A symbol for the square root of –1, an imaginary number. In engineering, j is an indicator, not a number. It doesn’t take part in the math. It just tells you that the data it’s attached to belongs to the quadrature (Q) rather than the in-phase (I) component of an I/Q system. Think of j2 as the minus sign rather than –1, and j–1 as j-inverse (moving from Q to I) rather than 1/j.
Mathematicians and physicists use i to mean the same thing.
A set of Japanese digital carrier standards based on AT&T’s DS-#. The J-1 and J-2 carriers are essentially the same as T-1 and T-2 (1.544 Mb/s with 24 DS-0 VGCs, and 6.312 Mb/s with 96 DS-0 VGCs), with some framing differences. A J-3 carrier is 32.064 Mb/s (five J-2 carriers, plus overhead), a J-4 is 97.728 Mb/s (three J-3 carriers, plus overhead), and a J-5 is 397.200 Mb/s (four J-4 carriers, plus overhead). Compare DS-#, E-#.
Java 2 Platform, Enterprise Edition. An industry standard, led by Sun Microsystems, for development of network applications. It’s based on the Java programming language.
(2017) An international standard protocol for underwater acoustic data communications, named for the two-faced Roman deity of gateways and beginnings. Because it uses sound waves rather than EM waves, its modulation is the relatively inefficient binary FSK (BFSK), with frequency hopping between 13 tone pairs. It encodes data as 64-bit packets: 56 bits of data plus an 8-bit CCITT standard CRC calculated from the polynomial p(x) = x8 + x2 + x1 + 1. It applies a rate 1/2 convolutional code with generating polynomials g1 = [110101111] and g2 = [100011101], so the channel-coded packets are 128 bits.
To open a connection, a transmitter sends a sequence of three tones at 9440, 11520, and 13600 Hz, pauses for 400 ms, and then sends an initial packet. The initial center frequency, 11.52 kHz, is meant for distances under 10 km. Once a connection is established, comunicating nodes can negotiate a different frequency and/or protocol.
A compendium of hacker slang and terminology along with related lore, also called the Hacker’s Dictionary. It’s hosted at http://catb.org/jargon/html/index.html, and mirrored elsewhere.
Jet-Assisted Take-Off. Using rockets to enable aircraft to take off from short runways or with heavy loads, starting in WWII. The “jet” part of the name is intentionally misleading – pilots of the time didn’t trust rockets, with good reason.
A Java-based component architecture similar to ActiveX. Unlike ActiveX controls, JavaBeans can run on any system, although without the home-field advantage that ActiveX has in Windows.
Just a Bunch Of Disks. A group of hard drives that aren’t configured according to RAID. (A RAID’s drives work together to provide improved performance and fault tolerance.)
Originally stood for Joint Electronics Device Engineering Council. The semiconductor standards body of the EIA since 1960. It’s responsible for, among other things, IC chip form factors.
IC chip standards begin with a base two letters specifying the general configuration, such as CC (chip carrier), FP (flat pack), GA (grid array), IP (in-line package), or SO (small outline). These two letters can be prefixed by a third letter describing the positioning of the connectors (e.g. BGA, LCC, PGA, QFP). If this is done, then a pre-prefix letter specifying the package material can also be used (e.g. CBGA, PLCC). After that it starts getting complicated.
There are many, many IC form factor standards. Older types were designed for pin-thru-hole circuit boards, as opposed to the now-dominant surface-mount technology. Some common or representative ones:
BGA – Ball Grid Array. A high-density, surface-mount package, in which an array of contact pads on the bottom of the chip have balls of solder so that they can be soldered into a matching socket. This can’t be done manually, and requires special equipment. Micro-BGA components have pitch less than 1.0 or possibly 1.27 mm.
CDIP – Ceramic Dual In-line Package. A ceramic-encased DIP.
CLCC – Ceramic Leadless Chip Carrier. Subtype of LCC with a ceramic case.
DFN – Dual Flat Non-leaded. Smaller and thinner than leaded packages. The surface-mount contacts are on just two sides, and don’t protrude beyond the chip edges.
DFN chip (underside)
DIP – Dual In-line Package. The most used IC chip form factor from the mid-1960s into the 1990s. As electronics have gotten smaller and faster, manufacturers have abandoned it for much more compact surface-mount (SMT) types. The DIP is made with many pin counts and densities; the most common is 0.1" pin spacing, with the two rows of pins 0.3" or 0.6" apart. The small notch at one end indicates the top of the chip for pin-numbering purposes. Pin 1 is at top left; numbering runs down that side and then up the other side.
12-pin DIP chipDIP switch with 8 switches
A DIP switch is a DIP form-factor component holding an array of tiny two-position switches. It’s a common way to manage user-selected hardware settings in digital circuits, including PC components.
