Tech Flashback: Telecom Australia 600-series Adapters & Extensions

In the present day where landlines are a dying breed and mobile telecommunications has become ubiquitous, the beige-cream coloured Telecom Australia 600-series socket remains a common sight in Australian households. While I have no idea when the 600-series socket came into common usage, it would have likely existed around the same time (or even before) the inauguration of Telecom Australia in 1975, making it over 42 years old.

A Telephone Extension Reel

When I was young, these telephone cord extension reels were quite popular and could be bought from any variety shop for a reasonable price. Similar in concept to a power extension cord reel, these were fitted with a 605 plug on one end and a 610 socket on the body of the reel. The reel had about 10 meters of flat telephone cable. The design with the holes mimicked the design of a rotary pulse dial of a telephone of the era. The whole unit was the same “beige” colour that typified telephone accessories, and was clearly labelled with the Telecom Australia Authorization Number of C85/1/45. As it was illegal to connect unapproved devices, this was a necessary marking to demonstrate to the consumer that the accessory was approved. Despite being fitted with Australian 600-series sockets, the unit was made in Hong Kong. This one was actually ours and had survived a number of house moves.

The 605 plug can be seen to have three blades with two contacts per blade. A wider non-conducting spigot is provided. As the extension leads are intended mainly for temporary use, the spigot has a “filled” core – some others intended for semi-permanent installation had a hole, allowing a screw to be fitted in the mating 610 socket to hold it in place. The plug itself has an approval RA85/121 as well.

Removing the single slotted screw allows the cover to pop off. Six contacts are clearly visible, to which three lines (each requiring a pair of connections) could be connected. That being said, only contact 2 (tip) and 6 (ring) are carried by this extension lead, which will limit the functionality of some devices, as noted later in the section on Mode 3 Adapter. The configuration of connecting 2 and 6 is most common, and is the minimum necessary to support Line 1 operation.

The design of each of the blades is quite simple – an insulating plastic form has a channel in which the contact pin sits. The end of the pin is flared out, to allow a crimped spade connector to be connected to. The whole pin-form-pin sandwich sits inside a cut-out in the socket, across a channel at the bottom which prevents the assembly from sliding back into the plug. It’s likely that this is a later design, owing to the thin profile of the pin contacts.

A partner 610 socket is used in the centre of the reel, but as I didn’t want to disassemble the reel and risk breaking it, I didn’t take any photos of it. The concepts are easily conveyed with the next few adapters.

Telecom Australia Mode 3 Adapter

A Mode 3 adapter is not something that is too often seen, but when it is, it is often incorrectly mistaken as a double-adapter that doesn’t work. On a recent trip to a local thrift shop, I came across these.

These are Telecom Australia branded accessories, item number 557/3, still in its original packaging. This really speaks back to the days where you’d rent a telephone from the telco. I like the tagline – “The Vital Connection”. The adapter is used to connect various types of communication equipment (Viatel terminals, older fax machines, alarm diallers, answering machines) which have a special Mode 3 connection. This type of connection passes the telephone line into the equipment through the line 1 terminals, and returns the line through the line 2 terminals. This allows the device to “pre-empt” any downstream devices and take them off the line, so as to seize the line without any potential interruptions, similar to how a dial-up modem may have a line and (switched) telephone socket. This is “neatly” packaged into an adaptor, for where a Mode 3 socket doesn’t already exist and Mode 3 functionality is desired.

It comes with a fold out instruction leaflet which details how to install the adapter, including the potential issue of meeting a semi-permanently installed plug.

The adapter is in the beige colour, as expected. The front has the Telecom logo, the rear has the text describing the function.

The adapter has a subtle, tapered shape. Some others were more angular, but I suppose this could be a hint as to its age. Each of the blades has a rounded profile with a large contact area.

A closer look at the front shows that the spigot has a stepped profile. I believe this was to key the connection to avoid inappropriate connections, although I’m not particularly clear as to how that would work.

Support for this is seen on the “input” side of the adapter. Notice the spigot holes are different shapes for the two ports. The lower port is used for the telephone, the upper port is used for the Mode 3 device – inserting them in their correct sockets is vital for correct operation.

The unit can be disassembled by prying out the front cover.

Internally, two different types of pins are pushed into the plastic holder.

Two of the pins at the ends are of the sort that I will term seriesing pins. For the upper device, it has its pin 2 and 6 wired connected directly. The pin 1 and pin 5 connect to pin 2 and pin 6 of the lower device. The lower device has its pin 1 and pin 5 connected directly.

This has the effect of routing line 1 through the upper (Mode 3) device, which returns the line through line 2 connection which is then given to the lower (telephone) device. The completion of the connections to the telephone allows the telephone socket to also work on line 2 connections.

To facilitate the downstream telephone working on Line 1 while no Mode 3 device is plugged in, the contacts for pin 1,2 and 5,6 can be bent toward each other so they short together when no device is inserted. I presume the device may have been designed in this way, but whether the springs have retained their original position is not known.

The pin used for the middle connection is what I term a straight through pin. This basically has things parallel wired. This means that line 3 connections are duplicated to both devices in corresponding fashion (i.e. pin 3 to pin 3, pin 4 to pin 4) without interference.

It can be seen how this lends itself to easy production of double adapters and Mode 3 adapters through the use of similar parts.

