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G B 3 Edge Hill
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Our Modular Logic : "Logic-8"

ML .1 Outline

The minimum purpose of a Repeater's Logic is to control the Transmitter so that it is only engaged when the protocol conditions are satisfied to provide active users with the intended relay service. This is achieved by engaging a base-band coupling between the Receiver and the Transmitter. At all other times when there is no user input at the Receiver, this coupling must be disengaged so that the Transmitter remains silent. Additionally, for identification purposes, the Logic must also control the Transmitter into delivering a periodic transmission that bears the allocated Callsign.

In the UK, Repeater Licence conditions, and general custom and practice, set a minimum specification on the 'behaviour' which a Repeater should exhibit on air. But beyond that, it is permissible for the Keeper's engineering team, usually in consultation with their regular Users, to provide their Repeater with its distinctive 'personality' based on additional signalling or minor departures from the minimum specification (subject to the approval of the Licensing Authority). Thus, no two Repeaters are likely to exhibit exactly identical traits, when encountered on the air. GB3EH is controlled by a Logic which meets the control specifications through a signalling scheme which is fully compatible with the CAIRO-8 transceiver interface.

For simplicity, it is called Logic-8.
It is intentionally a "minimalist's logic" !

We think it is a particularly elegant solution but, like all other Repeater teams, we would say that wouldn't we?
So, please judge for yourself ...

ML .2 Why CAIRO-8 Compatibility ?

CAIRO-8 is a "de facto" standard interface for the baseband signals outlet of any transceiver to operate with a range of active modules in a variety of fixed-frequency applications. It embodies many conventions and practices which also suit the specific Receiver and Transmitter radios of the permanent Repeater and its interface to that essential control module; "The Logic". In summary, it provides and controls an audio Relay where baseband signals, received from transmitting users on the Input frequency, are cross-coupled for re-transmission to receiving users, on a different Output frequency. Invariably these two frequencies have a nationally standardised, band-specific separation; -600 KHz for 2m, or +1.6 MHz for 70cms (in the UK). Once set to the allocated frequency pair, the two radios of a given Repeater resemble a single "transceiver", albeit one which is capable of duplex operation. (There is also the significant engineering issue of RF de-sensitisation which is addressed elsewhere; e.g. CAIRO Talk-Thro'.)

The necessary base band link between the Transmitter and Receiver radio pair, may be achieved simply by attenuating and isolating the CAIRO-8 Audio-Line output from the Receiver onto the microphone input to the Transmitter. The CAIRO-8 specification dictates that both of these should be standard levels; 1Vpk and -65dBV respectively, so the coupling may be achieved with either a Transformer or else an Opto-Isolator device. (These CAIRO techniques were developed for and evaluated in, various temporary Relay configurations.) CAIRO-8 also specifies a Receiver Squelch signal which is directly compatible with a Transmitter's PTT input. Thus, in principle, this is a simple coupling which has to be altered only slightly so that the Transmitter may be additionally "keyed-up" for Callsign delivery.

ML .3 Control Protocols

The essential difference between the Repeater, as a permanent facility, and the temporary or emergency Relay, concerns the conditions under which the PTT may be asserted for an Output transmission to occur. Repeater operations preclude the simple Squelch-to-PTT coupling of the temporary facility, to impose a more complex set of protocols which must be satisfied before the Transmitter becomes engaged. These protocols originate in events which may either be External, leading to the Relay mode as the controlled response to active users, or Internal, leading to the Signalling mode in which various transmissions may be issued to the receiving users. The protocol for External events ensures that only genuine user Input, from the continuously running Receiver, is relayed to the Output, so that spurious received signals cannot engage the Transmitter, nor hold it open excessively following genuine user Input. The protocols for the Internal events determine when and in what form, the Repeater may issue "prompt" signals or periodically enter its Beacon mode to send Callsign and related identifications. Typically, these identifications are delivered in Morse code, at rates which do not exceed 20 wpm, with the Callsign occurring at least once every fifteen minutes when the Relay mode is engaged.

Beyond the minimum statutory requirements, it is customary for the Logic to control a Repeater with various additional or secondary features which make it "user-friendly" on the air, as noted earlier. This secondary behaviour is neither prescribed nor proscribed in any detail, in the (UK) Licence, so Installers have the freedom (within reason) to configure their Repeater for a particular "personality" to suit their users. These "personality traits" arise as the Installers set the exact conditions which user Input must satisfy for the Relay mode to be engaged and maintained, and as they resolve the type, variety, rate and duration of the internally generated signals which may occur between the overs of active users and likewise, for the Beacon mode to be beneficial to users and listeners alike.