DLCC – Dual Leadless Chip Carrier. An LCC with contacts along just two sides.
FBGA – FineLine Ball Grid Array. A sub-type of BGA with 1mm ball pitch.
LCC – Leadless Chip Carrier. Surface-mount, with inset metal contacts spaced 0.05" apart on all four sides. It’s often mounted in a socket rather than soldered down.
LCC chip
LGA – Land Grid Array. A high-density, no-solder, ZIF package that’s latched into the chip socket to ensure contact, meaning it can be easily removed and replaced. Instead of pins, the bottom of the chip has an array of flat contacts called lands; the (short, stubby) pins are in the chip socket. Around 2006, Intel started using LGA rather than PGA for its desktop CPU chips. See Socket#.
MCR – Molded Carrier Ring. Fine-pitch type with a plastic strip covering the leads to protect them. The plastic strip is removed just before assembly.
MDIP – Molded Dual In-line Package. Another name for DIP.
MLP – Micro-Leadframe Package. Variants include MLP quad (MLPQ), with contacts on all four sides; MLP dual (MLPD), with contacts on two sides; and MLP micro (MLPM).
MSOP – Molded Small Outline Package.
PDIP – Plastic Dual In-line Package. A DIP with a plastic shell, so common that it’s usually just called DIP.
PGA – Pin Grid Array. High-density, with an array of pins projecting from the chip underside corresponding to a grid of holes in the socket. Like LGA, this is a removable, no-solder ZIF design, with latches holding the chip in the socket to maintain contact, and is common for CPU chips. See Socket#.
PGA chip and socket (12 × 12 grid)
PLCC – Plastic Leadless Chip Carrier. An LCC with a plastic shell, so common that it’s usually just called LCC.
44-contact PLCC chip and socket
PQFP – Plastic Quad Flat Pack. A QFP with a plastic shell, so common that it’s usually just called QFP.
PSOP – Plastic Small Outline Package. A SOP with a plastic shell, so common that it’s usually just called SOP.
QFN – Quad Flat Non-leaded. Smaller and thinner than leaded packages. The surface-mount contacts are on all four sides, and don’t protrude beyond the chip edges.
QFN chip (underside)
QFP – Quad Flat Pack. Very common high-density form factor, with surface-mount gull-wing leads on all four sides. The pitch (spacing) of the leads ranges from 1.270 to 0.020 mm. Chip thickness is typically 3.8 to 2.0 mm. TQFP (Thin QFP) is just 1 mm thick. There were no industry standard sizes until sometime after 2000.
QFP chip
QFP-SD – Stacked Die Quad Flat Pack. Two devices in one QFP chip.
SC## – Single Chip. Similar to but smaller than SOT, with 3 or more surface-mount, gull-wing leads in two rows. Sizes include SC70 and SC90. A final -# indicates the number of leads, e.g. SC70-5 meaning 5 leads.
SDIP – Shrink Dual In-line Package. A smaller version of the venerable DIP.
SOIC – Small Outline Integrated Circuit. Another name for SOP.
SOJ – Small Outline J-lead. Similar to SOP, but the pins are bent back into the chip to form loops.
SOJ chip
SOP – Small Outline Package. A flat, surface-mount type with gull-wing leads, as contrasted with the older pin-to-socket DIP for which it was an early replacement before being supplanted by even more compact types. The leads can be on two sides or all four sides. Variations include SSOP (Shrink SOP), TSOP (Thin SOP), TSSOP (Thin Shrink SOP), and VSOP (Very SOP). Also called SOIC.
SOP chip
SOT## – Small Outline Transistor. A family that use plastic cases and 3 or more surface-mount, gull-wing leads in two rows, and incorporate transistors or diodes. Popular sizes are SOT23, SOT143, SOT223, and SOT89. A final -# indicates the number of leads. E.g., SOT23-6 has 6 leads.
SSOP – Shrink Small Outline Package. A smaller SOP.
TDFN – Thin Dual Flat Non-leaded. A thinner DFN with contacts along just two sides.
TO-## – Transistor Outline. A family of sealed metal or plastic packages with leads projecting from one end. Commonly used for transistors.
TO-5 form factorTO-247 form factor
TQFP – Thin Quad Flat Pack. A thinner QFP with contacts along all four sides.
TSSOP – Thin Shrink Small Outline Package. A smaller, thinner SOP.
Japan Electronics and Information Technology Industries Association. Created by the merger of EIAJ and JEIDA (Japan Electronic Industry Development Association) on 1 November, 2000. Like EIAJ, this organization sets standards for IC chips, cooperating with JEDEC.
