Communications Audio Interface for Remote Operations


HH .1 Hand-Held Adaptors re-using commercial monophone headsets and their rig adaptors

If you already have a commercial monophone headset (e.g. Icom, Yaesu, Kenwood) and wish to make it CAIRO-compatible, cut the 'stereo'-jack plug from the headset cable and install a DIN-5 plug (with 0.001uF across electret's pin pair). Replace the PTT-adaptor's jack socket with grommet and short length (5cm, 2") of CAIRO Line cable to a DIN-7 line socket. Re-arrange internal wiring connections, as in Fig. 5, but with the adaptor's PTT switch wired in parallel with the on-going, CAIRO-line, pin-4 (red) core.

When converted, the items will still operate with that Rig, exactly as before, but the PTT unit is now a Rig-Adaptor for any CAIRO accessory, and the headset is good for any CAIRO configuration.

HH .2 PTT Diode Bias obtaining electret bias with two silicon diodes in the PTT line

Some transceivers - though not many these days - will tolerate a pair of silicon diodes in series with the PTT line, inside their Rig-Adaptor module, to generate a 2 x 0.7 = 1.4 V bias for the electret phantom-powering requirement, when no "voltage" pin is available.
(This was once an 'official' CAIRO suggestion but has since proved to be of limited practical success.)

HH .3 DIN Grommet Tie adding extra security to the line cable at plug-entry grommets

The Line cable is not a 'snug'-fit at the grommet of the (diecast body) DIN plugs or in-line sockets. To improve the mechanical integrity, e.g. at the Orange Reel's plug, introduce a small cable-tie (2.5 mm, black) around the DIN's grommet at the taper, just below the cable-entry collar-ring. It is also advisable to use heat-shrink sleeve (2.5mm) at all soldered connections involving CAIRO cable.

HH .4 Cut Bungs protecting line cables at window or door entry points

The Bored Rubber Demijohn (fermentation) Bung, e.g. Boots chemist 42-80-024, with a single radial cut taken down to the bore hole, may be slipped over the remote Line cable for protection as it enters via doors or windows which may have to be held or tied 'closed' while the room is in use. [The slit is just about visible in this HOIME.]

HH .5 'Weather-Man' PTT operating 'walk-about' in a control room or on rough terrain

Discard the ink-bladder from one of the thicker types of felt-tip marker pen and replace the 'nib' with a push-to-make switch, e.g. [337-942], with a twin-core cable emerging from the other end. The item fits snugly in the hand and might be used, with a monophone headset, when operating at a control-room wall-map or signals-team status-chart, or when 'pedestrian' in rough terrain.

HH .6 Headphones Adaptor avoiding plug-cutting on (cherished) "Hi-Fi" headphones

Normally, CAIRO accessories would be terminated directly with a DIN plug of the appropriate kind. However, quality headphones may be required for non-Cairo listening (e.g. Hi-Fi) when the original (moulded) jack is the correct termination. If so, take a diecast DIN-7 plug and replace its cable grommet with a 3.5mm stereo socket (e.g. Maplin FK03D) inside the DIN body - nut outside hole. This requires all internal pin-ends to be cut down to a minimum and fine wires to DIN pins 6,1,2. (Also consider including a 1/8W cross-feed Binaural Ambience resistor (680 ohms), just for CAIRO.) Keep this item with the headphones and also a 'one-peice' 3.5mm-1/4" jack adaptor, if required.

HH .7 Headset Convenience cutting a feed line to personal 'height' to avoid plug damage

When preparing headphones or headsets for use as personal items, cut the feed cable such that, when the set is slung around your neck (as for a short break from operating) the plug hangs at your ankle, when standing upright. This avoids dirt ingress and tripping, e.g. when going for a "/P" !

