CommunicationsAudioInterface for Remote Operations

CAIRO-8 - the Ultimate !

CAIRO-8 is a fully specified, 'de facto' standard interface for the signals outlet of transceivers which are to be operated in the widest possible range of fixed-frequency applications. While these applications still include Remote Operations, they also extend to Augmented Operations where active modules operate in close proximity to their 'host' transceiver to facilitate the 'mode', e.g. Talk-Through Units (temporary Repeaters), TNCs for Packet, and many others having relevance in emergencies, but beyond the scope of these pages to describe in any detail, e.g. net-traffic logging tape-recorders or modules which might interface between transceivers and (private-wire) telephone signalling.

C8 .1 Augmented Operations

Active modules for Augmented Operations generally benefit from up to three additional facilities which are not normally required in the 'conventional' Remote Operations of basic CAIRO. These are d.c.-Power, Audio-Line and Squelch outputs, being a PALS augmentation, as requested in the first development period by several committed CAIRO Users - the early friends of CAIRO ! These facilities substantially simplify the design of special modules for reliable operation without elaborate set-up procedures, e.g. TTUs, whilst also improving the overall performance of certain existing 'modules', e.g. TNCs. CAIRO-8 sets a full engineering standard for each of these outputs and introduces the Squelch signal which is assigned to pin-8 of an 8-pin DIN socket.

Specifically, this connector ensures that the first seven pins stay fully compatible with all DIN plugs of basic CAIRO, so that existing Lines or accessories may still plug into CAIRO-8 transceivers and operate exactly as before. In short CAIRO-8 is "downward compatible" to CAIRO.

C8 .2 Outlet Socket

To avoid any possible confusion with basic CAIRO,CAIRO-8 uses a simple convention whereby the physical presence of an 8-pin DIN socket on a transceiver (or Rig-Converter unit, see C8 .8) is itself a sufficient 'label' to identify that transceiver as a full CAIRO-8 host. Consequently, 8-pin sockets should NOT be installed on transceivers unless the specification is implemented in full, nor should they be employed as outlet sockets on any item of basic CAIRO hardware; all of which must have 7-pin DIN sockets, as declared already.
This rule ensures that any active module, which has been engineered for operation within the advanced specification and thus has an 8-pin DIN plug on its tail, will only mate with fully converted host transceivers or with other in-line active modules, similarly engineered for full CAIRO-8 compatibility.

The convention also ensures that existing or future users of CAIRO need not vary a single item of original CAIRO equipment, nor do they need to undertake a CAIRO-8 conversion, if at all, until they have the requisite skills and detailed knowledge of their transceiver's sub-systems.

Caution: The scheme uses DIN-8 connectors which have radial 45deg. pins (for DIN-7 mating),with pin-8 at 0.7 mm below centre (e.g. socket [476-299] and plug [473-256]),not the offset-pin versions available from some sources.


Transceivers converted to CAIRO-8 present the signals of basic CAIRO, at pins 1 to 5, and the PALS set, at pins 6, 7 and 8. These latter signals are not transported by the Orange Reel, or similar CAIRO Lines, to a remote operating point but are used only by modules working into the host rig.

Pin-7 is for the d.c.-Power supply; i.e. "12V". It is declared to be this in CAIRO but is not always implemented on simple Rig-Adaptors, being seldom required in basic Remote Operations. However, in CAIRO-8 it must be implemented for use by any module that is designed to take its power from the host rig (as befits the definition 'host').

Pin-6 is for an Audio-Line fixed-level output which is independent of the volume control. Modules which use this fixed-level output, e.g. TTUs or TNCs, should not require any pre-service set-up procedure (after initial 'bench' adjustment ) and will operate with any CAIRO-8 host. In basic CAIRO, this signal is declared to be a Secondary Audio to facilitate binaural headphone sets. The specified output is compatible with an earphone supply. Thus, there is no (electrical) conflict.

Pin-8 is for the Squelch signal. This does not occur in CAIRO and gives rise to CAIRO-8.

C8 .4 Squelch (Pin-8)

References to the 'internal squelch state', should be taken to mean all conditions which disable receiver audio, including internal tone-squelch and similar facility options, when fitted inside the rig.

