different spacing for the cable strain relief. Thus, using a WE/SS 8P8C crimp die set on a Tyco/AMP 8P8C plug will crush the top of the connector and damage the crimp die set, and vice versa. While the WE/SS compatible plug is produced by a larger number of manufacturers than the Tyco/AMP plug, it is still important to know what style is being used to avoid damaging the plug or tool during crimping.
Both types of 8P8C plugs are available in shielded and unshielded varieties, depending on the attenuation tolerance needed. Shielded plugs are more expensive and require shielded cable, but have a lower attenuation and can reduce signal noise.
Although a narrower 4-pin and 6-pin connector will fit into the wider 8-pin jack, the smaller connector can potentially damage the contacts of the larger, because the outside edges of the smaller connector press onto the contacts of the larger. The outside edges of an RJ11/RJ45 plug typically project out by about 0.5 to 1.0 mm further than the contact surfaces, and these edges press the outermost contacts of the larger connector further than if a full-size connector were plugged in. The smaller connector may therefore permanently bend pins 1, 8 or 2, 7 of the larger connector.
Applications
8P8C are commonly used in computer networking and telephone applications, where the plug on each end is an 8P8C modular plug wired according to a TIA/EIA standard. Most wired Ethernet network communications today are carried over Category 5e or Category 6 cable with an 8P8C modular plug crimped on each end.
The 8P8C modular connector is also used for RS-232 serial interfaces according to the EIA/TIA-561 standard. This application is common as a console interface on network equipment such as switches and routers. Other applications include other networking services such as ISDN and T1.
In floodwired environments the center (blue) pair is often used to carry telephony signals. Where so wired, the physical layout of the 8P8C modular jack allows for the insertion of an RJ11 plug in the center of the jack, provided the RJ11 plug is wired in true compliance with the U.S. telephony standards (RJ11) using the center pair. The formal approach to connect telephony equipment is the insertion of a type-approved converter.
The remaining (brown) pair is increasingly used for Power over Ethernet (PoE). Legacy equipment may use just this pair; this conflicts with other equipment, because some manufacturers previously short circuited unused pairs to reduce signal crosstalk. Some routers, bridges and switches can be powered by the unused 4 lines--blues (+) and browns (-)--to carry current to the unit. There is now a standardized wiring scheme for Power over Ethernet.
Different manufacturers of 8P8C modular jacks arrange for the pins of the 8P8C modular connector jack to be linked to wire connectors (often IDC type terminals) that are in a different physical arrangement from that of other manufacturers: Thus, for example, if a technician is in the habit of connecting the white/orange wire to the "bottom right hand" IDC terminal, which links it to 8P8C modular connector pin 1, in jacks made by other manufacturers this terminal may instead connect to 8P8C modular connector pin 2 (or any other pin). Labels and manufacturer's documentation should be consulted whenever an unfamiliar connector is first encountered.
8P8C modular connectors are also commonly used as a microphone connector for PMR, LMR, and amateur radio transceivers. Frequently the pinout is different, usually mirrored (i.e. what would be pins 1 to 8 in the TIA/EIA-568 standard might be pins 8 to 1 in the radio and its manual).
In landline telephony, an 8P8C jack is used at the point a line enters the building to allow the line to be broken to insert automatic dialling equipment, including intrusion alarm panels. In analogue mobile telephony, the 8P8C connector was used to connect an AMPS cellular handset to its (separate) base unit; this usage is now obsolete.
Commonly (and incorrectly) referred to as "RJ45", the physical connector is standardized as the IEC 60603-7 8P8C modular connector with different "categories" of performance. The physical dimensions of the male and female connectors are specified in ANSI/TIA-1096-A and ISO-8877 standards and normally wired to the T568A and T568B pinouts specified in the TIA/EIA-568 standard to be compatible with both telephone and Ethernet.
A similar standard jack once used for modem/data connections, the RJ45S, used a "keyed" variety of the 8P8C body with an extra tab that prevents it mating with other connectors; the visual difference compared to the more common 8P8C is subtle, but it is a different connector. The original RJ45S keyed 8P2C modular connector had pins 5 and 4 wired for tip and ring of a single telephone line and pins 7 and 8 shorting a programming resistor, but is obsolete today.
Electronics catalogs commonly advertise 8P8C modular connectors as "RJ45". An installer can wire the jack to any pin-out or use it as part of a generic structured cabling system such as ISO/IEC 15018 or ISO/IEC 11801 using RJ45 patch panels for both phone and data. Virtually all electronic equipment which uses an 8P8C connector (or possibly any 8P connector at all) will document it as an "RJ45" connector.
Crossover cables
A router to router crossover cable uses two 8 position connectors and a UTP (Unshielded Twisted Pair) cable with differently wired connectors at each end. Although a registered jack specifies the wiring pattern and corresponding form factor rather than just the pin assignments or the physical connector, crossover cables are often incorrectly marketed as "RJ45 crossover cables".
10P10C
The 10P10C connector is commonly referred to as an RJ50 connector, although this was never a standard registered jack in the Universal Service Order Codes. The 10P10C has 10 contact positions and 10 contacts.
The most common uses of the 10P10C connector are in proprietary data transfer systems, such as the Digiboard and Equinox Super-Serial multi-port TIA-232 adapters. 10P10C connectors are also used to implement RS-485 interfaces, and for data link connections in APC and Eaton uninterruptible power supplies. In the latter case, a keyed 10P10C plug with a protrusion on the pin 1 side near the back is used.
This connector is also used by some vendors, such as BOCA, for expansion modules of their multi-port RS-232C serial boards. For example, Cyclades (later absorbed by Equinox) used pin 1 as an "RI" (ring indicator) signal, which is seldom used, allowing an 8P8C plug to be inserted to their 10P10C socket for most applications. The Cisco Systems STS-10X terminal server features this connector. FordNet, a five-pair communications networking medium, also used the 10P10C between terminals.
Motorola uses the 10-pin connector as a microphone connector in several of their mobile radio product lines.
Polycom utilizes this connector to interconnect multiple SoundStructure audio mixers.
The 10-pin connector is also used by Demag Cranes AG in some pendant connections. National Instruments is also using the 10p10c connector for their NI 9237.
MTS Systems Corporation is using the 10p10c connector for their MTS FlexTest® Controller Family.
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