HH .8 Handset Ambience attenuating a telephone handset earpiece acoustic output level

Telephone handsets (and other complete accessories) may be used directly onto a remote Line, without an Outlet Box with a volume control. If so, the close-ear level of the earpiece could be excessive when the Line is driven for a 'monitoring' extension speaker. When preparing handsets, include an internal resistor (try 680 ohms at first) in series with the earpiece such that its close-ear acoustic level is about the same as that from a typical 8 ohms extension speaker, placed at 'arm's length'.

HH .9 Monophone Headset single-ear listening for control loggers or outstation operators

Monophone accessories, of the type which clip over the ear-lobe, are preferable for being safe, convenient and of good audio quality; e.g. AKG's K5, rather than those which enter the ear-canal.

HH .10X Electret Mikes - HOIME

HH .11 Speaker-Mike creating a simple and expendable home-brew speaker-mike

In an old (ex 'dynamic') 'fist'-mike case, with DPDT-type PTT switch, install a small speaker (e.g. 50mm, 8 ohms) and miniature audio transformer (e.g. = 1:10) wired as follows;

The Loudspeaker does both jobs! In the default switch position (as drawn, above), it is coupled to the Received Audio circuit between DIN pins 1 and 2. Conversely, in the momentary contacts' position, the PTT at pin-4 becomes grounded (to pin-2), while the speaker circuit is completed to be a transformer-coupled microphone. The audio quality, in the Microphone/Transmission mode, is far from perfect but the device is a handy arrangement for very ocasional transmissions; e.g. as a 'stand-by' mike for a Controller to use

HH .12 Black Outlets maintaining transceiver appearance with a 'colour'-matched socket

For most CAIRO or CAIRO-8 applications the chrome-finish DIN socket is generally favoured. However, some sources supply black-finish sockets which may be more suitable as a substitute front-panel outlet, e.g. RS [479-743], on black-painted transceivers, when converted to CAIRO-8.

HH .13 "Side-Winding" retaining external scanning functions on CAIRO-8 transceivers

Some FM rigs support a scanning option which is only accessible when operated with the original 'local' mike. To retain this facility, whilst otherwise converting the rig to CAIRO-8, an equivalent pair of Up/Down, press-to-make switches may be included on the (attachment) converter module.

HH .14 TNCs in CAIRO operating unmodified TNCs with CAIRO-adapted (not CAIRO-8) Rigs

TNCs, not yet modified for full (CAIRO-8) interface compatibility, may be operated with CAIRO rigs by employing a lead which is made-up to the "Hand-Held" interface, given in the TNC manual. This uses capacitive isolation to 'break' the premature grounding of the Tx-AFSK (= mike) output, and may be satisfactory directly into a CAIRO-adapted rig but NOT with other operator accessories.
(Use a PVC-body DIN plug for the TNC end of this lead so it is clear that this is not the CAIRO end.)

HH .15 Data-Link Cable exploiting "CAIRO-Line" as a TNC data terminal (RS232) link

The data interface, from a TNC to its VDU/Computer terminal, operates in a higher than 'normal' (for the RS232 spec.) RF environment at a Packet station. The four-core, individually screened cable (as for CAIRO Lines, etc.) is quite well suited for 'long' RS232 lines in this environment. The four cores are used for the TNC's TxD, RxD, RTS, CTS and the screens take signal ground (SG) between the D-type connectors at the TNC and the Terminal. It is not a full-specification lead, but puts to good use a left-over length of this cable, following general CAIRO engineering activities.

(With appropriate 'conversion'-leads (e.g. D-Types to DINs), RS232 signals may be run down a 'spare' fixed-installation CAIRO 'drop' (see CI .2), for "Clandestine" Packet from a User-Service HQ.)


HH .16 Able-Labels(TM) preventing personal radio and CAIRO belongings from going missing

The 1.5" x 3/4" self-adhesive labels, e.g. Able-Labels (Tel: 01604-810781), are an ideal size for identifying CAIRO (and other radio) items which may be used in multi-operator circumstances. Include Name, Callsign and Address, and position in a visible but not unsightly portion of the item. (Able-Labels may still be able to include the CAIRO Logo?)

