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G B 3 Edge Hill
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The ATIC Interface Module

IM .1 Outline

To ensure full compliance with CAIRO-8, the Logic-8 controller module (or Logic-80, etc.) must be accompanied by an Audio & Tones Interface Card; the ATIC module. The general purposes of this card have been outlined previously, both to clarify the overall design philosophy and to establish the demarcation of tasks between the constituent modules. Most of the ATIC sub-systems are based on well-established CAIRO interface techniques or the basic practices of digital electronics which are taken to be self-explanatory, in most cases. Nevertheless, constructors and installers may wish to consider the following engineering aspects in greater detail than was given in the earlier schematic, before embarking on construction.

ATIC Sub-System PCB

IM .2 Signal Conventions

The digital signals which pass between individual cards, or which pass between optional elements on a particular card, are all operated as 'active-low' controls with pull-up resistors on the using gate, if they cause critical actions. This ensures that if the sourcing logic is omitted (or becomes temporarily disconnected) the action will default to a benign state. In most cases, the digital signals are given names or mnemonics which declare their active level, or the action which a high-to-low transition will initiate. The Boolean inverse of any such signal, and the few which are active-high, are shown as ^_^ in these pages, e.g. ^SQLCH^ (= SQLCH = not-SQLCH (= not-'squelch')).

Likewise, digital ICs are shown simply by their functional designation; "74_xxx" (e.g. 74_04) or just " 'xxx " (e.g. '04), though it would be usual to employ the 74LS or 74HC series of devices in most cases. (The exception is the 74S287 , 4-bit PROM used in Logic-8.) [Six-figure references relate to RS Components.]

IM .3 Audio Coupling

The audio coupling, from the Receiver's Audio-Line output (1Vpk) to the Transmitter's microphone input (-65dBV), uses Opto-Isolation (i.e. the *OPTO* circuit) to comply with the general CAIRO requirement for isolated-pair microphone inputs. Beneficially, this results in an audio 'bus' which has two further opto-isolators (e.g. H11F1/3 [650-790]) for the various signalling tones to be superimposed. However, unlike the basic *OPTO* configuration, all isolators in this application require an additional control signal applied to their optical diodes. These signals are 74_07 buffered to remain as an active-low (pull-down) control on the cathode, to engage the d.c. path whenever coupling is required. In each case, a 2.2µF capacitor is needed to provide the a.c.-signal with a low-impedance shunt path to ground.

Because this method of control intentionally alters the d.c. conditions on the optical diode, it is essential to have the a.c.-coupling capacitor, at the anode, for any signal whose source may be susceptible to these d.c. changes. However, the coupling and attenuation impedance are such that a small, 1µF capacitor may be used before the required attenuation resistor (e.g. an 8.2K for the Audio-Line Opto) rather than the larger, 22µF item used after the resistor, as previously declared in the C8-TTU configuration.

The two opto-isolators for the signalling-tone injection, derive their input from a TTL gate where the signal swing is equivalent to a "2Vpk" source. The attenuation resistor for the "low"-level coupler is 76K, and is 33K for the "medium"-level coupler, though Installers may wish depart from these recommended values in some installations. When these two couplers are engaged together, for the "loud"-level to occur, it is as if a single 22K resistor was applied to a single isolator. To ensure that the tone signals are added in the correct phase under this condition, all three opto-isolators must have their bilateral-FET outputs wired in 'pin-for-pin' parallel, with a single 0.001µF capacitor across the bus, mounted close to the through-audio coupler pins. The control signals for these three couplers are designated TL-L, TL-H (Tone Level - Low, High) and EAR (Enable Audio Relay), respectively. To ensure full immunity from spurious signals on the main 5V power rail, the three opto-couplers must share a single *REG* regulator .

