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.


Ambox notice.png This explanation may be incomplete or incorrect: Created by a FIREWIRE TRIBUTE PIN. Should include a column for the actual purpose of the pin. Do NOT delete this tag too soon.
If you can address this issue, please edit the page! Thanks.

Electronics connecters 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 ezcitement 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. 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 whrn 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 Many actual pin configurations use +5 volts to supply power to a device. That's not what this means. 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. Voltages above 5 are not usually used in micorcircuitry cables. (See Pin 4) 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. In this case "Rx" is used as part of a pun on "Rx (prescription) only." 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 legal to insert the USB-C connector rotated by 180°. 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 interferences. This mixes that practice with a humorous reference to the notion of matter versus antimatter. Currently there is no such thing as antidata. 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 often have ports connecting to common sources of various supplies (oxygen, water, fuel, vacuum). 12 TMDS Clock-
+240V DC This is a reference to washing machines and other large home appliances, which use 240V AC for power. Additionally, many cable specifications try to provide a way to power devices over them, bit 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 help surveillance cameras or telephones to work with one connecting cable only.  But with this much voltage on such a small cable, a short or a peripheral drawing too much current would seem more likely to quickly cause a fire on such thin wires, melting the insulation and wiring in a sudden burst of toxic smoke.
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 for washing machines, not microcircuitry, and conversion between the the two kinds of power everywhere makes for transformers being embedded in most modern devices, taking extra materials to manufacture and losing a significant percentage of all electricity in the conversion. 15 SCL (I²C serial clock for DDC)
Amazon Copyright Pin HDMI can optionally be protected by a digital rights management (DRM) scheme, known as HDCP. This pin humorously implies the presence of a different DRM scheme specific to Amazon.com, 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 rrquests 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). 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, unless it is this one that supplies something like soil (like soup). 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

HDMI 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

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
GND A ground pin
GND A second ground pin
GND A third ground pin
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.
GND A fourth ground pin. A real micro USB only has one ground pin, in addition to the metal plug housing.


Label Explanation
+5V DC A common electronics supply voltage.
+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.
+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, producing smoke and melting the metal together such that the cable likely couldn't be removed. But 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.
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 docile 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 to accidentally connect 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.
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.
+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 thr 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. There 3.3eV/C would be 5.287183 * 10^-19 volts, or slightly more than 1/2 attovolt. It could also be read as +3.3 eV/c (per speed of light), in which case it is equivalent to a momentum of 5.3E-28 kg m/s, In high-energy physics, the momentum of particles is sometimes expressed in MeV/c or GeV/c units.
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.
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.
+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. NOTE: Positrons cannot be conducted through normal matter conductors as they would annihilate with the electrons.
Pin Roulette Presumably in the same vein as "Chat Roulette", 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.
GND Ground pin. Typically denoted as "GND".
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.
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. 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.

The fact that only half of the USB-C pins are documented might hint to an alternative way to manufacture connectors that can be legally 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°.


Label Explanation
Pin An 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.


[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)