2317: Pinouts

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The other side of USB-C is rotationally symmetric except that the 3rd pin from the top is designated FIREWIRE TRIBUTE PIN.
Title text: The other side of USB-C is rotationally symmetric except that the 3rd pin from the top is designated FIREWIRE TRIBUTE PIN.


Electronics connectors are designed to transport both information and power. A pinout diagram describes the function of each pin such as to communicate data, transport power, physical function (keying), etc. In this comic there is an absurd alternative to the actual pins used in connectors. The pin labels are references to many tech issues and attributes, and not all may be documented correctly here.

Hardware hobbyists might feel excitement at seeing a unified specification for these common connectors, but the comic is of course humorous. The real life diagrams are as follows: HDMI, Micro USB, USB-C.


Label Explanation Pin No. Actual purpose
+3.3V DC 3.3V is a typical voltage in digital electronics. The "+" Indicates a voltage positive with respect to ground. 1 TMDS Data2+
Data Most digital communications cables contain at least one wire that carries data. Serial communication protocols have one data line. Typically there will be a more descriptive name if there are multiple data pins. "Data" and "3.3V" set up the appearance of a normal pinout diagram to experienced readers. 2 TMDS Data2 Shield
-3.3V DC Negative voltages were used more frequently in the past; however, modern systems typically generate any negative voltages they might require internally from the given positive voltages. This could be a reference to frustration specification implementers have to go through when handling hardware quirks: negative voltages are very abnormal, and would require design alteration to meet, but the designers of a ubiquitous plug can do whatever they want. 3 TMDS Data2-
+5V 5V is a typical voltage in digital electronics. The "+" indicates a DC voltage, positive with respect to Ground. V is also the Roman numeral for 5. 4 TMDS Data1+
Tx "Tx" typically refers to pins used to transmit as opposed to "Rx" (receive). 5 TMDS Data1 Shield
+6VI V is usually used to represent volts, but here, VI represents the Roman numeral 6. (See Pin 4)
Voltages above 5 are not usually used in micro-circuitry cables.
The product VI (Voltage * Current) is equal to Power.
6 TMDS Data1-
Wx "Wx" does not typically exist in the "Tx"/"Rx" scheme. In the weather forecasting community, Wx means "weather". 7 TMDS Data0+
+7VII V is usually used to represent volts, but here, VII represents the Roman numeral 7, continuing the pattern from above. 8 TMDS Data0 Shield
Rx Only "Rx" typically refers to pins used to receive as opposed to "Tx" (transmit). This description might also be related to the fact that most Ethernet devices can exchange the Rx and Tx port automatically, if needed, in order to eliminate the need for crossover cables and the fact that it is possible to insert the USB-C connector rotated by 180°.
Additionally, "Rx", standing for the Latin "recipe" is a common abbreviation for medical prescriptions. Some medicines are only available by prescription and not over-the-counter, often abbreviated as "Rx Only".
9 TMDS Data0-
Antidata Some ports use differential signaling, where a signal and its inverse are sent over a pair of pins (e.g. D+ and D-). The combined signal is more robust to interference. This mixes that practice with a humorous reference to the notion of matter versus antimatter. Currently there is no such thing as antidata.[citation needed] Antidata pins could be needed in the future as low-entropy or high-entropy source in quantum communication connectors to securely dispose of data. 10 TMDS Clock+
Unknown Occasionally extra pins will be included for future use; however they will typically be labeled "reserved" to point out that their usage is not yet defined. The use of "unknown" suggests that this pinout is based on an incomplete reverse-engineering of the HDMI format, instead of on the official documentation, or maybe the official documentation doesn't explain it and this pin's function is being kept as a secret by the ones who designed it. 11 TMDS Clock Shield
Water Labs and hospitals often have ports connecting to common sources of various supplies (oxygen, water, “medical air”, pressurized air, natural gas, vacuum). It may also refer to the HDMI to garden hose meme. 12 TMDS Clock-
+240V DC This is a reference to heavy-duty home and commercial appliances, which use 240V AC for power. Additionally, many cable specifications try to provide a way to power devices over them (PS/2, USB, Power over Ethernet,) but these small communication cables usually carry only DC and not AC electricity.

