CommunicationsAudioInterface for Remote Operations

CAIRO Engineering

The success of CAIRO, as a scheme for Remote Operations under hastily summond situations, is that all items are engineered and tested prior to use, so that they are brought into service with "plug and work" simplicity. Providing that all the engineering details of the Interface have been honoured in the prior preparation of accessories and transceiver adaptors, stations may be constructed from any available item, including those of mixed ownership. The more widely adopted this ("de facto") Standard becomes, the more likely it is that operators, who may never have worked together before, may do so and deliver an effective service through mutually compatible equipment (and techniques).

As components are introduced into the discussion, links will be provided to the summary Components Page, e.g. [S7L], where (UK) Sources are identified but without prejudice for other sources and suppliers. If a section involves several example items, these may be listed thus;

" Items Check List : magnetic retrieve tool, [542-144] ; twin screened cable, [367-224] ; 9.5 mm heat shrink [398-177] ; Plug : [P5] "

CE .1 CAIRO Signals

The majority of (amateur radio) transceivers operate with very similar accessory signals, even though they may well employ widely differing connectors and signal-to-pin assignments !

Received audio is usually presented as a single-ended signal for a matching loudspeaker of 8 ohms, 2W 'handling'. This output is equally compatible with combinations of higher impedance earpieces, providing that the total impedance of the combination load does not fall below about 8 ohms.

Secondly, the PTT action is usually engaged by a switch-closure to system earth, while the microphone input is invariably designed for an electrodynamic transducer of about 600 ohms impedance and -65dBV sensitivity.

(Exceptions to these rule-of-thumb specifications are corrected by the Transceiver Adaptor item.)

In Remote Operations all three circuits are being harnessed in a single cable which, in many circumstances, will be much longer than in "conventional" operations, close to the transceiver. To permit this, CAIRO imposes 'good engineering practice' on the microphone circuit, which is a relatively high impedance and very low level signal compared with the speaker and PTT circuits. The very real possibility of detrimental cross-talk onto the microphone circuit when the speaker circuit is not de-powered, must be eliminated. This occurs in any 'intercom' mode, like cross-band duplex working, for example.

Elimination (actually a minimisation) is achieved by presenting all microphone elements as isolated, two-terminal devices on two conductors that are individually screened throughout the link; being a balanced or quasi-balanced circuit into the transceiver. Outside those screens, two further conductors are used for the speaker and PTT signals which then use the screens as their common return path to system earth.
This yields a minimum specification for the remote systems cable; two screened conductors with two more, outside that screening.

In practice, the lightweight 4mm diameter, 4-core (7/0.1mm), individually-screened signals cable is declared to be the standard cable - the Line - for most applications within the CAIRO scheme. [LC] (Please note that multicore overall-screened cables do NOT satisfy the requirement.)

The suggested colour assignment, with reference to signals and DIN-pins (see Fig. 4, below), is:





Speaker (Rx'd Audio)






Mic-High (+)






Mic-Low (-)


The two isolated microphone connections are designated Mic.-high (+) and Mic.-low (-).
These terms refer to the signal potentials in completed circuits, NOT two alternative impedances.
( Mic-High (+) at pin-3, is the core that carries the superimposed 'electret-bias' voltage (+5V) , see below.)

CE .2 CAIRO Connections

CAIRO uses 7-pin DIN sockets throughout and then 5-, 7- or 3- pin DIN plugs as appropriate.
The assignment of the electrical signals to the seven DIN pins has been influenced by two factors.

Firstly, there will be frequent use of binaural (dual-ear) headphones under Remote Operations, so two adjacent connector pins (1, 6) have been allocated to the received audio so that headphones may retain the independent wiring of their earpieces, allowing them to be supplied with different audio signals in some circumstances. These feeds are designated the Primary (1) and Secondary (6) audio circuits and usually correspond to the left and right earpieces, respectively. All such accessories require a 7-pin plug in the scheme. Socket pins 1 & 6 are normally both connected to the Line's received audio ('speaker') conductor, either directly or indirectly via mute switching or potentiometer control, as noted elsewhere.

