RJ45 – a name that will ring a bell with nearly everyone: the connector on Ethernet cables that joins a PC and router to the internet. Sometimes it might be called a Western connector or even Western modular connector, although it is not at all modular but consists of one piece of translucid polycarbonate with eight gold-plated contacts.
From 1877 there was the Bell Telephone Company in the USA – named after Graham Bell, the American inventor of the telephone. Taken over in 1899 by its own affiliate for long-distance calls AT&T (American Telephone & Telegraph), the whole conglomeration then called itself American Bell (aka Ma Bell), and held a monopoly for telephony in the USA.
In 1881 already AT&T took over the Western Electric Manufacturing Company, founded in 1856: a manufacturer of typewriters, alarm systems, and relays for Western Union. In 1925 Bell Telephone Laboratories (aka Bell Labs) emerged from equal parts of the Western Electric Research Laboratories and development departments of AT&T. This was where not only the transistor but also the hamming code, the Hall effect, the laser, CDMA modulation, Unix, C++ and lots more was developed and elaborated. Bell Labs lead the world with seven Nobel Prizes for Physics.
It all started in 1976 - RJ45 connector had special coding
Bell Labs submitted the concept for a very low-priced jack as a standard connection for telephones, this being registered by the Federal Communications Commission in 1976 and also identified by the name RJ (registered jack). The following numbering is the sequence of registration and thus purely coincidental – 2-pole RJ11 (contacts 4/5), 4-pole RJ14 (contacts 3/4/5/6), 6-pole RJ25 (contacts 2 to 7), and the 8-pole modular connector (contacts 1 to 8) as RJ45. The latter is standardized in detail in IEC 60603 for its high-frequency characteristics up to 2 GHz (in a shielded version), and in EIA/TIA-568 for 10/100BASE-T and 1G/10GBASE-T for Ethernet cabling.
The name »modular« is a sign of the times. Telephone sets were modularized so that by means of RJ11 on the telephone receiver, for example, this or its cord could be replaced without having to dismantle the basic unit plus dial.
The original RJ45 connector had special coding, and for high-speed modems featured only two contacts (4/5) and two additional bridged contacts (7/8). When you speak of RJ45 today, it is the familiar Ethernet connector with eight contacts, and the deficit that there are two twisted pairs on the contacts 4/5 and 3/6 (for reasons of the historic RJ11–>RJ14 compatibility), which led to difficulties in crosstalk compensation, especially at higher data rates.
The apparent backward compatibility, meaning you can actually plug RJ11, RJ14 and RJ25 connectors into RJ45 sockets, has to be treated with caution. The slimmer connectors have plastic ribs on the edges, so they can extra bend contacts 1, 2, 7 and 8 of an RJ45 socket, and damage them to the extent that after such attempts at plugging an RJ45 connector will no longer make the named contacts.
RJ45 connector versionsFig. 1-3: specially fitted RJ45 connectors
Nevertheless, RJ45 was not designed for tough environments. Although it proves the opposite, because there are billions of them in use worldwide. Unfortunately connectors – especially when they work well – are again and again stretched beyond specifications. That also happened to RJ45. There you had a standardized connector face, so it was plugged into environments for which it was not designed.
Still, somebody had a bad conscience, for which reason the connector was given an armored case (Fig. 1) to guard against damage. The locking mechanism of these RJ45 connector designs is much more stable, but at the same time it puts stress on the socket because of the extra weight of the armored case. So the connector has to be guided leading into the receptacle, which costs depth.
Other »special« designs address IP65 rated watertightness, or extremely thick CAT5 cable with AWG22 cores in the industrial sector, or they pack RJ45 with the ODVA quarter-turn fastener (Fig. 2). A producer of base stations came up with a value-for-money solution in terms of watertightness a few years back. IP55 rating is achieved by a rubber cap with sealing lips (Fig. 3). The (standard) RJ45 is first plugged, then the »rubber boot« is pushed over the connector from the cable end, and seals by the sealing frame of rigid plastic.
And why all the effort?
There is one simple reason for all the custom solutions: you want a service person anywhere in the world to be able to take signals off these interfaces with a plain patch cable and a laptop. That could even be possible in a bottling plant in Nepal!
