Why are domino plugs reversible?

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johnkenyon
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Why are domino plugs reversible?

Post by johnkenyon » Thu Apr 27, 2017 3:56 pm

I always wondered why you would have a connector which is deliberately unpolarised, and I found this web page:
https://www.vintage-radio.net/forum/sho ... hp?t=59591
which includes the text
There was also a 360 degree 5 pin (sometimes called a domino) used for headphones.
I reckon they used this plug so you could swap left and right on your "cans" by fiddling with the plug end rather than fiddling with the headphones...

john

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1024MAK
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Re: Why are domino plugs reversible?

Post by 1024MAK » Thu Apr 27, 2017 6:25 pm

DIN is a German standard (link).

3 pin DIN plugs (mono in / out) may have been deliberately designed to fit in 5 pin 180o sockets. 5 pin are for stereo in / out. Because round DIN plugs are often used for audio use. Another example is the 2 pin DIN connecter, it is often used for speaker connections.

More on round DIN connectors here ;-)

Mark

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ctr
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Re: Why are domino plugs reversible?

Post by ctr » Thu Apr 27, 2017 11:12 pm

If Acorn had wired the socket with RTS diagonally opposite CTS and RX diagonally opposite TX then you could have used the same cable as both a modem cable and null-modem cable by rotating the connector 180°. I think. But they didn't.

I always wondered if that was the point of the symmetrical connector and then someone wired it wrong.

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Re: Why are domino plugs reversible?

Post by Coeus » Fri Apr 28, 2017 12:27 pm

ctr wrote:If Acorn had wired the socket with RTS diagonally opposite CTS and RX diagonally opposite TX then you could have used the same cable as both a modem cable and null-modem cable by rotating the connector 180°. I think. But they didn't.

I always wondered if that was the point of the symmetrical connector and then someone wired it wrong.
That does seem like a missed opportunity.

Thinking of serial standards and missed opportunities it seems to me twisted pair Ethernet was similar missed opportunity. RS232 had the concept of DCE vs. DTE which came from an era when the equipment was generally dumb and in-band signalling would not have been done so one needed a connection for carrier detect etc. hence an asymmetric interface (three control signals one way, two the other, as well as one data each way) but it caused a certain amount of confusion and interoperability issues. For example given that a computer was generally configure as a DTE, would a printer be configured as a DCE on the basis that it would most likely be directly connected to a computer? or as a DTE expecting to connect to a modem? I think both were done so one had to work out if a straight through or crossover cable was needed.

Despite this trouble the designers of TP Ethernet retained the DTE/DCE model so, prior to autodetecting ports, it was not possible to connect two PCs back to back with a standard cable, or to daisy chain switches and hubs without either a dedicated "uplink" port or a crossover cable. It would have made much more sense to have one type of device so transmit is always on pin X and receive is always on Y, all cables are crossover and therefore any port on anything will connect to any port on anything else. Even cable extenders would not be problematic - a coupler for linking two cables together would also include a crossover so any extended cable would always have an odd number of cross over components, i.e. there would be one crossover end to end.

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1024MAK
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Re: Why are domino plugs reversible?

Post by 1024MAK » Fri Apr 28, 2017 1:27 pm

With regard to RS232 usage... As a printer is not DCE (data communications equipment), it should have been wired up as a DTE (data terminal equipment). Before computers became common and affordable, "dumb" VDU terminals and "line" printers were often connected via serial connections (including via private line modems and via PSTN "dial up" modems).

In theory, only modems and similar communications equipment should be wired as DCE.

The biggest problem with RS232 was that a lot of computer manufacturers did not use 25 way D-connectors and/or did not implement a sensible serial interface. So although there were "standard" crossover cables, they had limited use.

I'm not a network specialist, but each twisted pair in Ethernet cable (10BASE-T) has different twist ratios per metre. This is done to reduce crosstalk. When twisted pair Ethernet was developed, the primary use was as a star network (see star topology).

With both RS232 and Ethernet, as with a lot of electronic interfacing standards, the story is that of evolution. Where previous developments and systems are built upon.

More on the history of Ethernet here Ethernet physical layer.

Mark

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