High voltage power is only sometimes used in small communications connectors. It might help to keep the current of power lines low to avoid generating excess heat. With +240V DC inside the HDMI cable, monitors would not need any longer a separate power plug. This is similar to Power-over-Ethernet, which does not exceed 60V, or ISDN, which goes up to 110V depending on country, and helps surveillance cameras or telephones to work with one connecting cable only. But with this much voltage on such a small cable, a short could lead more likely to melting the insulation and wiring in a sudden burst of toxic smoke. The produced thermal power through a short with resistance R_short that is building up is U²/R_short, increasing with the square of the voltage (also the breakdown voltage of the isolation is more easily reached). However, a high voltage reduces the chance of a peripheral drawing too much current, which could cause a fire on thin wires otherwise: The current through the cable for a device, needing a given power of P, is I = P_dev/U, the thermal power of the cables with resistance R is P_cables = R_cables*I² = R_cables*P_dev²/U², so it is reduced by the square of the voltage.

13 CEC
Vacuum Labs often have ports connecting to common sources of various supplies (oxygen, water, fuel, vacuum). 14 Reserved (1.0-1.3a), Utility (1.4+, optional)
5V AC Pins often supply low voltage direct current to devices. This pin supplies 5V alternating current, which is not typically supplied. AC is used in electrical transmission because it can be stepped up to high voltage to minimize transmission losses, then stepped down to more useful voltages right before loads. DC power is required for logic circuitry like in computers. Conversion between the two kinds of power everywhere would make for transformers being embedded in most modern devices, taking extra materials to manufacture and losing a significant percentage of all electricity in the conversion. Having both is not entirely unheard of: the Commodore 64 power supply provides both DC, for the logic of the motherboard; and AC, for powering the tape drive. 15 SCL (I²C serial clock for DDC)
Amazon Copyright Pin HDMI can optionally be restricted by a digital restrictions management (DRM) scheme, known as HDCP. This pin humorously implies the presence of a different DRM scheme specific to Amazon the company, as well as poking fun at the fact that copyright is an explicit part of the HDMI protocol (although it is not assigned to a specific pin). This is likely a reference to how chips and cables and specifications often have specific strange additions or functionality in them to meet the requests of various bodies invested in the design process: additionally after reverse engineering many strange things have been found laying around in the functionality of hardware that do unexpected special things. 16 SDA (I²C serial data for DDC)
GND GND typically refers to "ground" on pinout diagrams. Remarkably, this is correct. 17 DDC/CEC Ground
Decorative Decorative elements are pieces of an assembly that serve only an aesthetic purpose without any technical function. This would not be particularly useful inside a connector, as almost no one will ever see it--however, in HDMI, pin 14 was reserved for future use in versions 1.0–1.3a (and was only assigned an official use in 1.4). A decorative pin is also a popular piece of jewelry. In electronics unneeded pins are often marked NC (Not Connected). 18 +5V
Ground Compared to Pin 17 ("GND"), this appears to be the same ground pin. However, the presence of both GND and Ground seems to imply that GND represents something other than the standard "ground" pin. Alternatively, this pin could supply "Ground" as in soil (like the "soup" electrical outlet from 1293). Some systems have different grounds for analog and digital sections, but they would typically be disambiguated by terms like AGND. Many pinouts have multiple grounds or DC supplies with no apparent explanation, seen more prominently on Randall's USB connector. 19 Hot Plug Detect

The HDMI interface uses four pairs of shielded twisted-pair connectors, along with seven other connectors. (Twisted pair means a wire is wrapped with the other wire that returns the current to the original device, thus minimizing electromagnetic noise. Shielding refers to wrapping a cable with a conductor to absorb the energy of noise.) Three of these pairs are for data (TMDS Data0, Data1, and Data2) and the other is a clock. These pairs take up three pins as one of them is a ground pin for the shielding wrapped around each pair. TMDS stands for "Transition-minimized differential signaling" and is also used in the DVI standard.

DDC stands for "Display Data Channel" and is based on the I²C serial standard. It is used to allow the transmitting device to learn what formats of data the receiving device can accept.

CEC stands for "Consumer Electronics Control" and is supposed to allow a single remote control to control multiple devices.

"Hot Plug Detect" refers to hot-plugging, where a cable is connected to a device already turned on. The device should then ideally detect that the cable has been plugged in and respond appropriately.

Micro USB

Label Explanation Pin No. Actual Purpose
GND A ground pin 1 +5 V
GND A second ground pin 2 Data- (Differential signalling)
GND A third ground pin 3 Data+
USB Apparently the only data pin in this connector. This could be a play on how the USB specification tries to squeeze all data onto a single channel as if it is a software protocol rather than specializing the structure of a physical cable to provide for rapidly carrying the information likely to flow over it. USB is an external packet network similar to Ethernet, but uses a totally new design without any reuse of prior similar specification. Ironically, in the real Micro- and Mini-USB standards, this pin is not connected to a wire in the cable. 4 On-The-Go ID, does not have a wire in the cable.
GND A fourth ground pin. A real micro USB only has one ground pin, in addition to the metal plug housing. This is the location of the actual ground pin. 5 Ground.