A 1998 Sun Microsystems software technology, now open-source, that links clients with network services they need through a Jini Lookup Service on a server. A client, in this context, can be anything – device, person, organization. The resulting system is endlessly customizable, and can be applied almost anywhere. It’s similar in concept to Bluetooth, except that it relies on a server rather than being peer-to-peer.
Just-In-Time. For software, this refers to just-in-time compiling, the practice of compiling parts of executables from platform-independent code to platform-specific code only as needed. It’s a compromise between fast but inflexible traditional compiling, which generates one platform-specific executable, and slow interpreted code, which requires an interpreter to access the source code and generate platform-specific executables. Java, JavaScript, and .NET are among the languages/environments that use JIT compiling.
Named for Welsh physicist Brian Josephson. Two superconductors separated by a thin, non-superconducting gap so that superconducting current, which consists of Cooper pairs of electrons, can tunnel in both directions across the gap. This allows two superimposed quantum states – a quantum bit, or qubit – to exist in the junction.
The SI standard unit of work, representing 1 newton acting over a distance of 1 meter. One joule therefore equals one N-m (newton-meter) or W-s (watt-second).
Joint Photographic Experts Group. An ISO & ITU format for compressing and storing graphical images. It uses a lossy compression algorithm, but produces very small files with a .jpg or .jpeg filename extension.
The original JPEG applies a discrete cosine transform (DCT) to 8×8 pixel blocks, quantizes the resulting coefficients, and uses variable-length coding to represent the results. Transmission demands an error-free channel – an error in the JPEG header will render the data stream undecodable, and an error in the image data causes obvious errors in the displayed image. JPEG-2000 compression is based on wavelets instead of DCT.
JavaScript Object Notation. A standard format that Web servers use for structured, human-readable data that they pass to client-side scripts. It’s simpler than XML, which is also used for data exchange. The syntax is based on JavaScript, but it’s just ASCII text, so any scripting language can use the data.
A JSON object contains one or more key:value pairs. Scripts use the keys to read the associated values from or write them to the JSON object. A key is a uniquely identifying text string. A value can be just a text string as well, or can be a number, a Boolean condition (true or false), another JSON object, or an array of any or all of those things. JSON objects are enclosed in {curly braces}, and arrays are enclosed in [square brackets]. Text strings, including those used as keys, are wrapped in "double quotes". Numbers and Booleans have no wrapping.
For example, a simple JSON object could contain just a single array called “Simpsons”, with each member of the array being a JSON object:
JavaServer Pages. A Java-based technology that generates real-time content for incorporation into Web (HTML) pages. These pages commonly have a .jsp rather than .html extension.
Joint Test Action Group. A circuit testing and debugging bus created by an industry consortium of the same name, and adopted in 1990 as IEEE Standard 1149.1, after which the JTAG organization disbanded. The problem JTAG addressed was that physical access (for testing purposes) to dense, complex, multi-layer circuit boards was becoming increasingly difficult.
The solution was to build test circuitry into the chips, and access it through a board-wide bus that can program, monitor, and debug the chips. A JTAG-compliant device has pins for power, ground, and four signals: test clock (TCK), test mode select (TMS), test data in (TDI), and test data out (TDO). Multiple JTAG-compliant devices in a circuit can be tied to the four-wire bus to form a single scan chain.
The test access port (TAP) controller, which is external to the circuit under test, is a state machine with 16 states: test logic reset, run test idle, select DR scan, capture DR, shift DR, exit 1 DR, pause DR, exit 2 DR, update DR, select IR scan, capture IR, shift IR, exit 1 IR, pause IR, exit 2 IR, and update IR. (DR = data register, IR = instruction register.) State transitions are driven by the combination of TMS signal (0 or 1) and current state. An optional fifth line (TAP reset, or TRST) can be included to provide an asynchronous reset to the TAP controller. The process of reading and setting the values of pins in JTAG-compliant devices via the JTAG bus is called boundary scanning.
JTAG and the similar BDM assume that the microcontrollers and microprocessors they monitor use off-chip memory. On-chip memory is becoming common, requiring new debug technologies such as Nexus.
Joint Tactical Radio System. A defunct program to develop an all-services radio system for the US military, to replace all the tactical radios currently in use. This embraces many different roles and operating environments, translating to different power levels, frequencies, ranges, and weight limits. That’s probably why the project, launched in 1997, was canceled in 2011 without ever delivering.
Jonzy’s Universal Gopher Hierarchy Excavation And Display. A 1990s search engine, now obsolete, that searches and indexes Gopher sites by keyword. It’s similar to but less powerful than Veronica, because it searches only high-level menu items and doesn’t include file-name menus.