To achieve universality, for this otherwise highly rig-specific internal signal, the squelch output is defined as a 'metallic contact' closure, between pin-8 and ground (pin-2), when the squelch is 'open' - the contacts to have a minimum d.c. rating of 0.5 A.

This definition is consistent with any PTT input such that, in a simple Talk-Through, say, the Squelch output, at pin-8, of an Rx-Rig may be coupled directly to the PTT input, at pin-4, of a Tx-Rig - alongside a 'through-audio' path - and vice-versa for full back-to-back coupling.

Typically, the Squelch output requires a form-A relay, e.g. reed [345-561], with its coil supplied from an internal d.c. rail, e.g. "12V", and activated by a transistor whose base is then controlled by the rig's internal squelch-state signal. The transistor could be any general-purpose switching device, either an NPN or PNP type depending on the sense in which the available internal signal operates. Fig. 12 shows an NPN device energising the relay in the presence of a positive (w.r.t. ground) 'squelch-open' signal. A PNP would be used with an active-low signal. (Back-e.m.f. protection by shunt diode at the coil is usually included in the encapsulated relays.) Transceivers which have a "Busy" lamp (not an LED) may accept the relay coil in parallel with the lamp as an increased, but still tolerable, load for an existing transistor (or similar) switch.

C8 .5 d.c.-Power ("12V": Pin-7)

This is a 'raw' but protected 13.8V +/-15% supply capable of delivering 1A maximum to power external active modules. Protection should be by fuse, e.g. 1.25A 'slow-blow' (cartridge), or an over-current trip, and this supply should turn off with the transceiver's On/OFF switch.

Individual active modules are unlikely to draw power to the limit of this provision but it is permitted, in the design philosophy, for up to 5 low-power modules to be connected in a 'chain' to a single host transceiver, with the chain being then terminated by remote operator accessories. Each module should be designed to provide a single and 'simple' function to enhance the operational facilities of a station, but only when required. When two or more functions are required, the relevant modules would be plugged in-line with each other, choosing carefully the order in which they are configured because this may determine the priority of one function over another.

The typical active module requires only a short signals tail (e.g. 0.5 m - 1 m) with an 8-pin DIN plug, and a single (chassis) DIN-8 socket. Because the tail is short, multicore overall screened cable, e.g. 8-cores [367-460], may be used. Every plug signal must pass through the module, pin-for-pin, to the DIN socket, to support the chain connecting scheme. In effect, this forms a signals bus, passing from module to module, for final termination at the remote end.

Within a given module, the sensing or generating circuits for that modular function should be T-connected onto the relevant bus lines. The circuitry should be designed to present negligible loading and inject negligible currents onto the bus lines, except during active signalling periods. In short, an active module must be 'transparent' to all signals, both when it is de-powered (by its own On/OFF switch) and again when it is ON, but not itself signalling.

Two of the cores of the suggested cable should always be used together for the pin-7 power bus to provide adequate current handling in all circumstances, Fig. 9. (DIN pins are rated at 2 A.)

Fig. 9

Fig. 10

Active Module Signals Bus

*Reg* Regulation

C8 .6 d.c.-Regulation

In many "temporary" operations, transceivers are frequently operated in vehicles with the vehicle battery being the overall power supply. Increasingly, vehicle "electrics" are hostile to this supply, injecting several forms of unsuppressed fluctuations; alternator whine, start-up loading, electronic ignition, headlight dimmers, etc., some of which may give rise to several volts of noise.

When designing active modules, all reasonable steps should be taken to protect the module from these fluctuations, particularly when they could affect circuits which generate a mike input. There can be no guarantee that every host transceiver will provide adequate filter protection, on its "12V" rail output, against such spurious interference, in all circumstances !