A ground pin is commonly found on USB and other pin connectors. At least one ground is necessary to complete the circuit, and some cables use multiple ground lines to distribute current or to support twisted pairs. However, there is no purpose served by having many more ground pins than data pins. Therefore, it seems rather silly for the micro USB to have 4 ground pins and only 1 functional "USB" pin. It also does not give much information about what the "USB" pin would do, as opposed to a standard pinout diagram. This diagram also leaves out the +5V power pin that is present in the real micro USB connector, which would render most USB peripherals unable to function.

The ordering and count of the pins may be an allusion to Monty Python's "Spam" sketch, in which one of the many Spam-related menu items is "Spam, Spam, Spam, egg, and Spam".


Label Explanation Pin No. Actual Purpose
+5V DC A common electronics supply voltage. A12 Ground
+3.3V DC Another common electronics supply voltage. This pin is not present on USB type C, mostly because the voltage is too low to supply useful levels of power at the current limit of the pins. A11 Rx2+ (Differential Signalling for Superspeed or alternate modes)
+120V AC Residential supply voltage in the United States; its use on an electronics connector would be very unusual, as it would burn out any unguarded transistor-logic electronics, letting out the magic smoke and melting the metal together such that the cable likely couldn't be removed. This high a voltage might be an allusion to the high voltages USB-C screens and similar devices can request in order to get enough energy without exceeding the cable's current limit. A10 Rx2-
Boobytrap Pin (Pure solder) Solder is a metal alloy with a low melting temperature (typically around 360°C, but in special cases melting points between 90°C and 450°C or above are available), used to bond electronic components together permanently. Most solder materials are quite ductile so it might be too easy for the forces a connector is subject to to permanently change their shape. Along with the fact that the resistance of solder is typically way higher than that for copper the pin under heavy electrical stress could overheat and melt, thus bonding the connector to its receptacle, and thereby "trapping" the receptacle. Additionally putting a pin that might slightly change their shape directly next to a high voltage pin means risking accidentally connecting two power rails with entirely different voltages together (causing an overvoltage in the lower-voltage rail that is likely to break the circuit, as has happened with some Apple devices). In this case the +120V AC pin might be able to deliver enough power to actually fuse the solder-only pin. This could also be a reference to increasing publicity around the fields of electronics security: hobby reverse engineers have been finding ways for some time now to evade the blown fuses in microcircuitry preventing them from being reprogrammed, using glitching techniques.

Another issue is that the heat required for anyone to solder a wire to the cable-side of that pin would cause the entire pin to melt and coagulate, triggering the boobytrap, and causing the pin to disappear and possibly make a mess next to the other pins.

A9 VBUS (+5V)
Mechanical All connectors include portions whose job is to ensure a solid connection between the cable and the port. This is typically not the job of the pins, however. In real USB type C connectors, this task is performed by the center tongue of the female connector. It could also imply transmission of energy or signal through a mechanical principle, such as torque or reciprocating motion. A8 SBU1 (Sideband Use)
+3.3eV/C Electron-volts per coulomb would be a very unusual unit to see in a pinout. These units are used in particle physics, but the appearance is very similar to the common 3.3V supply. It would be a unit of electric potential equivalent to approximately 1.60217662 × 10-19 volts. Thus 3.3eV/C would be 5.287183 × 10-19 volts, or slightly more than 1/2 attovolts. It would be extremely difficult for most consumer electronics equipment to determine the difference between this and 0 volts. It could also be read as +3.3 eV/c (per speed of light), in which case it is equivalent to a momentum of 5.3 × 10-28 kg m/s, In high-energy physics, the momentum of particles is sometimes expressed in MeV/c or GeV/c units. A7 D- (USB 2.0 Data)
Candlepin Randall is making a pun on the word pin, which refers to a electrical connector pin as well as a thing to knock down in bowling. Candlepin bowling is a form of bowling. A6 D+
Facebook use This would imply that Facebook had a hand in specifying USB type C, and had a pin dedicated to their use. This could be seen as strange given Facebook's primary business is web technology and would have little need for a dedicated pin in the USB standard, but a lot of back doors have been found in electronics. A5 CC1 (Configuration Channel)
+5V (Positrons) Positrons are the antimatter counterparts to electrons. So this pin is supplying +5V, but does so by sourcing positrons into the device rather than sinking electrons out. Positrons cannot be conducted through normal matter conductors as they would annihilate the electrons. A4 VBUS (+5V)
Pin Roulette Presumably in the same vein as "Chatroulette", this pin's purpose is not rigidly defined and is instead left to chance. According to the title text, this pin's counterpart on the other side of the connector is the "FireWire Tribute Pin", so this cable would only be truly rotationally symmetric (which is the whole point of the USB-C connector) when the pin roulette ball lands on that same function. A3 Tx1- (Differential Signalling for Superspeed or alternate modes)
GND Ground pin. Typically denoted as "GND". A2 Tx1+
SKY Reference to the ground pin, which refers to the common grounding on the larger metal body. There is no corresponding "sky" pin, although sky is often thought as the opposite of ground. A1 GND
FireWire Tribute Pin In title text: FireWire is Apple's version of IEEE 1394 which is a 6 pin connector that has a ground pin, a power pin and two pairs of data pins. It was something of a competitor to USB, and had faster data rates than USB 2.0, but is much less popular now, and has essentially been replaced by Thunderbolt. Presumably the writers of the USB-C standard (which far outpaces its data rates) wished to memorialize it somehow, although it's not clear how the pin actually works for that purpose. B3 Tx2-

