CAIRO is a RAYNET standard for mic/speaker connections for rigs. It specifies the connectors, the pin-out and (to a minor extent) the signal levels. The intention is that as many RAYNET operators as possible adopt it. Those who do can use each other's equipment with relative ease: my mic and Fred's headset can be plugged into your rig without trouble. Some people will modify their rigs by installing a CAIRO connector, but most will make an adaptor lead which plugs into their rig's mic and speaker sockets, and presents a CAIRO socket to the world.
CAIRO has been around for decades and its take-up by RAYNET members and groups has been variable. Those who have adopted it have probably seen benefits. Speaking personally, I adopted it long before I joined RAYNET and have found it very useful, even just as a personal standard.
Members of Leeds RAYNET have recently expressed a lot of interest in CAIRO and it seems that it is being adopted as a Group standard. At the moment there seems to be just one manual (mine) to serve the entire Group! I am therefore printing the CAIRO connector standard here for everyone's benefit. This brief article is not an introduction to CAIRO but little more than a reminder of what the pin-out is.
There are two versions of the standard, the original CAIRO and CAIRO-8. The latter is better but hard to implement. It requires signals which can probably only be provided by hacking into your rig - it is unlikely you can provide them through an adaptor lead. The CAIRO-8 standard demands that it is implemented fully: if you can't do it all, give up and implement basic CAIRO. Basic CAIRO is perfectly adequate for most speech purposes. The real advantages of CAIRO-8 are in building talk-through boxes (which we own anyway) and using TNCs (where the benefit is minor). For Leeds RAYNET purposes, I doubt whether much will be lost in sticking to basic CAIRO. The two versions are fairly compatible - basic CAIRO equipment will connect to CAIRO-8 kit and get virtually all the facilities it would normally expect.
The basic CAIRO standard provides five signals: Mic, PTT (push-to-talk), Speaker, Second speaker and +12V. The last two are optional and not much equipment provides them. The second speaker line never seems to have been used much and can probably be forgotten about.
The connector is the 7-pin audio DIN connector. The socket is connected to the rig; mics, speakers, etc., are wired to plugs. In most cases the plugs can be 5-pin ones; the loss of the extra two pins rarely matters. However, all sockets should be 7-pin to take the occasional 7-pin plug. I strongly recommend that you only use good quality metal-cased connectors, not cheapo plastic ones. The CAIRO standard specifies non-locking connectors. The solder connections for the plug are shown in the diagram (not reproduced on this Web page); the back of the socket is, of course, the mirror-image of this. There are also some recommended wire colours for four of the signals: Speaker blue, PTT red, Mic earth yellow and Mic high white.
The standard also specifies a separate earth (sometimes called signal return) line for the microphone earth. This signal should never travel along the sheath of the screened cable; doing so can introduce interference. It is often very tempting to ignore this part of the standard, especially when the cable you can get has one core too few! It is important, though. If you ignore it you could generate strange cross-talk effects when you plug into other people's CAIRO gear. Needless to add, this would only become apparent when you are trying to set up an important station, possibly at a live incident!
This single page has really presented just a reminder of the connections. To understand how to make rig adaptors, you should really consult the full CAIRO document. If you have WWW (World-Wide Web) access, it can be found at http://www.eeap.aston.ac.uk/pjb/cairo/manual-1/index.html
Chris Trayner G4OKW
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G4RDL