For this reason, it is recommended that a fixed-voltage regulator, e.g. 78L05 [648-488], is used in any CAIRO circuit which may be susceptible to fluctuations on the 'raw' d.c.-Power line. This will also prevent a given module from injecting its own spurious signals onto the supply to the detriment of other modules in chain connections. The full regulation is shown in Fig. 10 where it may be noted that the two de-coupling capacitors should be mounted in very close proximity to the regulator's leads. The 4K7 resistor is a minimum-load conditioner for the regulator, should the intended load be disconnected or be of such high impedance that it draws insufficient current.
(For simplicity, in later schematic figures, this regulation will be shown as a 3-terminal block - *Reg*.)

n.b. This *Reg* arrangement is also recommended for providing the electret bias in the Rig-Adaptors of basic CAIRO, by substituting it for the 10K resistor of Fig. 6 ("G" to ground), and omitting the 10 uF capacitor.

C8 .7 Audio Line (1Vpk : Pin-6)

The CAIRO-8 Audio-Line is declared to be a fixed output at a level which closely relates to the '0dBm' professional audio standard and is therefore sufficient for most practical purposes. It is obtained by installing an audio amplifier IC, e.g. LM380, which derives its input from the 'top' of the receiver volume control, assuming this to be a squelch-gated source of received audio. If not, a squelch-gated source elsewhere in the receiver chain must be identified since this output, like the pin-1 speaker signal, must be silent during transmit or when squelch conditions apply, except in dual-band transceivers in their "duplex" mode.

The Audio-Line amplifier should have a 1M potentiometer (multiturn, PCB-mount, e.g. [160-152]) at its input and a series resistor at its output so that the precise open-circuit output level may be set. This is 1 V peak (2V peak-peak) for a received sine-wave, at 1 KHz, transmitted at 100% deviation in FM systems. The rms value of this 1Vpk calibration sine-wave is 707 mV.

The output impedance must be no greater than 600R so that all active-modules which use this signal, must present a high impedance load, not less than about 5K. In practice, the output impedance is set to 220R with a resistor so that, if ever binaural headphones are connected to a CAIRO-8 socket, the typical 32R earpiece will have a working, but not excessive, listening level.
(This gives rise to the alternative designation for this pin-6 output as the Earphone Audio Level.)

Fig. 11

Audio-Line Amplification
to fixed-level -

All 'central' pins of the LM380 DILIC are grounded for the tracks to act as a heat-sink.

The Audio-Line output matching, together with the d.c.-Power provision (1 A max.), sets an upper limit of 5 on the "Fan-Out" drive into active modules which are chained together.

C8 .8 Engineering CAIRO-8

All CAIRO-8 specifications must be honoured in full 'behind' the 8-pin DIN outlet socket.

Typically, the additional circuitry would be installed inside the transceiver (on a PCB), for the 8-pin DIN socket to replace the original mike connector - as a 'Grey-Box' conversion.
Because of this, CAIRO-8 is not normally applicable to hand-helds and other very compact rigs, nor to any other transceiver for which a manufacturer's warranty agreement remains in force.

However, it is admissible to construct an external module - a "Rig-Converter" - for transceivers which may have the extra PALS signals available, either at a non-CAIRO 'back-panel' interface or brought out directly to such a module via a short length of multicore signals cable. This approach to CAIRO-8 is then similar to the use of a Rig-Adaptor to achieve basic CAIRO. Almost always it will be essential to secure this converter module mechanically to its transceiver.

If the transceiver's original 'front-panel' mike connector is retained, the module should have a short signals tail (with matching plug) such that this is the ONLY connection for the converter's mike (and PTT) feeds. Then, for CAIRO-8 operations, it becomes essential to plug-in this tail, in place of the 'local' mike, and so prevent electrical contentions on the rig's mike input.

Fig. 12

Transceiver Conversion

schematic only

It is also recommended that any CAIRO-8 conversion should include a mute switch, in series with the rig's internal speaker, for use after operational set-up procedures are complete. The acoustic output of a remote transceiver might be a source of irritation to nearby personnel and 'universal mode' active modules, designed to operate with non-FM host rigs (e.g. SSB), may well require FM rigs to be left un-squelched - a major irritation if not muted !
Likewise, message traffic at emergencies may involve sensitive information (about casualties) which bystanders near an unattended remoted vehicle, say, should not be able to over-hear.
(This more general case would suggest that muting should somehow be achieved, even in basic CAIRO.)