The two sides of a USB C connector are labeled "A" and "B". These are rotationally symmetric, mostly. For example, B10 and B11 are Rx1, a separate twisted-pair for receiving information in Superspeed mode compared to A10 and A11's Rx2. This gives two Rx/Tx pairs for Superspeed use. CC1 and SBU1 are mirrored to CC2 and SBU2. However, the D, VBUS, and GND pins are perfectly mirrored.

The fact that only half of the USB-C pins are documented might hint to an alternative way to manufacture connectors that can be inserted rotated by 180°: Make the receiver use only the right side of the pins and make the sender connect both the left and the right side so all Pins that might match a function are connected correctly no matter if the cable is rotated by 180°. However, doing this would result in only having one Rx/Tx pair for Superspeed use.


A coaxial RF connector has two contacts - one pin, and the shield; typically the whole connector is labeled with whatever function/signal is carried by the pair. The joke here is that the label is technically correct (the best kind of correct),[citation needed] but not very useful to the end user, as it does not specify the voltage rating, impedance, connector size, or other useful information about the cable. Some serial data transmission systems, such as Ethernet, used coaxial cable early on as a low cost, widely available solution, however most of these have largely become obsolete. A common coaxial cable still widely in use is RG-6, which is typically used to deliver satellite television, cable television, and cable Internet services in the United States and Canada.


[Caption at top]
Quick Reference Guide
[Four common connectors are depicted - vertically, rather than the usual horizontal orientation.]
[The first connector is a 19-pin HDMI connector.]
[The nine pins on the left are labeled:]
  • Data
  • +5V
  • +6VI
  • +7VII
  • Antidata
  • Water
  • Vacuum
  • Amazon Copyright Pin
  • Decorative
[The ten pins on the right are labeled:]
  • +3.3V DC
  • -3.3V DC
  • Tx
  • Wx
  • Rx Only
  • Unknown
  • +240V DC
  • 5V AC
  • GND
  • Ground
[The second connector is a 5-pin Micro USB connector.]
[The five pins are labeled:]
  • GND
  • GND
  • GND
  • USB
  • GND
[The third connector is a 24-pin USB-C connector, with only the right side labeled.]
[The twelve pins on the right are labeled:]
  • +5V DC
  • +3.3V DC
  • +120V AC
  • Boobytrap Pin (Pure Solder)
  • Mechanical
  • +3.3eV/C
  • Candlepin
  • Facebook Use
  • +5V (Positrons)
  • Pin Roulette
  • GND
  • SKY
[The fourth and final connector is a 1-pin COAX connector.]
[The one pin in the center is labeled:]
  • Pin

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Can we add this one to a new category, "Comics that Randall makes just to screw with xkcd wiki contributors"? I can think of plenty of candidates for this category! Cosmogoblin (talk) 21:42, 8 June 2020 (UTC)

Wouldn't be more useful to make a category for comics that Randall DOESN'T make to screw with xkcd wiki contributors? Might be smaller ... -- Hkmaly (talk) 22:26, 10 June 2020 (UTC)

The claim that a coax has only one conductive part is incorrect. It has two. The pin is the inner conductor. The shield is the outer conductor. Without both it wouldn't work.

yes, but it's a "pin" out. Hence, "pin" WhiteDragon (talk) 21:37, 9 June 2020 (UTC)

I'd also say that the claim at the top that a pin can have only one bit or one voltage of power at a time is incorrect. Power over Ethernet is a perfect example of power and data at the same time. There are also plenty of types of signals which transmit multiple bits at once. A simple example would be a signal using four voltage levels to transmit two bits simultaneously, but there are many more fancy analog encodings that use phase and frequency and other characteristics to transmit data. Plus, you can often included two signals on the same conductors. For example, ADSL combined a normal phone signal and a higher frequency data signal on the same lines. Also cable TV combined many signals on one set of conductors.