But there are alternatives for Ethernet interfaces. A new connector face, called RJ Point Five, addresses three weak spots of conventional RJ45:
- The space problem when many connections have to be housed side by side in a »pizzabox» – a 19-inch rackmount. The system to date has 13.97 mm spacing in a daisy-chained case, and up to 19 mm if shielded sockets are set singly in the front panel.
- The »historical« contact occupancy of the wire pair 3/6 with more crosstalk to neighboring contacts means compensation problems in socket and connector.
- Piercing termination of wire pairs in the connector is critical if wire structure and wire insulation are not optimally matched to blade contacts of the connector.
A few years back TE Connectivity designed RJ.5 (Fig. 4), with a 7 mm center line and – because of thumb width – providing release straps on the connector to unlock. But there are optical fibers or LEDs in the socket, lengthened by indicators in the connector to simply check port activity.
Three connector typesFig. 4-6: RJ.5, dual-row Mini I/O and M12
Given the density of the pins, the sockets are produced in press-fit technology. Wire contacting in the field-installable connector is by improved piercing technology with AWG26 wire guiding. Of course, because of connector proximity (see double-decker with 2 × 8 ports in Fig. 4), RJ.5 is mainly intended for routers, switches and storage.
A further variant, the Mini I/O for industry, appeared on the market in 2016. The connector face with eight contacts (Fig. 5) in two rows of 1.27 mm spacing has two coder variants. Light indicators are not provided; on the other hand the Mini I/O only needs a front-panel cutout of 11 x 9.5 mm, nor is there a screen join to the front panel. The connector allows connection of 8 × AWG26 in piercing technology with wire guiding or 4 × AWG22 soldered on. The highly stable snap lock is between the metal collars of the shields, and – similar to SFP – is released by a plastic slider in the connector. Extra to vertical and angled receptacles for SMT processing, there are also cable couplings that can be used as front-panel lead-throughs. These can also be installed in the field by piercing with AWG26 wire guiding.
M12 – the »workhorse«
The screw-locked or speed-locked M12 round plug connector (Fig. 6) has become a genuine workhorse for industrial electronics applications. In the meantime there are more than ten codings that avoid false insertion on pole counts of 3 to 12. That also includes 8-pole X-coding, with the four wire pairs required for Ethernet, neatly separated in shielded chambers of this round plug connector, and allowing CAT6A operation up to 10 Gbit/s. All this is even available in an IP67 rated watertight version with metal casing and insulation displacement for fast cable connection.
The drawbacks are obvious: M12 is larger (20 mm diameter because of the locking), and costlier than the named alternatives. Joining the angled receptacle to the circuit board of the device is a headache because of the rear attachment and the tolerances of multiple ports. The X-coded M12 solution nevertheless gained ground; probably also because aspects like watertightness and robustness are unbeatable arguments. That takes us back to the beginning, and the question remains unanswered: »Does RJ45 make sense for industrial applications?«
The answer can only be »yes –but«. Yes, because the cost of RJ45 is unbeatable, its universality is unique, and the variety of products (including RJ45 with integrated transformer plus 3+6 compensation) covers the widest range of all. But, since there are simply applications that demand too much of RJ45: because of the density of ports, because of watertightness, because of cable pull, or because of the needed wire cross-sections in an industrial application.
You have to think of all these things when using RJ45 in industrial design. But as a developer you must also remember that a Nepalese may only have a laptop with an RJ45 patch cord when they want to inspect their system. If there were an interface with a more suitable connector face on this system, it is doubtful if they had a suitable adapter cord.
after working as a radio and TV technician, studied communications engineering at Nuremberg College of Technology. His career first took him as a project and sales engineer in process automation to AEG in Nuremberg. In 1975 he moved to the components division of AEG Telefunken, as a product manager for tantalum and film capacitors. This was followed by positions at TRW and Labinal Components & Systems before in January 1994 he became division manager responsible for worldwide marketing of connector products in telecommunications at Molex in Heilbronn. Since July 2017 he has been occupied as a freelance consultant for connectors, under the logo ConConsult.