Fig. 3 : Modified CAIRO-8 *OPTO* for Controlled Bus Mixing

IM .4 Tone Generators

The ATIC has two tone generators, each based on a crystal oscillator and a counter-divider (e.g. 74_4060 [632-679]) to provide the selection of tones for the on-air Signalling. The '8MHz' chain actually has an 8.1920 MHz crystal so that the taps, at 212, 213 and 214 (IC-pins 1, 2 and 3), provide exact 2K, 1K and 500Hz outputs. The '7MHz' chain has a 7.1680 MHz crystal so that its equivalent taps provide exact 1750, 875 and 437.5 Hz outputs, respectively.

If required, Installers are at liberty to use a substitute '7MHz' crystal (between about 7.0 and 8.5 MHz) for the taps to yield alternative tones, but at pitches which lie within the communications audio pass-band (i.e. 400Hz - 2.5KHz). Conversely, the '8MHz' chain is further divided with a 74_90, ÷10 IC, to yield a 50Hz (20mS) output, CLOCK, as an additional reference signal for several mechanisms in Logic-8. Therefore, it would not be usual to use a substitute crystal in this chain, unless the consequences of doing so had been fully considered.

The six tone-taps, from the two chains, become the inputs (1 to 6) to an 8-to-1 Multiplexer (e.g. 74_151, [307-654]) for a single, selected output square-wave (at 2Vpk) into the tone-couplers. This assignment requires inputs 0 and 7 to be grounded (logic-'0') so that the full set of outputs map onto the multiplexer select lines as follows ('x' = don't care);






Fout Hz























































The most-significant line - C(3)- selects the primary tone chain ('0' = "8", '1' = "7" MHz) while the other two lines select the tone taps within each chain. Inverse symmetry in this assignment, ensures that the two default conditions, either all-0's or all-1's, give a benign "no-tone" output. The Multiplexer also has an overriding control input - STROBE - to enable or disable its output. This is used to apply the train of pulses which "key" the selected tone into Morse Code (or pips).

IM .5 Tone-Burst Detector

The presence of the user-input "Access" tone-burst is detected from the Audio-Line, by an NE567 IC [307-294]. This is a standard Phase Locked Loop (PLL) device with an internal free-running oscillator which attempts to align itself with any input oscillation which is close in frequency to its own, and deliver a two-state output accordingly ('0' = locked, '1' = unlocked). The CR-"stack" (see IM.6, below), at pins 5 and 6, determines the free-running oscillator frequency. With the values shown in Fig. 4 below, the centre frequency, as observed by a frequency counter at pin-5, can be set exactly to 1750Hz.

Fig. 4 : Tone-Burst Detector using NE567 PLL

The PLL's pin-2 capacitor is a low-pass filter which determines the capture bandwidth. With the value of 10µF, this allows tones to be in error of 1750Hz by about ±2%. The pin-1 capacitor is an output filter. This determines how long the PLL must lock before the output will be asserted (low) and it also determines how long the output remains asserted when the PLL unlocks. With a 47µF capacitor it requires a minimum of ??mS to lock (and the output holds for ??mS after unlock). The 0.47µF input capacitor upholds the CAIRO-8 rule that the Audio-Line must be a.c.-coupled to any using device. In this case, the pin-3 input has an internal d.c.-bias present which alters as the PLL locks or unlocks, so this capacitor is essential. (How many hours we lost discovering that one !!) The 4.7K resistor, at pin-8, is a minimum load on the output to give the signal, TONBST, its TTL-compatibility properties. (For construction purposes, please note that the NE567 IC is prone to radiate its oscillations onto nearby tracks and other ICs and should therefore, be sited well away from the opto-isolators, etc.)