So, anyway, I'd remove the claim. Mootstrap (talk) 23:00, 8 June 2020 (UTC)

Just because it’s interesting: DCC with RailCom+ allows some cool stuff. It allows many-to-many high-power power transmission, robust many-to-many bidirectional data transmission, hot-swap with automatic configuration and collision resolution, physical position tracking of the connected devices, some way of short-circuit resolution with continued communication, mixing with other protocols, and all with only two pins, which may be arbitrarily interchanged at any time. Admittedly it has a much lower data rate than Power over Ethernet and terrible EMI, but potentially much higher power. 08:22, 9 June 2020 (UTC)

For that matter, the RF cable connecting a regular TV antenna, or the wire in a car that connects the radio antenna, carries the signals of all the channels. 02:20, 9 June 2020 (UTC)

I think "Pin Roulette" is a pun on Penn Jillette, the talkative half of the Penn & Teller magic act, and maybe also a reference to chatroulette. Barmar (talk) 23:10, 8 June 2020 (UTC)

Possible, but I'd stick with the simple explanation - that the "Pin Roulette" pin selects a random function when the connector's plugged in. 23:18, 8 June 2020 (UTC)
Modern conectors additionally tend to have multi-purpose pins, which might be dangerous if you guess the current meaning of the pin wrong.Gunterkoenigsmann (talk) 06:15, 9 June 2020 (UTC)

In addition to pins being able to carry both data and power, or to carry multiple bits at a time, some pins function as clock signal pins that indicate bit boundaries rather than themselves carrying data; therefore I also think the claim should be either omitted or changed entirely. Vaedez (talk) 23:33, 8 June 2020 (UTC)

Firstly, no Soup? Secondly, GNDN might easily have been referenced. Thirdly, would a pin made of solder melt, as pins connected to wires/boards by solder do not melt the solder (under proper range of use). 23:38, 8 June 2020 (UTC)

i think the implication is that it could melt, which is a trap--Vaedez (talk) 23:48, 8 June 2020 (UTC)

Perhaps we should add the actual usage of the pins to help those who actually want to know? 00:08, 9 June 2020 (UTC)

Wouldn't 3.3eV/C be a tiny fraction of 3.3V, since a columb is a much greater value of charge than that of the electron?-- 00:24, 9 June 2020 (UTC)

Depending on how you read it, the third pin from the top might match the 120V AC. This would make it a different kind of "tribute" to FireWire... EHusmark (talk) 07:52, 9 June 2020 (UTC)

AFAIK FireWire allows many-to-many communication, while USB never did. The FireWire tribute pin could be a way to establish many-to-many communication. Alternatively, FireWire allows daisy-chaining, while USB supports only a tree network trough hubs. The FireWire pin could be somehow physically strange, so a second USB-C cable could be connected to it. 08:22, 9 June 2020 (UTC)

The "FireWire tribute pin" bit actually probably was intended to be about the 110V pin. Providing 110V is, of course, absurd, but FireWire was (I think) the first computer bus to use a relatively high bus voltage to send lots of power over the wire. The spec actually requires hardware to handle up to 30 VDC. In fact, I once heard lore about an early prototype PowerMac G4 (I think) that was nicknamed "FireBurner". Apple built it at one point in the distant past, and actually provided 30V worth of bus power. They didn't ship that configuration to the public as far as I know, but they used to make it available to companies who wanted to test their FireWire hardware for compatibility. Unfortunately, a lot of hardware manufacturers in the early days didn't pay attention to that 30V number and assumed that the hardware would always provide 12V like the Macs that had shipped up to that point. When they actually encountered 30V, a lot of those early devices didn't survive. Dgatwood (talk) 19:49, 6 August 2020 (UTC)

Added a little more description to the coax cable section, just in case it wasn't obvious to a layman what an example of the cable would be or why it was included with a cartoon about digital data cables.-- 22:46, 10 June 2020 (UTC)