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Comment by bobmcnamara | Hacker News Reader

Most 8P8C other connectors are incompatible with RJ45.

Why wouldn't you say RJ45?

RJ45S and RJ45M are ordering codes for so-called “registered jack” configurations for terminal connections to the U.S. telephone network. These codes were defined until 2000 in the FCC Rules (47 CFR § 68.502(e)) and later in the TIA/EIA-IS-968 standard, and they refer to single and multiple arrangements of two wires and a programming resistor on a miniature eight-position keyed jack.

https://www.govinfo.gov/content/pkg/CFR-2000-title47-vol3/pd...

Unfortunately, the “RJ45” part of these codes has become a metonym for the unkeyed version of the miniature eight-position jack and plug, now widely used for Ethernet and other purposes, but strictly speaking, RJ45 refers to a different connector with totally incompatible wiring.

Specifically, what is colloquially an "RJ45" or "Ethernet" connector is an 8P8C "Bell System Miniature Plug/Jack" (AT&T's original name; it is a smaller version of the older Bell System connectors) / "miniature plug/jack" (FCC genericization of the name by removing "Bell System", even though the word "miniature" is no longer meaningful without context) / "modular jack" (ANSI/IEC genericization). That is what is meant when just "8P8C" is said.

Pedantically speaking, RJ45 (as first defined by AT&T internally[1], and later by the FCC's 47 CFR part 68) is not that. The RJ45 socket is a keyed 8P8C modular jack, not a regular 8P8C modular jack. Here is a photo: https://commons.wikimedia.org/wiki/File:RJ45_female_connecto...

[1]: The "RJ45" designation was originally an AT&T "USOC" (Universal Service Order Code). In the '70s, the FCC told AT&T that they had to allow interoperability from other companies, so the FCC had to publish a bunch of specifications; the meaning of "RJ45" became publicly specified in Bell System Communications' Technical Reference PUB 47101 "Standard Plugs And Jacks" (1979, though I think there might be an older number/revision from the early '70s that I haven't been able to track down). That (in combination with a few other technical references, such as PUB 47102), later became part of the Code of Federal Regulations, as 47 CFR part 68.

RJ45 is a specific AT&T USOC order code to slap a normal 8P8C jack on someone's wall to provide something like multi-line analog telephone service.

RJ45 is a keyed 8-position jack, not a normal 8-position jack. ("Keyed" means that there's a notch in the side making it a different shape; you would not be able to fit an "Ethernet" connector into it.)

Closer is RJ38X, which is a series 8-position jack, not a normal 8-position jack. ("Series" means that the jack shorts pint 1 to pin 4 and pin 5 to pin 8 when there's not a cable plugged in to it; you would be able to fit an "Ethernet" connector into it, but even so it's probably not what you want.)

AFAICT (skimming 47 CFR part 68, and the historical AT&T documents that became 47 CFR part 68), there is no RJ-number for a normal 8-position jack.

> you would not be able to fit an "Ethernet" connector into it

Because of the size being different? Surely a keyed female plug will take a male connector with or without the key. Or did you mean you couldn't fit a RK45 connector into a Ethernet plug because then the key would interfere?

RJ45 is a mechanically (slightly) different connector, but indeed all RJ specs were for phone lines, with RJ45 focused on several lines for high speed modem connections.

The regular ethernet 8P8C connector was defined by both an ANSI and ISO spec, neither of which gave the connector an actual name as it covers modular connector designs. :/

I used to do phone support for a phone company / isp and I have no idea what you just wrote.

To be fair, 5 out of the 6 phone support agents I talked to at Optimum (an ISP) did not know what IPv6 is, so saying you used to do phone support for an ISP isn't really saying much.

you tell him!

RJ45 is a connector with a key notch sticking out and a "programming" resistor joining two of the pins. It won't work for Ethernet at all, the plug side can't even fit in the 8P8C socket Ethernet uses. If you grind off the key it'll still not work, because of the embedded resistor. Also the pinout is totally wrong, so even if you didn't have the resistor it wouldn't work. None of the RJ connectors have the correct pinout for Ethernet.

He said RJ45 is an AT&T part/reference number. Just like RJ11 which is your small phone plug that had 6pins, more rare.

Nit: RJ11 has two contacts.

RJ11, RJ14 and RJ25 all used the same 6P housing though, making them 6P2C, 6P4C and 6P6C connectors, respectively.

Things sold as RJ11 is often 6P4C, making for another error. The rule of thumb is that anything referred to as RJ-something is likely wrong.

Well you definitely SHOULD say RJ45. We do a lot of networking at my job and if I asked for an 8P8C connector, I would get confused stares. Say Ethernet cable, Cat 6 cable (or whatever cat), or RJ45. Sometimes being correct isn’t the right thing to do.

If a contractor installed exactly what he asked for, an RJ45 jack which would be unusable for his needs he would have no grounds to stand on to demand it be corrected without paying more. By specifying the technically correct name as well as the colloquially recognized name he is being precise and accommodating.

If a contract requires RJ45 terminated Ethernet patch cables and the contractor delivers keyed RJ45, they have not delivered because RJ45 doesn’t even have the correct conductor layout to act as an Ethernet cable. Contracts call for RJ45 all the time and there are no mixups. You’d probably find it quite difficult to even find vendors for keyed RJ45

I know this feels like a technically correct gotcha, but in fact is not. Do some parametric searches on digikey and flip through some manufacturer catalogs. If you go out of your way to misinterpret industry standard jargon you won't be paid for your work and you'll lose the contract.

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