Secondly, Remote Operations may sometimes involve 'active' items or units which require d.c. power. Therefore, pin-7 has been allocated for a (limited-current) 12V supply, although it is not essential to honour this provision in items for basic CAIRO.
In boxes, panels and modules where the 12V provision is not being implemented, it is strongly recommended that 'pin-7' is physically removed from all sockets during construction. This prevents the pin from otherwise working loose later in the life of the item, perhaps to cause shorting across other socket pins. It also labels the item, to the keen-eyed user, as not having the d.c.-interlink provision.
But this recommendation does NOT extend to CAIRO-8 where the 12V supply is an essential provision.

The remaining pins of the 7-pin format have each been allocated to the microphone and PTT, in such a way that a complete accessory only requires a 5-pin DIN plug, while a loudspeaker (or monophone earpiece) only requires a 3-pin DIN plug.

It is again stressed that ALL sockets MUST be of the 7-pin variety, to accept any system plug, even though remote Lines (and extension leads) only transport the signals that are defined by the first five pins.

Fig. 4

The CAIRO Interface DIN Pin Assignment

Pin numbers used throughout are always the 'official' DIN designations.

(It is useful to observe that the two mike pins are at maximum distance from the two speaker pins; being 'guarded' by the two d.c. pins (12V, PTT) and system ground (at pin-2). Also, failures due to adjacent-pin shorts are all resonably benign.)

CE .3 Mechanical Integrity

When used in CAIRO, DIN connectors routinely experience many more insertions and removals than in the audio applications for which DIN was first developed and is still widely encountered (e.g. Hi-Fi system hook-ups). Therefore, it is most strongly recommended that all plugs are selected from the diecast (but non-latching) product ranges, NOT the PVC-insulated ones, despite the initial extra expense. (The first "heavy-boot" incident will soon justify this 'warning', if not heeded ! )

Items Check List: Sockets : [S7L] , [S7N] , [S7F] ; Plugs : [P3] , [P5] , [P7]

CE .4 Transceiver Adaptors

It is assumed that few users will perform modifications inside a transceiver to achieve CAIRO compatibility, particularly if warranty agreements or "Type Approval" conditions might be jeopardised by such actions. Instead, external Adaptors may be used which plug into existing transceiver socket(s) to interface the signals onto a CAIRO DIN socket.
Generally, just one of two forms of Adaptor will be required.

CE .5 The Series Adaptor

For transceivers which employ the series mic.-PTT input circuit, i.e. most hand-helds, the required adaptor is a simple pair of short leads from appropriate plugs (often miniature and sub-miniature jacks) to a single in-line DIN socket, with a strap between pins 4 and 5 (see Fig. 5, below).
Any system accessory presents itself to that transceiver as a series connection by exploiting the isolated, two-conductor connection required of all microphone accessories in the CAIRO scheme.

In these series inputs, the PTT switch action completes a d.c. path for the microphone circuit to have a bias which is sufficient to operate electret accessories, as well as 'dynamics', etc. Almost always the d.c. path for electrets and a.c.-coupled mikes must be provided by a shunt resistor, 'external' to the transceiver. If so, this is included as a permanent provision, "behind" the DIN socket, being of no detriment to the performance of 'dynamics' when they are used instead. Generally, 4K7 will be correct, as shown, but some hand-helds may require a higher value (e.g. 20K), so please check with the rig manufacturer's handbook for this detail (and see EQ.1).

Fig. 5

Series Transceiver Interface Adaptor

(with remote-end terminations shown)

CE .6 The Active Adaptor - phantom powering

The electret microphone gives excellent audio quality with good sensitivity (-60dBV) from a very small and inexpensive capsule item that lends itself to miniature accessory designs. With separate mic. & PTT transceivers (i.e. most 'base', mobile and "handbag" rigs) a bias voltage is not normally present on the microphone input circuit.

However, it is operationally desirable and therefore a special requirement within CAIRO, that electret-based accessories should be usable in all circumstances, with ALL transceivers having the properties of the hand-helds in this respect. For this reason, the importance of which cannot be overstressed, the "Rig Adaptor" is an active arrangement which superimposes a permanent bias, between 1.5V and 10V (4.5V is optimum), onto the remote microphone circuit. This general technique is known as "phantom powering". Figure 6 depicts a schematic approach but it MUST be interpreted in conjunction with the handbook for each transceiver and the resulting adaptor MUST only then be used with that rig.
Transceiver Adaptors are the only CAIRO items that are NOT general-purpose.
(If your transceiver already has electret-bias phantom-powering, please see EQ .3 .)