HH .17 X The Bus Speaker - HOIME

HH.18 X The Desk-Top Console - HOIME

Exceptions and Quirks

These CAIRO Pages describe techniques which are as widely applicable as reasonably possible.
While investigating these techniques, the following exceptions were noted (or reported by others):

EQ .1 Screen/Screen PTTs not all Hand-Helds are exactly alike with respect to PTT control

Most Hand-Held transcievers may be CAIRO-adapted on the basis of Fig. 5 and the text of Page 12. However, a few makes, e.g. Trio 2500/3500, require the PTT to operate between the mike's screen and the speaker's screen, i.e. between the 'outers' of the two jack sockets on those Rigs. In these cases, the CAIRO Earth-Screen (DIN pin-2) is wired ONLY to the speaker tail's return conductor, while the mike tail's screen is connected ONLY to the 'strap' between pins 4 & 5, Fig. 5.

EQ .2 Mike 'Head' Amps not all Rigs work to the "-65dBV" microphone sensitivity level

The Active Interface Adaptor (Fig. 6, Page 13) or the CAIRO-8 Conversion (Fig. 12, Page 25) arrangements assume that a transceiver has "standard" signals, particularly for mike (-65dBV). Some transceivers, e.g. the ICOM series - IC 251, 751, 271, 3210E, etc. - have head amplifiers in their microphone accessories and an equivalent is required in the CAIRO interface, as well as the electret-bias scheme. A single transistor (e.g. BC108) pre-amp will do this. (Contact G4ODM)

EQ .3 Biased Mike Rigs be wary of Rigs with a mike bias which only occurs during Tx

Some commercial transceivers may already have an 'electret-biased' mike input but the voltage is ONLY present when the rig is keyed into Tx. This is NOT sufficient for CAIRO which specifies a permanent bias, even during Rx (though the bias may drop from a 'high' Rx to a lower Tx level). Such rigs will require the full interface adaptation/conversion (which 'ignores' the Tx-only bias) so that the external mike circuit is always biased, for full compatibility with CAIRO-TTUs, etc.

EQ .4 TNC Death-Throws short, blank Tx burst from C8-TNCs during power-down

When a C8-TNC is powered-down, at its own front-panel switch, while still connected to an operating host rig, the rig will sometimes transmit for a short (~ 1 sec.) period. This is due to the bias-off conditions of the PTT transistor 'collapsing' before the SIL relay supply - the 12V rail - has decayed with the capacitors in the TNC's supply filter arrangements. It is of little consequence.


Electret Capsules

Lavalier Microphones

Transceiver 12V d.c.-Power supply input has been given a form of 'standardisation' by the National Raynet Committee (1990). This is a T-connection system (AMP 'spades') where female, shrouded-pin sockets 'present' 12V sources and semi-exposed, male-pin plugs connect each unit. The horizontal pin (T-'top') is Positive (+) and the vertical pin ('down-stoke') is Negative (-).


Aerial Connections to transceivers can be a source of frustration for the lack of commonality. "Good quality", e.g. silver-plated, 50ohms BNC connectors exhibit negligible insertion losses up to nearly 4000 MHz at RF power levels well in excess of amateur "legal limits". They are available in both 'genders' (male or female, line or chassis) and a variety of 'geometries' (straight, right-angle, bulkhead, etc). Versions can now be obtained for large diameter coax (e.g. UR67), and 'one-piece' adaptors from BNC to other RF-connection systems, e.g. PL259/SO239 or N-series. Consider them for 'personal' unification on aerial feeders' rig-ends and inter-unit patch-leads.






Supplementary Material


Remote Operations

CAIRO Basics

CAIRO-8 Basics

Raynet Packet

CAIRO Installations

CAIRO-8 Relays

CAIRO Relays

Relay Installations

Summary Configuration

Data Card

Handy Hints