It is the task of the Logic, as a CAIRO-8 active module in the case of GB3EH, to mechanise these protocols.

ML .4 CAIRO-8 Compatibility

A variety of radio pairs may be suitable for a particular Repeater to operate in a specific band and achieve an acceptable "footprint" of service coverage into its target locality. Once selected, such Receiver and Transmitter radios should be capable of modification so that their respective received audio and squelch signals, and transmit microphone and PTT signals, meet the CAIRO-8 specification in full. In the case of ex-PMR equipment, the required CAIRO-8 signals and levels will often be found to be exactly what the equipment was designed to provide and will therefore, require little more than one or two minor "lid-off" adjustments. These can be undertaken when installing the single DIN-8 outlet socket that is required to accept a CAIRO-8 'loose-tail' as the single umbilical to the Logic module.

ML .5 Paired-Radio Compatibility

If the two radios are housed as separate items, the Receiver should be furnished with a single DIN-8 socket as the outlet for its Audio-Line (pin-6), Speaker (pin-1), Squelch (pin-8), 12V d.c. Power (pin-7) and Ground (pin-2) only, with no connections to socket pins 3, 4 and 5.

Conversely, the Transmitter should be furnished with a single DIN-7 socket as its CAIRO inlet for the isolated Mic.-High (pin-3) and Mic.-Low (pin-5) signals, the PTT (pin-4) and Ground (pin-2), only.

When the Logic module is to be used in this 'paired-radio' configuration, it should have a 'split tail'. This may be formed from two lengths of CAIRO-line cable (four individually-screened cores) which emerge from a single Audio-8 line-socket to a DIN-8 plug (for the Receiver) and a DIN-5 plug (for the Transmitter). Please note that it is the Receiver which supplies power to the Logic module, both for consistency with the other signals which are all feeds into the module, and also to allow the Logic to be activated even before the (final) connection is made to the Transmitter. This arrangement also ensures that if ever the Receiver becomes deprived of its power supply, the Logic module is similarly deprived so that the Transmitter can never be activated, even when it has a separate supply which remains live.

Also note that the Ground (or earth-screens) returns will become common in the Logic Module, so the radios must be compatible in this respect. (This condition may well be imposed by considerations of the aerial bonding requirements.)

ML .6 Talk-Through Tests

Once the radios have been engineered to CAIRO-8 (and tuned for their allocated Channel) they may be bench-tested, well before the Logic has been constructed, by using a single CAIRO-8 Talk-Through Unit (C8-TTU) from Receiver to Transmitter (on a dummy load). This coupling is sufficient to ensure that the Audio-Line output level is set correctly, at 1Vpk, and that the input (microphone) sensitivity is correct at "-65dBV" and has the electret-bias present (at 5V). For this last reason, it is preferable to use the *OPTO* version of the C8-TTU because this is the method of interface isolation which is used in the Logic-8 module to achieve a simple, yet very effective means of controlled mixing between the Relay-mode audio and the Signalling-mode tones. With radios which are housed together as a single item, use the Talk-Through module in its self-loop configuration. With radios which are separate units, use a C8-TTU from the Receiver to the Transmitter.

ML .7 Modularity - in principle

To achieve maximum flexibility in the engineering, installation and subsequent operation of a Repeater in CAIRO-8, the Logic scheme adheres to the usual CAIRO practice of modularity. This allows a minimum configuration - Logic-8 - to be installed at first, to be followed by an upgrade to a more advanced configuration as time, finance and enthusiasm may permit.

The basic Logic-8 scheme uses a few discrete digital components to determine the onset, duration and rate of the main signalling events, to meet the well-known KISS criteria ! This achieves a sufficient controller for a (new) CAIRO-8 Repeater to be legitimate on the air while the Installers adjust the aerials, evaluate the coverage "footprint" and so forth, and the Users establish themselves as a coherent group. This last activity is considered to be a non-trivial aspect of the approach because it allows the Keeper to discover the full "personality" which the Users would like, if and when an upgrade is considered. A significantly upgraded controller might be designated Logic-80 if its more complex control protocols are achieved by a Microprocessor (e.g. a Z80) or other modern programmable logic devices. (Please note, we are simply using the term 'Logic-80' to signify any controller which exceeds the minimum, Logic-8 control specification.)