Fig. 6

an identical pair of Rig Adaptors

Active Interface Adaptor

schematic diagram only

A resistor, of value about 10% greater than the transceiver's actual input impedance (often '600R' so 680 ohms is typical) acts as the external drain load for the electret's integral FET, via the mic.-high (pin-3) line. It is supplied from any transceiver pin with a permanent positive voltage present; 'V', scan control, etc.

If the supply is less than 8V the feed can be a 10K resistor, decoupled (10uF) to system earth, as shown. But if the available voltage is above about 8V, e.g. the "12V" rail, this 10K resistor is omitted to be replaced by a 5V regulator device - *REG* (see C8 .6).
Only if the d.c. supply is at the genuine "12V" level, should a connection be made to DIN socket pin-7. Generally this will not be the case, so it is expedient to remove that pin to prevent it working loose, gradually, and to serve as a tell-tale, to a subsequent user, that there is no 12V provision.

It should also be noted that the isolated conductor for mic.-low (at pin-5) may be grounded within the adaptor (quasi-balancing) unless the transceiver itself has a two-pin (balanced) input for mikes.

CE .7 Interface Module

The components of this interface adaptor are not easily installed inside the body of an in-line DIN socket. Instead, they may be housed in a small module, e.g. [RAM], which is modified to have a single-nut DIN socket fixed into one end and a transceiver microphone plug case soldered directly to the other end. As a complete item, it will protrude firmly from the rig, like an oscilloscope-probe adaptor does. Otherwise, it may be left as an in-line item, with short tail(s) passing to the transceiver plug(s). (The HOIME above depicts two identical adaptors seen from opposite ends.)

Before constructing the full active adaptor for a transceiver, a simple passive adaptor may be prepared for preliminary testing with "dynamic" microphone accessories only. This will require short wire links from microphone and speaker plugs to an in-line DIN-7 socket, somewhat like the series adaptor for hand-helds, described earlier (see CE .5).

When adapting 'Grey'- or 'Blue'-Box transceivers, the single-nut ('professional') DIN-7 socket is an exact-fit replacement for most circular multi-pin microphone sockets, for the interface components then to be installed within the transceiver itself. However, for these rigs, consider CAIRO-8.

CE .8 Dual Operator Box

The principal functions of the Single and Dual Operator (Outlet) Boxes have been described. The internal wiring and components of the Single Operator Box may be deduced as being 'half' of the following, in a small diecast box with two chassis DIN sockets on one long side. The more complex Dual Operator Box comprises two half sections, each with its own 100R, 2W (cermet) volume control and small knob. The "big switch" operator selector (a DPDT switch for mic.-high and PTT) is mounted in the front (114 x 55mm) face for the paired sockets to occupy the two (89 x 55mm) sides of the recommended box (see Fig. 3). The small (SPDT) mute switches are set at the forward edge of these sides. The internal hook-up wires need not be screened if the various speaker links are laid apart from the mike circuit links. The screens of the incoming signals' tail must be earth-bonded to the box chassis at one point only, to an earth tag at the strain-relief cable-clamp or one of the bolts anchoring the Auxiliary socket.

Items Check List : single box [SOB] ; dual box [DOB] ; volume pot. [VCP] ; pot. knob [VCK] ; operator select switch [OSS] ; mute switch [LMS] .

Fig. 7

within the
Dual Operator Box

Figure-7 shows that the Auxiliary socket pin-6 is wired to the mute switches as an in-feed from a secondary audio source, which must be single-ended with respect to system-earth (pin-2). It is usual to make-up a lead having a 7-pin DIN plug on one end and a 3-pin DIN plug on the other for this optional use of the Auxiliary socket. Secondly, it shows that all pin-7s are connected together as a voltage interlink. For this option, a fused lead may be constructed to feed 12V via the Auxiliary socket whenever an accessory, needing a d.c. supply, is used at the operators' sockets.
(If prepared with very great care, a combined in-feed but "split" lead may be considered.)
n.b. This is the only point in CAIRO where in-feeding, via plugs, is admissible.

(In practice, the pin-7, 12V provision is so seldom used (in basic CAIRO) that it is increasingly reasonable for a constructor to omit this wiring and remove pin-7 from all sockets.)