IM .6 Time-Constant "Stacks"

It is also useful to note here that several sub-systems in Logic-8 use devices which require an external 'CR'-pair for a time-constant which must then be capable of on-site adjustment. Generally, the 'C'-capacitor is chosen as a single, preferred value item to set the broad period, while the 'R' is implemented as a fixed resistor in series with a variable resistor or "pot" to allow for adjustments. The value of the fixed resistor is at least the minimum which the device requires, whilst the "pot" (e.g. a PCB-mount, multi-turn) has a value which, at maximum travel, allows the pair to achieve, but not exceed, the maximum value which the device will tolerate. This results in three series components; hence the term "stack". In some cases, as in this PLL circuit, the fixed resistor may be the larger value of the two, so that the "pot" provides only a small range for a non-critical final adjustment, within its multiple turns.

IM . 7 User-Speech Detection

In some protocols for initial user Access, it may be a desirable requirement for tone-burst to be followed by a period of user-speech before the Repeater will latch open. In turn, this requires a detector which "listens to" the Audio-Line to signify, with a two-state output, that audio is present. This is less specific than the PLL's task, which is to signify only when 1750Hz audio is present, but more specific than the CAIRO-8 Squelch signal which only signifies that there is a carrier. In effect, it must work between these two extremities to show when the carrier Input is not "blank".

Cairo-ists will recall that Audio-Gating or "VOX" arrangements can be troublesome, in the general CAIRO scheme which may be applied to any radio, for not knowing precisely the audio response and tailoring of any particular receiver. In the case of the CAIRO Talk-Through units, where user-traffic has to be detected, this is reliably achieved by deriving a two-state signal from the high-level noise output of an un-squelched (CAIRO) receiver as it drops to a lower-level when a carrier occurs. Strictly, this is not an exact user-speech condition but it is sufficient for the requirements of the temporary Talk-Through, nonetheless.

However, as a result of continuing developments and extensive evaluations, a circuit has emerged which exhibits very good "VOX" properties when supplied with signals at the CAIRO-8 Audio-Line level (1Vpk). It is based on a pair of 555 timers, from the single 556 package, working in a 'Master-Slave' configuration which we call the "partner-discharge" pair. This arrangement seems to have escaped the attention of the authors of 555 "Cookbook" publications, yet it is so simple and versatile that it also forms the basis of the major timing mechanisms in the Logic-8 module. (It is assumed that the reader has some familiarity with the basic operations and usual applications of the 555 and 556 timer ICs.)

IM .8 Partner-Discharge 556 Timers - 'Prime-then-Fire' actions

In the partner-discharge configuration, the master-slave partnership has the properties of a 'prime-then-fire' mechanism. The Slave (the "B-side" 555) is configured with a CR-stack that is coupled to its TRigger and THreshold inputs, for a normal monostable action (t = 1.1xCR), while the Master (the "A-side" 555) controls the CR-stack with its DIScharge output, to inhibit that action. When an appropriate initial condition occurs at the Master, it discharges the stack capacitor to 0V. This level is below the Slave's TR-comparator "V/3" reference, as determined by the internal ladder of three similar series resistors. Thus, the Slave's latch becomes set, to 'prime' the partnership into a high ('1') OUTput. When subsequent conditions at the Master cause it to release the stack, a voltage 'ramp-up' begins and this eventually reaches the Slave's TH-comparator "2V/3" ladder reference level, to reset the Slave's latch and 'fire' the partnership for a low ('0') OUTput (VOX = 1). However, at any time during the ramp, the Master may respond to other input conditions and discharge the stack again, thereby holding the 'fire' until the target condition has been stable for the complete monostable period. By this means, the partnership overrides all short-duration perturbations in the input signals and only signifies when the target condition is established and maintained. This allows the whole configuration to reject a much larger set of false occurrences at its input, than would otherwise be rejected by a single 555 (and many other timing devices !).

Fig. 5 : VOX-Gate ; Master-Slave "Partner-Discharge" Pair

The action of the Slave 555, with respect to the CR-stack, can be likened to a Schmidt-gate working with a switching signal which has a large voltage margin between its low and high levels. For this, it is sufficient to operate with the internal resistance ladder and accept the default references of 1.7V and 3.4V (approx., for a 5V rail) by leaving the Control-Voltage input (CV-pin), at the "2V/3" junction, unused.
However, in a high RF-pollution environment, such as a Repeater, this pin [3, 11] must be decoupled; 0.1µF. When required, further (active-low) inhibitory control may be applied to the RESet input of a 555 latch or else this may be tied directly to the rail, if not required.