If this adapter was plugged onto the end of the extension lead above, it would work just fine with the exception of having no line 2 or 3 service. However, if this adapter was plugged into the wall, and the extension lead above was used to connect a Mode 3 device, then everything breaks, because the Mode 3 device will attempt to return the line via the line 2 connections, which are not wired through on the cable. As a result, a four-core extension cord with line 1 and line 2 wired is necessary to extend a Mode 3 device (assuming it doesn’t use line 3 at all).

AWA Telephone Double Adapter

In a case of being in the right place at the right time, I found this in the same thrift shop in the same pile. I suspect they may have come from the same place, but this one is branded AWA (another bygone Australian brand). This is a double-adapter intended for connecting two telephones to a socket. Note how as a third party brand, it displays the necessary authorization numbers (C87/1/252). In this case, all the instructions are shown on the rear of the package, but note the similarity of the font between this and the adapter above.

This adapter has a hole through its spigot, allowing for semi-permanent installation. This has an AWA logo moulded into the same place as the Telecom logo was in the above (which implies the same manufacturer may have been involved). Because it is a double adapter, there is no distinction between the two outlets, both of which have a square spigot hole.

It should come as no surprise that it comes apart in the same way and consists of three straight-through modules, duplicating each pin in perfect correspondence.

RJ12/RJ45 Adapter

The RJ12 adapter is a common device which is often found bundled with any telephone, modem, fax machine sold in Australia. The RJ45 version is a little rarer. This one I found “dumped” outside during a council clean-up, so I decided to take it home just to take it apart.

Being a much more modern device, the full plug body is now omitted, and no screw is provided as the adapter is glued shut. An Austel approval is present on the side, and the pin for line 3 operation is omitted entirely as well.

With cost-reduction in full swing, the contacts for line 2 are omitted as well, making this a line 1 only adapter – the most common type and sufficient for most uses as the RJ12 connector is rarely used for two or three-line service.

The adapter itself accommodates an RJ45 style plug natively, but works with RJ12 plugs through a reducing sleeve. Only the centre two contacts are used.

The construction follows very similar principles to the wiring in the extension lead, with some corrosion appearing on the lower pin. This is probably because it had been exposed to water or chemical vapours in the past.


It’s interesting to think how we ended up with this “odd” connector that isn’t really used anywhere else (to my knowledge). Despite being a British “outcrop”, we could have just as easily adopted the BT socket just as New Zealand (across the pond) did, but we didn’t. This may have something to do with the Postmaster General or Telecom Australia’s commercial interests. That being said, the US RJ12-style and RJ45-style sockets eventually became the preferred means of telephone device connection, being more compact, cheaper and quicker to terminate but also pesky with the tab that likes to snap off.

Despite the old appearance and the limited market, it seems that the 600-series connectors will live on in some households as RJ12 adapters are ubiquitous, and the landline may be used to supply VDSL2 services over the new “mixed-technologies network” NBN that has been a disappointment all round. That being said, I don’t think such old connectors are optimized for impedance or high frequency operation, and all those connections would only serve to impede the connection quality, so maybe it’s a good idea to change over your wall plates and get rid of the beige.

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One Response to Tech Flashback: Telecom Australia 600-series Adapters & Extensions

  1. David Myers says:

    I used to work for a company that sold telephone equipment back in the 1980s, and I recall the various permutations of these connectors well.

    Originally the PMG (and later Telecom) owned all telephone equipment, including telephones in a subscriber’s premises. Subscribers were able to own and purchase their own equipment, but it was strictly regulated. This equipment had to undergo rigorous approval tests and had to have plugs that were incompatible with standard telephone wall sockets. These were the days when answering machines were expensive office equipment and not the mass market devices of today.

    Normally a PMG/Telecom phone was provided with a 603 plug which mated with a 610 wall socket. These had a square plastic key.

    Approved attachments had a 604 plug which mated with a 611 socket. These had a large stepped plastic key, and were not compatible with the 610/603, even though they looked externally similar, apart from a pair of semi-circular mouldings around the screw head on the side. An owner of one of these attachments had to get the PMG/Telecom to wire and install a 611 socket specially for their device. The technician would wire the new socket for mode 1 or mode 3 as required. An ordinary telephone with a 603 plug will not fit into this socket.

    Later in the 1980s, devices such as answering machines and cordless phones became mass market consumer items and it became totally impractical to get Telecom to add 611 sockets for every installation.

    To get around this, the 605 plug and 612 socket was introduced. The 605 plug had a small stepped key which could fit into either the 610 or 611 socket (as well as it’s mating 612 socket). The 605 plug was now fitted to most approved attachments, meaning it could be plugged in anywhere. The 612 socket however, would only accept the 605 plug. Double adaptors were also made available now that extra sockets for attachments were no longer provided. Mode 1 being a straight parallel connection and mode 3 being the switch over type as described in your article.

    As a boy, I remember our town converting over to an automatic exchange back in 1964. We had new phone wiring installed, complete with a 610 socket, and a new phone with a 603 plug, so these were around at least as far back as the early 1960s. These early 603 plugs were slightly different to the later ones in that the contacts were the full width of each prong rather than being a narrow insert as they are now. These are still fully compatible with the later design. Our previous phone was hard wired in to a black terminal box on the wall, which was probably the usual way of wiring in telephones before the 600 series plugs appeared.

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