The volume control for the Auxiliary socket's pin-1 (primary audio) output is shown to have an extra 'pedestal' resistor. If implemented, this ensures that, when the control is set to zero, the output is never turned fully off, for a small residual signal to always pass to any speaker plugged here. The assumption is that if a speaker is in use it may well be the only listening device and so should never be fully muted, to ensure that operators will always be aware of radio traffic.

Please note again that the audio links, to pins 1 & 6, are cross-coupled between adjacent pairs of side sockets so that operators may choose which earpiece to favour for headphone muting. Ensure that the central switch is correctly mounted so that its toggle points to the active sockets.

CE .8.X Construction Notes for The Dual Operator Outlet Box ->

CE .9 Accessory Engineering

Having constructed the CAIRO adaptor for a transceiver, it is usual to embark on the gradual preparation of a personal set of alternative accessories and an Orange Reel extension Line. As these are assembled and tested, the aluminium-type 'photographic' case (Argos retail one) is a useful 'grab-bag' , though some items may well suit your everyday (i.e. 'shack') operating meanwhile.

First amongst the accessories might be an extension speaker now furnished with a 3-pin DIN plug. It does not require screened cable, though it is essential to ensure that any exposed metal is bonded to the conductor which is assigned to pin-2. The audio feed is taken from pin-1 (pin-3 is unused).
However, do consider the Bus Speaker approach.

Next, prepare a combined microphone and PTT accessory, e.g. a 'fist'-mike, being sure that the microphone element is wired as an isolated device. This CAIRO rule MUST be obeyed and may mean that some accessories will need rewiring to comply. If the original cable is a harness with a screened conductor and two more conductors outside the screening, the connections will be: screened-core (mic.-hi) to pin-3, screen (mic.-lo) to pin-5, outside conductors (PTT switch) to pin-4, and to pin-2 again ensuring that this is the conductor which earths any exposed metal.

Headphones, as for 'portable stereo', are well worth preparing for personal use in CAIRO. Low impedance earpieces of about 30R each are most suited for CAIRO matching compatibility. By removing the often moulded stereo miniature jack, a 7-pin DIN plug is used instead with the left earpiece wired to pin-1 (Primary audio) and the right earpiece to pin-6 (Secondary audio). The screens or common return conductor(s) are taken to DIN pin-2. (All other pins are unused.) For an inexpensive, lightweight binaural headset (with electret 'boom' mike) check Maplin. The "Trample-To-Talk" (TTT) is a convenient device to use with headset operating.

Items Check List : trample PTT switch [TTT] ; headset Maplin LH375

CE .10 Binaural Ambience

A subtle improvement for binaural headphones and headsets that are wired for dedicated use as CAIRO accessories, is to include "cross-feed admittance". This is a single resistor installed between the 'hot' feeds to the two earpieces.The purpose is to retain a low residual audio feed, from one earpiece circuit to the other, whenever single-ear muting is used, as available to operations with the Dual Operator Outlet Box.
Normally, when one ear is muted, the received sound shifts completely to one side of the listener's head. With this resistor included, the sound still predominates at one ear but is perceived to remain 'inside the head' for a listening ambience that will be very much less fatiguing in lengthy duties.

The required resistor value should be selected by personal experiment - try 680R at first - and installed between the pins 1 & 6 feeds. To avoid disturburing the existing wiring into a headset the resistor can be included inside the DIN-7 plug.

CE .11 Electret Capsules

All electret-based accessories require a shunt capacitance of 0.001uF across the element to ensure immunity to RF breakthrough and to tailor the h.f. audio response which may be "toppy" into some transceivers. If the capsule is not accessible, it is sufficient to solder this capacitor across pins 3 & 5, inside the DIN plug, and solder the mike conductors to its short exposed leads. Monaural headsets with electret 'boom' mikes, e.g. YH-1 or YH-2, only require terminating in a 5-pin DIN plug to render them directly compatible with CAIRO without the further expense of the manufacturer's rig-adaptor. Although these often include a convenient PTT switch, the internal connections are seldom compatible with the required microphone isolation and PTT separation.

Small two-terminal electret capsules are widely available, e.g. Maplin's FS43W, and are well suited to upgrading second-hand accessories if the original microphone insert is found to give inferior quality transmissions. This capsule is so much smaller than other forms of transducer that it may be installed as a substitute, perhaps using a re-folded paper-clip as a mounting spider. Always ensure that the capsule body (the FET Source) is isolated from system-earthed metalwork. Here's how ...