IM . 9 Partner-Discharge VOX-gate

(The VOX-gate is our most intriguing use of the partner-discharge idea, so you might like to study a more straightforward approach first ?, e.g. Time-Out.)

For the VOX detector, the Master is configured as a Schmidt-like gate for analogue signals. The CV-pin is used to lower the reference voltages, e.g. with a diode (Germanium, in this case), so that the margin between the "2V/3" and "V/3" levels becomes only 0.2V and 0.1V (approx.) and is thereby consistent with the "swing" of the typical (1Vpk) a.c.-signal on the Audio-Line. The THreshold input is tied high to force the master latch into a reset condition but this is overridden by the TRigger input which is tied low to force the latch to remain set, instead. (This is the normally-avoided "indeterminate" condition of an SR-latch where " Q = ^Q^ = 1 "). Thus, in the absence of an a.c.-signal to alter this contention, the master's OUTput will be high ('1') and its DIScharge pin will be high-impedance. This contention only alters when the frequent positive-going peaks in user-speech cause the a.c.-coupled Audio-Line to make excursions above the "V/3" (= 0.1V) level and remove the setting condition, for the reset condition to predominate. In turn, this causes the master DIScharge to clamp the stack capacitor and prevent the slave from completing its intended monostable action. (In effect, the DIScharge pin has become a direct open-collector output from the TR-comparator.)

As noted already, the monostable action may only reach completion if the Audio-Line stays silent long enough for the stack to ramp past the ("2V/3" = 0.2 Volts) THreshold at the Slave. Therefore, if the stack period is of adequate fixed duration (e.g. 0.5 Sec.), this VOX-gate will span the normal pauses in user-speech for the Slave's OUTput signal (^VOX^) to remain primed ('1'). For consistency with other ATIC signals, this output is 74_04 inverted to become the distributed VOX signal.

Additional control is applied to the Slave by using ^SQLCH^ at its RESet input. This74_04 inverted signal is high when the Receiver squelch is open and goes low when a user finishes a transmission. It is included so that in this circumstance, the VOX signal is quenched immediately. It may also be noted that this particular Partner-Discharge configuration does not use the Slave's (open-collector) DIScharge pin to control the stack. Instead, it may be used as an output which operates in the same sense as the main output. One application of this, though not required here, is to pull-down a LED to show when the partnership is "fired but not primed". (This is the partnership's inactive condition but it often has significance for a follow-on circuit.)

In all applications of the 556 IC, it is essential to decouple its supply pins (7 and 14); 47µF.

IM . 10 Eurocard Layout

Apart from the RF-pollution problem which may well occur when the ATIC is coupled to a suitable Logic card at a Repeater site, as noted earlier, the ATIC sub-systems may co-reside on a single Eurocard without mutual interference, Fig. 6. Generally, the components and devices for the analogue signals which are directly associated with the CAIRO-8 interface, should be positioned close to the edge connector pins, for the digital devices to be suitably spaced elsewhere on the card. The crystals should be positioned well away from the opto-isolators to minimise any coupling to the CAIRO mike-input bus, since its wires or PCB tracks will not be screened.

Fig. 6 : Layout of the ATIC as a Euro-Card PCB

The conventions and practices for power supply in the scheme, require each card to derive its requirements from the 12V d.c.-power line; ATIC pin-2 (CAIRO-8, DIN pin-7). The main 5V-rail to the digital ICs (and a LED-display) uses a 7805 (5V @ 1A) regulator (e.g. [648-422]) with 100nF and 1µF, decoupling and smoothing capacitors mounted in close proximity to its input and output leads, respectively. This regulator will generate heat (especially when the LED-display is powered and the Repeater is active) so it requires a clip-on heat-sink, e.g. 40 °C/W [402-260]. As noted already, a separate single *REG* regulator is required for the three opto-isolators.