One method, in CAIRO-8, is to replace the speaker jack, if one exists, with a suitable switch of similar shank dimensions. It is no longer necessary to retain this jack because the equivalent output is now presented at the CAIRO-8 socket (pin-1).

Fig. 12 shows the elements of a full conversion as it might appear, inside the transceiver (or rig-converter), looking towards the rear of the 8-pin DIN socket. Purposely, the details of the Squelch interface are left undeclared because they will vary from one transceiver to another and must be designed in detail by the owner, in conjunction with the manufacturer's handbook.

C8 .9 Surge Protection

The signal circuits, between a host transceiver and the active modules, being harnessed together in single, multicore cables and multi-pin (DIN) connectors, run the small but finite risk of accidental shorts occurring either to the power lines or to other signal lines of higher potential. To protect the host transceiver from these and similar surges, it is advisable to include two series pairs of general-purpose silicon diodes, connected in anti-parallel across 'sensitive' signal lines, Fig. 13. Such protection should be included between the Audio-Line amplifier output and ground, and again across the isolated input pair for the mike. Likewise, this simple protection should be included in active modules based upon particularly surge-sensitive (semiconductor) devices.

The operational requirements of a Talk-Through Unit usefully illustrate the engineering applications of the 'extra' PALS, as specified and provided in the CAIRO-8 augmented standard. (The full explanation of Talk-Through schemes is given on a subsequent page.) However, the circuits which follow also incorporate engineering considerations that make them applicable as sub-systems of a wider range of active modules for operation with CAIRO-8 hosts.

Fig. 13

Fig. 14

Signal Surge Protection

Transformer Isolated Through-Audio (*XFMR*)

C8 .10 Isolated Audio

To accompany the cross-link between Squelch and PTT, a Talk-Through-Unit also requires an attenuated path (for ~44 dB's loss) between the Rx-Rig's audio-line output, at ~0.7 V rms (1Vpk), and the Tx-Rig's mike input, at ~4.5 mV rms (-45 dBm), both at the nominal impedance of 600R at 1 KHz. Simple resistive attenuation is inappropriate because detrimental d.c.-loop currents could pass between the audio-line amplifier of the Rx-Rig and the electret-bias on the Tx-Rig's mike input. To avoid this, and to comply with the general CAIRO scheme for cross-talk minimisation and for compatibility with 'series' rigs, any 'through-audio' path should incorporate a means of isolation.

The audio transformer, e.g. 600R, 1:1 [208-822], offers the basis for one solution, Fig.14.
The primary winding is supplied from the audio-line, via a series resistor, whilst the secondary, as the isolated-pair 'mike' output, has a symmetrical pair of resistors giving a fixed attenuation. With this arrangement, the 'primary'-side impedance exceeds the minimum 5K load specified for the audio-line amplifier of a host Rx-Rig and the 'secondary'-side impedance is sufficiently high never to compromise the electret-bias arrangement on the mike input of any Tx-Rig transceiver.

The basic TTU which results is an uncomplicated modular item such that a pair of TTUs, working with CAIRO-8 host rigs, serve several alternative configurations (section TT .8).

(For simplicity, in later schematic figures, this isolation will be shown as a 4-terminal block - *XFMR*.)

C8 .11 Opto-Isolation

An alternative approach, for compact and lightweight TTUs, uses opto-isolation, Fig. 15. The isolated bilateral analogue FET output closely resembles an electret device in its compatibility with the biased input of any CAIRO-adapted transceiver, as the Tx-Rig, while the photo-diode operates with d.c.-power and audio-line output from a CAIRO-8 host transceiver as the Rx-Rig. The Rx-Rig's d.c.-Power line provides the standing bias on the diode, via full regulation, *Reg*.

Fig. 15

Opto-Isolated Through-Audio Coupling


The opto-isolator (e.g. H11F1/3 [650-790]) and the a.c.-coupled series resistor provide ~44 dB's attenuation with the non-critical component values (10% types will do) as shown above. For a reliable implementation of this carefully evaluated circuit, use only a 'postage-stamp'-sized PCB, to keep all component leads short, and remove or 'stop' excess track on Vero-type boards.
(For simplicity, in later schematic figures, this isolation will be shown as a 5-terminal block - *OPTO*.)