Fig. 1 : Modular Repeater Logic


ATIC Front-Panel

ML .8 Modularity- in practice

For modularity to have practical benefit, the sub-system functions are allocated to separate Cards, to allow for interchangeability. In either scheme ('8' or '80'), there has to be a CAIRO-8 compatible Through-Audio coupling path (between the Receiver and Transmitter), together with a set of free-running audio tone generators which are selectively gated onto this path for the Callsign and other indicators to be transmitted. These elements are all assigned to a single, essential Audio and Tones Interface Card - ATIC - which is either a Eurocard prototype board or a specific PCB but with Eurocard dimensions and (DIN 41612) compatible edge connectors.

The second modular Card, in either scheme, implements the "Logic" mechanisms and does so entirely on the basis of digital signals which control the ATIC board. Thus, the Logic-8 or 'Logic-80' cards are readily interchangeable, in terms of electrical signalling to the ATIC, even though the mechanisms which they implement may be vastly different.

Further rationale for this level of modularity anticipates that some Installers may wish to develop the control well beyond the basic level of Logic-8 by the use of a Microprocessor single-board system (SBS) or other modern programmable logic devices. This observation has influenced the positioning of signals at the edge connectors so that, as far as possible, they are grouped in 8's for compatibility with the typical SBS's Parallel Input/Output (PIO) devices.

Modularity, in terms of separate cards for separate hardware functions, also carries the usual engineering benefit of electrical separation and isolation between analogue and digital signals to avoid spurious couplings which might otherwise degrade the performance of the whole.

ML .9 Interface Card - ATIC

The ATIC (Audio Tones & Interface Card) uses well-established CAIRO techniques, most of which are explained on the CAIRO Web Site.

Opto-isolators provide the microphone-level input to the Transmitter, as the Through-Audio coupling from the Receiver's Audio-Line output, together with the Signalling tone sequences which are periodically generated by the Logic. These separate sources are 'mixed' together on the audio bus which results when the bilateral FET outputs are wired in parallel as the Mic.-High and Mic.-Low lines. In this general arrangement there can be as many opto-isolators (i.e. *OPTO* circuits) as necessary to couple several independent sources. One *OPTO* is used as the Through-Audio path while further *OPTO* circuits, each having a different fixed attenuation, add the Signalling tones at selectable "deviation" levels. By this means, the periodic or Beacon Callsign may be sent at full level when the Repeater is idle, but at a significantly lower, background level whenever there is an active user. If the two couplers are engaged together, a "loud" level is achieved and this may be appropriate for some warnings, e.g. the time-out alert.
Each coupler is independently controlled by a logic signal at the cathode of its optical diode.

For compatibility with the remaining CAIRO-8 signals, the ATIC includes a relay which converts the logic PTT signal into a 'metallic-contact' output, and a buffer gate which converts the 'metallic-contact' Squelch input into a logic signal. The Audio-Line signal is used by a pair of hybrid devices ('567, '556) which detect the presence of tone-burst and user-speech on the Repeater input. Their digital outputs, TONBST and VOX respectively, are available to the mechanism which implements the initial-access protocol.

Fig. 2 : Schematic of the Audio & Tones Interface Card - ATIC

The ATIC module also includes a pair of frequency generators which are based on two crystals running at "7" and "8" MHz. These feed into two divider-counter ICs to provide a selection of square-wave oscillations for the on-air Signalling tones like the periodic Callsign, the "over-K" and time-out warnings, etc. The 8MHz chain has ' 2n ' divisions at 2K, 1K and 500 Hz while the 7MHz chain has ' 2n ' divisions at 1750 Hz, 875 Hz and 437.5 Hz. This latter set of tones is provided to benefit users with "musical ears" who may wish to check and perhaps tune, their own radio's tone-burst circuits (1750 Hz in the UK) against any Signalling which the Repeater delivers at these pitches. A multiplexer IC is then used to select one of these tones at a time, and to "key" it when the tone is delivered as a Morse code sequence.

Finally, the ATIC has buffers for the main internal logic signals to be displayed on LEDs for the benefit of on-site engineering adjustments. These LEDs are all supplied by, and individually pulled down from, a rail that may be switched off to conserve power during unattended operation.

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G B 3 Edge Hill
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Please proceed to The ATIC Module