IM . 11 Engineering Display

For engineering purposes, either on site or in the constructor's workshop beforehand, it is useful to have a simple display of significant signals, to augment the "hand-held" radio as the primary test instrument ! For this, a LED-display daughter-board, which is eventually mounted on the front-panel of the Module case, shows the principal two-state activities; Squelch ("RX"), 1750-detect ("TB" = Tone-Burst), PTT ("TX"), Thro'-Audio-active ("TA") and Internal Tones transmission in progress ("IT"); being the Morse train at the ATIC multiplexer Strobe input. The LED-display daughter-board is connected to the ATIC where the displayed signals are 74_07 buffered to minimise the load on their respective sources. A common supply rail (5V) includes a front-panel switch so that power may be conserved during normal unattended operation. A further LED, which is always grounded, is provided to show when the display is powered; "ON".








LP (e.g VOX)

showing :



Tone Burst


Thro' Audio

Internal Tones

Logic Probe

lit-up means :

powered up

is open

1750Hz detected

Tx is keyed up

coupling is engaged

Callsign or Pips

"Wild Card"

ATIC edge-pin








The display also includes a seventh, "probe" LED to anticipate any augmented scheme e.g. Logic-80 (and to alleviate any oversight on our part !). It is provided as a "wild-card" which Installers are at liberty to patch to any other logic signal which might usefully show a significant Internal occurrence within the Logic; e.g. the VOX-circuit detecting user-speech. This LED is also buffered on the ATIC but has an uncommitted active-low input, PROBE, on the ATIC edge-connector (pin-23). This will align it, via the mother-board, with the PIO-C4 output of the Logic-80 card but Installers may run a wire directly on the ATIC or on their Logic-8 card, to any sub-system they wish to observe, either as a permanent connection to a device or as a wander-lead for use as a simple logic-probe tool ("LP").

IM .12 ATIC Edge-Pins : A separate Table shows the assignment of signals to pins on the mother-board. Pin Table

IM .13 Module Case

Most Logic configurations can be implemented with a pair of Eurocard PCBs; one for the ATIC and the second for the Logic-8 or the (single-board) "Logic-80" prime-mover. These are coupled together with a pair of 32-way edge-connectors linked by a mother-board which may be formed from a suitably cut piece of Vero. These cards may be housed in an "interlocking" box; e.g. Maplin's CCN220 [YN52G] (220 x 100 x 40). There is sufficient surface on one end-panel for it to be drilled across the upper-half with seven, equi-spaced holes for the LEDs, mounted on a Vero-strip daughter-board, and in the lower half, the display switch on the Left, an Audio-8 chassis plug on the right, and a dual-lug DIN-8 socket in the centre; Fig. 7.

Fig. 7 : Illustrated Layout of the Module Front Panel

This CAIRO-8 socket is wired in parallel to the Audio-8 plug as an Engineering outlet for any CAIRO accessory, e.g. a telephone handset, to be plugged in for on-site testing. However, it is essential to unplug any such accessory, prior to unattended operation, because the microphone pins are permanently live into the Transmitter. The purpose of the connectors should be familiar or self-explanatory to most Operators, i.e. those notified as having responsibility for Close-Down, but it may be advisable to label the switch as "Display Power ONLY". The Logic itself, and hence the Repeater, may be disabled simply by unplugging it from the radio(s) or turning off their supply.

The back-panel of the case remains blank for the simple Logic-8 controller. Conversely, in any 'advanced' configuration, a further connector, e.g. a 15-way D-type, can be installed on this rear panel for connection to any additional module (e.g. an MSF receiver and decoder for precise Time-of-Day control).


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G B 3 Edge Hill
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Please proceed to Logic-8 Module