It is also useful to note that this arrangement, though slightly more expensive than transformer isolation, has two further advantages. Firstly, the output impedance is sufficiently high to make it applicable in any module which generates a mike signal onto the input 'bus' which will be formed when several modules are chain-connected. Secondly, the through-audio path may be controlled if the diode is returned, not to ground as shown*, but to the output of a Logic gate (TTL or similar), for the Low output state to sink the diode current and enable the coupling, while any other input condition, leading to a High output state, will mute the through-audio path. Typically, this control is useful in modules which generate transmissions only occasionally, e.g. 'pip'-tones or 'quiet-time' indicators. The opto-isolator is normally held in its muted state to be activated, as required, perhaps by the same control signal that is used to assert the PTT.

The switch which is shown here (Fig. 15) in the Squelch/PTT path, gives the manning operator control over the Talk-Through facility. This will be an optional component in the final TTU modules, which are described in more detail later (sections TT .8 and CR .7), because the insertion/removal of the Tx-Link tail connection will achieve the same purpose.

C8 .12 CAIRO-8 Loose-Tails

The general rule for CAIRO connections is that all DIN plugs, being semi-exposed male pins, "look" towards the transceivers and all DIN sockets, being shrouded female pins, present live signals for onward connection towards the terminating accessories, either directly or via boxes or modules. Maintaining this rule in CAIRO-8, requires any active module to have a hard-wired signals tail, furnished with a DIN-8 plug, and, if the module is not a full terminating unit, a pin-mapped DIN chassis socket for the onward connection. Clearly, this rules-out the use of DIN connectors to create 'loose-tails', as removable feed lines into modules, in order NOT to introduce confusion over which plug is which, on the loose-tails, and which socket is which, on the in-line modules. Further, the 12V d.c.-power facility, being routinely transported through CAIRO-8 connections, makes any form of in-feeding, via DINs, an intrinsically unsafe practice. When hook-ups might be made in haste, e.g. Raynet, and so perhaps in the 'wrong' sequence, electrical damage, e.g. shorts, could occur at the semi-exposed pins of the un-plugged 'far-end' DIN plug of a loose-tail.

But strict adherence to the general CAIRO rule for DINs will always prevent these hazards; the permitted exception being occasional in-feeds to the auxiliary socket of a Dual Operator Box. However, some existing terminal equipment, e.g. TNCs, even when modified for full compatibility with CAIRO-8 signalling, may still require in-ward connectors to overcome unavoidable mechanical constraints; e.g. a chassis already punched-out for an in-feeding connector.

For these applications, CAIRO-8 uses the 8-pin circular audio ('mike-type') connector, as its standard for creating loose-tails. These connectors are particularly well suited to this task. They have a pin-geometry which is very similar to the DIN-8 format and the pins are rated at 5 A. The "plugs" (as called) are actually line-sockets with protected female pins, e.g. Maplin FK30H, and the chassis "sockets" (as called) are plugs with semi-exposed male pins, e.g. Maplin FK29G. The screw-collar allows the mating to be locked, for a semi-permanent capture of the loose-tail, and the integral cable-clamp is particularly robust for clamping the prescribed 8-way cable. Hence these connectors, no longer required on rigs or accessories - where their pin 'direction' was incorrectly applied ! - are 're-used' as CAIRO-8 loose-tail connectors, Fig. 16 A, with a pin assignment which mimics very closely, the CAIRO-8 DIN format, Fig. 16 B , although the actual pin-numbers differ - beware !.

Fig. 16 A

Fig. 16 B

Convention for Loose-Tail Connections

Audio-8 Connector Pin Format
(viewed from the rear of male pins)

Cable Core Colours

DIN-8 : 1 2 3 4 5 6 7 8
  Blue Screens White Red Yellow Violet Black
Audio-8 : 2 4 6 3 5 1 7 8
Signal :














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