Because any transceiver, which is encountered in a CAIRO configuration, will provide a biased microphone circuit for electrets, by virtue of its Rig-Adaptor's phantom-powering provision, it is possible to create a whole range of bespoke accessories, for remote operations, based on the electret capsule, small speakers and miniature PTT buttons.
One such application is the in-vehicle "hands-off". Drivers should NOT use hand accessories when travelling (UK Highway Code) and it is advisable for mobile radio operators to be suitably equipped. CAIRO accessories fixed about the vehicle offer a good practical solution which may be less expensive than some proprietary "hands-off" apparatus.

CE .12 Dual Rig Selection Box

Operations from the vehicle or the 'shack', where two transceivers might be available, may be enhanced further by the use of a Dual Rig Selection Box. This permits a single set of accessories, e.g. the "hands-off" set, to access one or other transceiver, via a pair of switches.

The box has two (short) signals tails which emerge from either side for connection to the CAIRO adaptors of the nearby transceivers. At the rear, a central socket provides the speaker, mike and PTT connections for the (remote) line to the operator accessories. On the front panel, two purposely dissimilar sideways-mounted switches support rig selection. One is a large DPDT switch that selects the source "receiver" ('Rx-Rig') from either the left or right transceiver, as pointed to. The smaller toggle switch is a sideways-mounted 3PDT type that determines which "transmitter" ('Tx-Rig') is operated in respect of mike and PTT connections. If both switches point left, that transceiver is worked simplex and likewise the right transceiver when both toggles point to it. Alternatively, with the switch-toggles opposing each other, one transceiver is for reception and the other is for transmission. With appropriate transceivers at the tails, this allows cross-band 'duplex', or split-frequency in-band working. Note that no combination of switches allows simultaneous transmission on both rigs.

Also at the rear, but offset from the central socket, the box provides an auxiliary socket, wired in the now familiar convention for cross-linked primary (pin-1) and secondary (pin-6) audio. Primary audio at the centre socket, is the selected Rx-Rig while primary audio at the auxiliary socket is the deselected transceiver (and vice versa), with the selection switch being wired as a changeover for this purpose, Fig. 8. This allows a speaker to be used, at the auxiliary socket, to monitor the deselected transceiver, as required by good operating practice and Licence conditions. All remaining pins are parallel wired so that, in all other respects, these are paired sockets for combination connected accessories.

Lastly, it is important to note that this box (and others like it which access signals from paired rigs) unavoidably connect together the earth-screens of the two transceivers, so they must both be of the SAME polarity with respect to ALL power-supplies and other shared items; aerials, etc., in the entire configuration.

Items Check List : box [502-348] ; 'large' Rx switch [316-519] ; 'small' Tx switch [317-190]

Fig. 8

within the
Dual Rig Selection Box

CE .13 CAIRO Augmentations

Within its declared standard for signals and connections, CAIRO enables the careful construction and use of a selection of augmenting Modules. Typically these contain active circuit electronics; IC op-amps, timers, etc., to add a function or feature which is not normally present in all transceivers but which may be required in some instances. For this reason, the typical module is constructed as an in-line unit with a signals tail and plug, and a single socket, so that it may be inserted only when required. All such modules must be designed to be totally transparent to the Line signals when they are de-powered or inactive, and they must NOT load any signal nor vary its characteristic, except when the module itself is signalling. Such modules will require a low current d.c. supply, hence the provision for this (at pin-7) in the basic CAIRO connector format.

Again, the range of useful Modules is almost endless and some examples illustrate this; PTT-sensing indicators (on-air/standby status), microphone tailoring (e.g. "compression"), "pip"-tones (to signify the end of overs), station identifiers, all-stations alert tones, DTMF-type lock and key units and, of course, Talk-Through Units -TTUs- for baseband switching and control.

Talk-Through Units take the received audio from one transceiver (the 'Rx-Rig'), attenuate it, and pass it as the microphone input to a second transceiver for re-transmission (the 'Tx-Rig'), and vice-versa for cross-band Relays. Normally, a Talk-Through Unit should only activate the PTT into the Tx-Rig when there is audio present at the output of the Rx-Rig. One secure way to achieve this is by the use of a signal which is related to the Rx-Rig's (FM) receiver Squelch state, being either "Open" or "Closed".





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